Genetic causes of sporadic and recurrent miscarriage

Approximately 80% of miscarriages happen within the first 12 weeks of gestation. More than half of early losses result from genetic defects, usually presenting as abnormal chromosome numbers or gene rearrangements in the embryo. However, the impact of genetics on pregnancy loss goes well beyond embryonic aneuploidy. For example, the use of big data has recently led to the discovery of specific gene mutations that may be implicated in sporadic and recurrent miscarriages. Further, emerging data suggest that genetic factors play a role in conditions for which there is a causative association with recurrent pregnancy loss. Here, we summarize the evidence on the genetics of miscarriage and provide an overview of the diagnosis and prevention of genetic causes associated with sporadic and recurrent pregnancy loss. Approximately 80% of miscarriages happen within the first 12 weeks of gestation. More than half of early losses result from genetic defects, usually presenting as abnormal chromosome numbers or gene rearrangements in the embryo. However, the impact of genetics on pregnancy loss goes well beyond embryonic aneuploidy. For example, the use of big data has recently led to the discovery of specific gene mutations that may be implicated in sporadic and recurrent miscarriages. Further, emerging data suggest that genetic factors play a role in conditions for which there is a causative association with recurrent pregnancy loss. Here, we summarize the evidence on the genetics of miscarriage and provide an overview of the diagnosis and prevention of genetic causes associated with sporadic and recurrent pregnancy loss. Chromosomal abnormalities are diagnosed in over 50% of first-trimester miscarriages, becoming less prevalent in second- and third-trimester losses. On karyotyping, most chromosomal abnormalities are numerical (termed aneuploidy, usually because of chromosomal nondisjunction during meiosis), including autosomal trisomies (30%–60%), triploidy (11%–13%), monosomy X (10%–15%), and tetraploidy (9%), whereas only a minority result from structural chromosome rearrangements (2%–6%) and mosaicism (8%) (1Muñoz M. Arigita M. Bennasar M. Soler A. Sanchez A. Borrell A. Chromosomal anomaly spectrum in early pregnancy loss in relation to presence or absence of an embryonic pole.Fertil Steril. 2010; 94: 2564-2568Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 2Soler A. Morales C. Mademont-Soler I. Margarit E. Borrell A. Borobio V. et al.Overview of chromosome abnormalities in first trimester miscarriages: a series of 1,011 consecutive chorionic villi sample karyotypes.Cytogenet Genome Res. 2017; 152: 81-89Crossref PubMed Scopus (98) Google Scholar, 3Eiben B. Bartels I. Bähr-Porsch S. Borgmann S. Gatz G. Gellert G. et al.Cytogenetic analysis of 750 spontaneous abortions with the direct-preparation method of chorionic villi and its implications for studying genetic causes of pregnancy wastage.Am J Hum Genet. 1990; 47: 656-663PubMed Google Scholar, 4Lomax B. Tang S. Separovic E. Phillips D. Hillard E. Thomson T. et al.Comparative genomic hybridization in combination with flow cytometry improves results of cytogenetic analysis of spontaneous abortions.Am J Hum Genet. 2000; 66: 1516-1521Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar, 5van den Berg M.M.J. van Maarle M.C. van Wely M. Goddijn M. Genetics of early miscarriage.Biochim Biophys Acta. 2012; 1822: 1951-1959Crossref PubMed Scopus (196) Google Scholar). Most chromosome anomalies in sporadic miscarriages appear to arise de novo. During gametogenesis, abnormalities may occur because of nondisjunction, translocations, deletions, duplications, or insertions of a chromosomal segment. These lead to numerical or structural chromosomal imbalances in either the oocyte or sperm, resulting in chromosomally abnormal embryos that are more likely to miscarry. It is important to note, however, that a plethora of embryonic chromosomal imbalances remain compatible with life, including some aneuploidies (e.g., trisomy 21 and monosomy X) and structural translocations (e.g., balanced translocations) (6Hassold T. Hunt P. To err (meiotically) is human: the genesis of human aneuploidy.Nat Rev Genet. 2001; 2: 280-291Crossref PubMed Scopus (1841) Google Scholar). In addition to chromosome abnormalities, defects in individual or multiple genes involved in meiosis regulation, DNA repair, and cell proliferation may impair gamete as well as embryonic development. Such changes may result directly from the expression of abnormal genes, rendering the fetus nonviable, or stem from epigenetic modifications that alter the regulatory mechanisms involved in gene expression. Furthermore, there is evidence suggesting that imprinting disorders arising during gametogenesis may be associated with miscarriage (7Tomizawa S.I. Sasaki H. Genomic imprinting and its relevance to congenital disease, infertility, molar pregnancy and induced pluripotent stem cell.J Hum Genet. 2012; 57: 84-91Crossref PubMed Scopus (77) Google Scholar). A systematic review of 19 studies investigating cytogenetic findings of pregnancy tissue after miscarriage identified that the pooled prevalence of fetal chromosomal anomalies in women with recurrent pregnancy loss (39%, 95% CI 29%–50%, 6 studies) was comparable to that found in sporadic miscarriages (45%, 95% CI 38%–52%, 13 studies) (5van den Berg M.M.J. van Maarle M.C. van Wely M. Goddijn M. Genetics of early miscarriage.Biochim Biophys Acta. 2012; 1822: 1951-1959Crossref PubMed Scopus (196) Google Scholar). This suggests that as the number of successive pregnancy losses increases, factors other than embryonic genetic anomalies must be at play in recurrent miscarriage by rendering the endometrium inhospitable (8Ogasawara M. Aoki K. Okada S. Suzumori K. Embryonic karyotype of abortuses in relation to the number of previous miscarriages.Fertil Steril. 2000; 73: 300-304Abstract Full Text Full Text PDF PubMed Scopus (429) Google Scholar). These nonembryonic factors include uterine malformations, endocrine disorders, and heightened localized or systemic immunity secondary to inflammation or infection (9Turocy J.M. Rackow B.W. Uterine factor in recurrent pregnancy loss.Semin Perinatol. 2019; 43: 74-79Crossref PubMed Scopus (32) Google Scholar, 10Vomstein K. Feil K. Strobel L. Aulitzky A. Hofer-Tollinger S. Kuon R.J. et al.Immunological risk factors in recurrent pregnancy loss: guidelines versus current state of the art.J Clin Med. 2021; 10: 869Crossref PubMed Scopus (36) Google Scholar). However, in 2%–5% of cases of recurrent miscarriage, there are underlying parental chromosomal rearrangements increasing the risk of further pregnancy loss (11De Braekeleer M. Dao T.N. Cytogenetic studies in couples experiencing repeated pregnancy losses.Hum Reprod. 1990; 5: 519-528Crossref PubMed Scopus (245) Google Scholar, 12Bender Atik R. Christiansen O.B. Elson J. Kolte A.M. Lewis S. et al.ESHRE Guideline Group on RPL[ESHRE guideline] ESHRE guideline: recurrent pregnancy loss: an update in 2022.Hum Reprod Open. 2023; (2023)hoad002Google Scholar, 13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar). Although chromosomal imbalances are a well-documented cause of miscarriage, there is a paucity of data on specific parental and fetal gene mutations that may increase the risk of pregnancy loss. In 2017, a systematic review of 428 case-control studies identified an association between unexplained recurrent miscarriage and 21 variants in parental genes involved in immune response, coagulation, metabolism, and angiogenesis, although the evidence was mostly of low certainty (14Pereza N. Ostojić S. Kapović M. Peterlin B. Systematic review and meta-analysis of genetic association studies in idiopathic recurrent spontaneous abortion.Fertil Steril. 2017; 107 (9.e2): 150Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar). More recently, however, large datasets of pregnant women have allowed for the identification of one locus on chromosome 13 associated with sporadic miscarriage (rs146350366) and 3 loci for recurrent pregnancy loss on chromosomes 9 (rs7859844), 11 (rs143445068), and 21 (rs183453668) (15Laisk T. Soares A.L.G. Ferreira T. Painter J.N. Censin J.C. Laber S. et al.The genetic architecture of sporadic and multiple consecutive miscarriage.Nat Commun. 2020; 11: 5980Crossref PubMed Scopus (32) Google Scholar). Female age is by far the strongest risk factor for miscarriage, with the probability of pregnancy loss being highest at the extremes of women’s reproductive lives (i.e., <20 years and ≥40 years) (16Magnus M.C. Wilcox A.J. Morken N.H. Weinberg C.R. Håberg S.E. Role of maternal age and pregnancy history in risk of miscarriage: prospective register based study.BMJ. 2019; 364: l869Crossref PubMed Scopus (313) Google Scholar, 17Quenby S. Gallos I.D. Dhillon-Smith R.K. Podesek M. Stephenson M.D. Fisher J. et al.Miscarriage matters: the epidemiological, physical, psychological, and economic costs of early pregnancy loss.Lancet. 2021; 397: 1658-1667Abstract Full Text Full Text PDF PubMed Scopus (372) Google Scholar). Although it is thought that the increased risk of miscarriage in teenage pregnancies may be associated with a plethora of contributing factors, including substance abuse (18Tung I. Beardslee J. Pardini D. Chung T. Keenan K. Hipwell A.E. Adolescent childbirth, miscarriage, and abortion: associations with changes in alcohol, marijuana, and cigarette use.J Child Psychol Psychiatry. 2020; 61: 104-111Crossref PubMed Scopus (6) Google Scholar), because female age advances beyond 40 years, the frequency of genetic abnormalities in the oocyte and embryo rapidly becomes the leading cause of pregnancy loss, resulting in an exponential rise in sporadic and recurrent miscarriage rates (16Magnus M.C. Wilcox A.J. Morken N.H. Weinberg C.R. Håberg S.E. Role of maternal age and pregnancy history in risk of miscarriage: prospective register based study.BMJ. 2019; 364: l869Crossref PubMed Scopus (313) Google Scholar, 19Brosens J.J. Bennett P.R. Abrahams V.M. Ramhorst R. Coomarasamy A. Quenby S. et al.Maternal selection of human embryos in early gestation: insights from recurrent miscarriage.Semin Cell Dev Biol. 2022; 131: 14-24Crossref PubMed Scopus (23) Google Scholar). The effect of paternal age on the risk of miscarriage appears to increase with time, and male partners aged ≥40 years exhibit on average 69% higher odds of miscarriage compared with those aged 20–29 years (odds ratio 1.69, 95% CI 1.18–2.43) (17Quenby S. Gallos I.D. Dhillon-Smith R.K. Podesek M. Stephenson M.D. Fisher J. et al.Miscarriage matters: the epidemiological, physical, psychological, and economic costs of early pregnancy loss.Lancet. 2021; 397: 1658-1667Abstract Full Text Full Text PDF PubMed Scopus (372) Google Scholar). This results partly from an overall decline in reproductive function, including lower testicular activity and altered reproductive hormone secretion as men age. However, male age also has a crucial effect on sperm chromosome number and structure (20Sartorelli E.M.P. Mazzucatto L.F. de Pina-Neto J.M. Effect of paternal age on human sperm chromosomes.Fertil Steril. 2001; 76: 1119-1123Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar, 21Kaltsas A. Moustakli E. Zikopoulos A. Georgiou I. Dimitriadis F. Symeonidis E.N. et al.Impact of advanced paternal age on fertility and risks of genetic disorders in offspring.Genes. 2023; 14: 486Crossref PubMed Scopus (11) Google Scholar), DNA integrity (22Alshahrani S. Agarwal A. Assidi M. Abuzenadah A.M. Durairajanayagam D. Ayaz A. et al.Infertile men older than 40 years are at higher risk of sperm DNA damage.Reprod Biol Endocrinol. 2014; 12: 103Crossref PubMed Scopus (56) Google Scholar, 23du Fossé N.A. van der Hoorn M.P. van Lith J.M.M. le Cessie S. Lashley E.E.L.O. Advanced paternal age is associated with an increased risk of spontaneous miscarriage: a systematic review and meta-analysis.Hum Reprod Update. 2020; 26: 650-669Crossref PubMed Scopus (116) Google Scholar), gene mutation rates (24Ma R.H. Zhang Z.G. Zhang Y.T. Jian S.Y. Li B.Y. Detection of aberrant DNA methylation patterns in sperm of male recurrent spontaneous abortion patients.Zygote. 2023; 31: 163-172Crossref PubMed Scopus (1) Google Scholar), and epigenetic defects (21Kaltsas A. Moustakli E. Zikopoulos A. Georgiou I. Dimitriadis F. Symeonidis E.N. et al.Impact of advanced paternal age on fertility and risks of genetic disorders in offspring.Genes. 2023; 14: 486Crossref PubMed Scopus (11) Google Scholar). Although the mechanisms underlying these associations remain incompletely elucidated, data suggest a putative role for increased reactive oxygen species compounded by impaired antioxidant and DNA repair mechanisms as age advances (23du Fossé N.A. van der Hoorn M.P. van Lith J.M.M. le Cessie S. Lashley E.E.L.O. Advanced paternal age is associated with an increased risk of spontaneous miscarriage: a systematic review and meta-analysis.Hum Reprod Update. 2020; 26: 650-669Crossref PubMed Scopus (116) Google Scholar, 25Johnson S.L. Dunleavy J. Gemmell N.J. Nakagawa S. Consistent age-dependent declines in human semen quality: a systematic review and meta-analysis.Ageing Res Rev. 2015; 19: 22-33Crossref PubMed Scopus (246) Google Scholar). There is a strong association between recurrent pregnancy loss and underlying maternal conditions thought to render the decidua inhospitable to the implanting embryo. Such hostility to the blastocyst may result from local anomalies in the endometrial immune-endocrine environment or from a generalized state of heightened systemic immunity. Examples of chronic disorders associated with recurrent miscarriage include inherited and acquired thrombophilia, subclinical hypothyroidism, thyroid autoimmunity, polycystic ovary syndrome (PCOS), and prolactin disorders (13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar). This section examines the genetic mechanisms underpinning the etiology of these conditions in the context of miscarriage. Inherited thrombophilia includes Factor V Leiden mutations, protein C and S deficiencies, antithrombin deficiency, and prothrombin gene mutations, all of which exhibit an autosomal dominant inheritance pattern (26Liu X. Chen Y. Ye C. Xing D. Wu R. Li F. et al.Hereditary thrombophilia and recurrent pregnancy loss: a systematic review and meta-analysis.Hum Reprod. 2021; 36: 1213-1229Crossref PubMed Scopus (38) Google Scholar, 27Khan S. Dickerman J.D. Hereditary thrombophilia.Thromb J. 2006; 4: 15Crossref PubMed Scopus (170) Google Scholar). Collectively, this group of conditions affects approximately 5% of the general population (26Liu X. Chen Y. Ye C. Xing D. Wu R. Li F. et al.Hereditary thrombophilia and recurrent pregnancy loss: a systematic review and meta-analysis.Hum Reprod. 2021; 36: 1213-1229Crossref PubMed Scopus (38) Google Scholar). Although the mechanisms through which inherited thrombophilia may increase the risk of miscarriage remain understudied, it is thought that an underlying hypercoagulable state exerts a prothrombotic effect on the placental microvasculature, leading to placental failure (28Quenby S. Booth K. Hiller L. Coomarasamy A. de Jong P.G. Hamulyák E.N. et al.Heparin for women with recurrent miscarriage and inherited thrombophilia (ALIFE2): an international open-label, randomised controlled trial.Lancet. 2023; 402: 54-61Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar). However, the evidence of this association is stronger for second-trimester losses than for those occurring before 12 weeks of gestation (13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar, 28Quenby S. Booth K. Hiller L. Coomarasamy A. de Jong P.G. Hamulyák E.N. et al.Heparin for women with recurrent miscarriage and inherited thrombophilia (ALIFE2): an international open-label, randomised controlled trial.Lancet. 2023; 402: 54-61Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar). In addition, heparin treatment has not been shown to decrease the risk of miscarriage in women with inherited thrombophilia (28Quenby S. Booth K. Hiller L. Coomarasamy A. de Jong P.G. Hamulyák E.N. et al.Heparin for women with recurrent miscarriage and inherited thrombophilia (ALIFE2): an international open-label, randomised controlled trial.Lancet. 2023; 402: 54-61Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar). Antiphospholipid syndrome (APS), an acquired thrombophilia marked by the presence of autoimmune antiphospholipid antibodies, has no recognized pattern of inheritance. Yet, despite not being directly passed on from parents to their progeny, there is evidence to suggest a genetic predisposition for APS. This includes, for example, the existence of familial clusters of APS cases, affected monozygotic twins, and an increased prevalence of antiphospholipid antibodies in family members of people with APS (29Barinotti A. Radin M. Cecchi I. Foddai S.G. Rubini E. Roccatello D. et al.Genetic factors in antiphospholipid syndrome: preliminary experience with whole exome sequencing.Int J Mol Sci. 2020; 21: 9551Crossref PubMed Scopus (8) Google Scholar). Associations between specific genes and recurrent miscarriage in the context of APS remain elusive, however, and difficult to tease out in diseases of multifactorial etiology. Yet, recent data suggest a causative role for the human leukocyte antigen system, located on the short arm of chromosome 6, as well as genes involved in hemostasis, the immune response, apoptosis, and thyroid function (29Barinotti A. Radin M. Cecchi I. Foddai S.G. Rubini E. Roccatello D. et al.Genetic factors in antiphospholipid syndrome: preliminary experience with whole exome sequencing.Int J Mol Sci. 2020; 21: 9551Crossref PubMed Scopus (8) Google Scholar, 30Sugiura-Ogasawara M. Omae Y. Kawashima M. Toyo-Oka L. Khor S.S. Sawai H. et al.The first genome-wide association study identifying new susceptibility loci for obstetric antiphospholipid syndrome.J Hum Genet. 2017; 62: 831-838Crossref PubMed Scopus (21) Google Scholar, 31Sipak O. Rył A. Grzywacz A. Laszczyńska M. Szymański S. Karakiewicz B. et al.Molecular analysis of HLA-G in women with high-risk pregnancy and their partners with regard to possible complications.Int J Environ Res Public Health. 2019; 16: 982Crossref PubMed Scopus (10) Google Scholar, 32Królik M. Wrześniak M. Jezela-Stanek A. Possible effect of the HLA-DQ2/DQ8 polymorphism on autoimmune parameters and lymphocyte subpopulation in recurrent pregnancy losses.J Reprod Immunol. 2022; 149103467Crossref PubMed Scopus (4) Google Scholar). Subclinical hypothyroidism and the presence of thyroid autoantibodies have well-established associations with recurrent pregnancy loss (13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar, 33Rao M. Zeng Z. Zhou F. Wang H. Liu J. Wang R. et al.Effect of levothyroxine supplementation on pregnancy loss and preterm birth in women with subclinical hypothyroidism and thyroid autoimmunity: a systematic review and meta-analysis.Hum Reprod Update. 2019; 25: 344-361Crossref PubMed Scopus (87) Google Scholar). However, there is an overall lack of data on the genetic mechanisms predisposing women to thyroid dysfunction concurrently with recurrent miscarriage. Recent evidence suggests that women with moderately high thyroid-stimulating hormone levels (>2.5 mIU/L) combined with low plasma mannose-binding lectin levels and HLA-DRB1∗03 positivity may be associated with increased odds of positivity for at least one thyroid autoantibody and spontaneous miscarriage, but such an association has not been identified in recurrent pregnancy loss (34Yang Y. Li J. Zhou Y. Dai W. Teng W. Shan Z. TSH/miR-17-5p/ZNF367 axis is related to spontaneous abortion in patients with TSH above 2.5 mIU/L.J Mol Endocrinol. 2021; 67: 121-134Crossref PubMed Scopus (2) Google Scholar). A recent systematic review showed a 59% average increase in the odds of miscarriage in women with PCOS compared with women without PCOS (odds ratio 1.59, 95% CI 1.11–2.28) (35Bahri Khomami M. Joham A.E. Boyle J.A. Piltonen T. Silagy M. Arora C. et al.Increased maternal pregnancy complications in polycystic ovary syndrome appear to be independent of obesity-A systematic review, meta-analysis, and meta-regression.Obes Rev. 2019; 20: 659-674Crossref PubMed Scopus (101) Google Scholar). The etiology of PCOS is thought to be multifactorial and closely associated with that of obesity, hyperandrogenism, hyperinsulinemia, and endometrial cancer (36Joham A.E. Norman R.J. Stener-Victorin E. Legro R.S. Franks S. Moran L.J. et al.Polycystic ovary syndrome.Lancet Diabetes Endocrinol. 2022; 10: 668-680Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar). Data suggest genetic predispositions to PCOS that are highly heritable, but there is a paucity of research on specific genes involved in the etiology of miscarriage in the context of PCOS (37Gorsic L.K. Dapas M. Legro R.S. Hayes M.G. Urbanek M. Functional genetic variation in the anti-Müllerian hormone pathway in women with polycystic ovary syndrome.J Clin Endocrinol Metab. 2019; 104: 2855-2874Crossref PubMed Scopus (50) Google Scholar, 38Brower M.A. Hai Y. Jones M.R. Guo X. Chen Y.I. Rotter J.I. et al.Bidirectional Mendelian randomization to explore the causal relationships between body mass index and polycystic ovary syndrome.Hum Reprod. 2019; 34: 127-136Crossref PubMed Scopus (57) Google Scholar). Importantly, miscarried fetuses of women with PCOS appear to exhibit a higher rate of chromosomal anomalies compared with non-PCOS controls (61.3% vs. 52.7%, respectively), although the mechanisms underlying this association remain unclear (39Li Y. Wang L. Xu J. Niu W. Shi H. Hu L. et al.Higher chromosomal aberration rate in miscarried conceptus from polycystic ovary syndrome women undergoing assisted reproductive treatment.Fertil Steril. 2019; 111 (43.e2): 936Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar). Abnormal serum prolactin levels have been implicated in recurrent miscarriage (13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar). Although there is a growing body of evidence investigating the genetics of prolactin disorders (40Chang S. Copperman A.B. New insights into human prolactin pathophysiology: genomics and beyond.Curr Opin Obstet Gynecol. 2019; 31: 207-211Crossref PubMed Scopus (7) Google Scholar), we did not find studies focusing specifically on the genetics of miscarriage in the context of hyperprolactinemia. Genetic tests enable the identification of chromosomal and subchromosomal genetic anomalies responsible for pregnancy loss. Historically, karyotyping has been the most commonly used genetic test for the identification of chromosomal imbalances resulting in miscarriage. It involves obtaining parental blood or pregnancy tissue, culturing cells, and analyzing their chromosomes in the metaphase stage of mitosis. In the context of recurrent miscarriage, karyotyping of pregnancy tissue yields inconclusive results in up to 20%–40% of cases because of the absence of fetal tissue or contamination with maternal cells. In addition, karyotyping for cytogenetic testing of pregnancy tissue after the loss of euploid female fetuses can lead to false-negative results in up to 22%–33% of samples (4Lomax B. Tang S. Separovic E. Phillips D. Hillard E. Thomson T. et al.Comparative genomic hybridization in combination with flow cytometry improves results of cytogenetic analysis of spontaneous abortions.Am J Hum Genet. 2000; 66: 1516-1521Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar, 13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar, 41Bell K.A. Van Deerlin P.G. Haddad B.R. Feinberg R.F. Cytogenetic diagnosis of “normal 46,XX” karyotypes in spontaneous abortions frequently may be misleading.Fertil Steril. 1999; 71: 334-341Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar). Chromosomal microarray analysis (CMA) diagnoses submicroscopic genetic abnormalities that remain undetected using karyotyping. It can be performed using array comparative genomic hybridization or single nucleotide polymorphism genotyping. Compared with karyotyping, CMA offers improved resolution, detecting molecular gains or losses of DNA down to 10 kilobases within the genome. It is also less likely to yield inconclusive or false-negative results. In a systematic review of 9 studies comparing conventional karyotyping and CMA, the investigators identified a 13% increase in the detection rate of chromosome abnormalities using CMA (42Dhillon R.K. Hillman S.C. Morris R.K. McMullan D. Williams D. Coomarasamy A. et al.Additional information from chromosomal microarray analysis (CMA) over conventional karyotyping when diagnosing chromosomal abnormalities in miscarriage: a systematic review and meta-analysis.BJOG. 2014; 121: 11-21Crossref PubMed Scopus (26) Google Scholar). These reasons have led the European Society of Human Reproduction and Embryology to recommend array comparative genomic hybridization as the preferred method for cytogenetic testing of fetal tissue (12Bender Atik R. Christiansen O.B. Elson J. Kolte A.M. Lewis S. et al.ESHRE Guideline Group on RPL[ESHRE guideline] ESHRE guideline: recurrent pregnancy loss: an update in 2022.Hum Reprod Open. 2023; (2023)hoad002Google Scholar). In recent years, cytogenetic testing has evolved to include next-generation sequencing, whole genome screening, and whole exome screening, featuring enhanced resolution to identify single nucleotides in small amounts of tissue using high-throughput assays. This yields large volumes of data that are often difficult to interpret. Such tests are also expensive, and their applicability to cytogenetics of miscarriage has not been fully elucidated (12Bender Atik R. Christiansen O.B. Elson J. Kolte A.M. Lewis S. et al.ESHRE Guideline Group on RPL[ESHRE guideline] ESHRE guideline: recurrent pregnancy loss: an update in 2022.Hum Reprod Open. 2023; (2023)hoad002Google Scholar). Cytogenetic analysis of fetal tissue should be offered to individuals sustaining their third and subsequent miscarriages. This often yields a merely explanatory result that warrants no further action when the risk of recurrence in future pregnancies is low (e.g., trisomy). Where unbalanced translocations are identified on fetal cytogenetics, however, parental karyotyping should be undertaken to rule out balanced chromosome rearrangements, which increase the risk of further losses (13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar). Parental chromosome rearrangements are involved in approximately 2%–5% of recurrent miscarriage cases, of which most are balanced translocations (12Bender Atik R. Christiansen O.B. Elson J. Kolte A.M. Lewis S. et al.ESHRE Guideline Group on RPL[ESHRE guideline] ESHRE guideline: recurrent pregnancy loss: an update in 2022.Hum Reprod Open. 2023; (2023)hoad002Google Scholar, 13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar). The cost-effectiveness of parental karyotyping in the absence of fetal cytogenetics is unclear because the odds of having a subsequent healthy child without assisted conception remain favorable (43Barber J.C. Cockwell A.E. Grant E. Williams S. Dunn R. Ogilvie C.M. Is karyotyping couples experiencing recurrent miscarriage worth the cost?.BJOG. 2010; 117: 885-888Crossref PubMed Scopus (37) Google Scholar). However, where fetal cytogenetic testing renders inconclusive results or is impossible because of a lack of suitable tissue, guidance recommends parental karyotyping. In addition, couples found to have a genetic abnormality should be referred for genetic counseling for discussion of the risks of future pregnancy and management options, including preimplantation genetic testing (PGT), prenatal testing (e.g., amniocentesis, chorionic villi sampling, and noninvasive testing), gamete donation, adoption, fostering, or remaining childless (12Bender Atik R. Christiansen O.B. Elson J. Kolte A.M. Lewis S. et al.ESHRE Guideline Group on RPL[ESHRE guideline] ESHRE guideline: recurrent pregnancy loss: an update in 2022.Hum Reprod Open. 2023; (2023)hoad002Google Scholar, 13Regan L. Rai R. Saravelos S. Li T.C. Royal College of Obstetricians and GynaecologistsRecurrent Miscarriage - Green-top Guideline No. 17. BJOG, 2023Google Scholar). There is no known intervention to prevent embryonic genetic defects. In the context of medically assisted reproduction, however, it is possible to perform PGT before deciding which embryos to transfer. This usually involves a biopsy of cells taken at the blastocyst stage in the laboratory, followed by embryo cryopreservation while awaiting PGT results. In the context of unexplained recurrent pregnancy loss, the evidence suggests that performing PGT for aneuploidies does not accrue any clinical improvement in live birth and is not cost effective (44Murugappan G. Ohno M.S. Lathi R.B. Cost-effectiveness analysis of preimplantation genetic screening and in vitro fertilization versus expectant management in patients with unexplained recurrent pregnancy loss.Fertil Steril. 2015; 103: 1215-1220Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). In addition, studies report a rate of embryo mosaicism ranging between 4% and 90%, raising uncertainties about discarding possibly normal embryos (19Brosens J.J. Bennett P.R. Abrahams V.M. Ramhorst R. Coomarasamy A. Quenby S. et al.Maternal selection of human embryos in early gestation: insights from recurrent miscarriage.Semin Cell Dev Biol. 2022; 131: 14-24Crossref PubMed Scopus (23) Google Scholar). Preimplantation genetic testing for monogenic disorders (PGT-M) or PGT for chromosomal structural rearrangements (PGT-SR) is an option for couples with genetic defects linked to an increase in the risk of miscarriage. The use of PGT-M virtually eliminates the chance of having children affected by the condition for which the embryos are tested. For PGT-SR rearrangements, questions of cost-effectiveness remain unanswered, although the evidence suggests a reduction in miscarriage rates (12Bender Atik R. Christiansen O.B. Elson J. Kolte A.M. Lewis S. et al.ESHRE Guideline Group on RPL[ESHRE guideline] ESHRE guideline: recurrent pregnancy loss: an update in 2022.Hum Reprod Open. 2023; (2023)hoad002Google Scholar). In England, the National Health Service funds up to 3 cycles of in vitro fertilization with PGT-M and PGT-SR in couples carrying genetic abnormalities, provided the female partner’s age is lower than 40 years, even in the absence of an absolute cause of infertility. Those with previously affected children remain eligible for state-funded treatment (45Kay A.C. Wells J. Hallowell N. Goriely A. Providing recurrence risk counselling for parents after diagnosis of a serious genetic condition caused by an apparently de novo mutation in their child: a qualitative investigation of the PREGCARE strategy with UK clinical genetics practitioners.J Med Genet. 2023; 60: 925-931Crossref PubMed Scopus (3) Google Scholar). The past 3 decades have witnessed rapid improvements in the diagnosis and prevention of genetic defects causing miscarriage. Current technology for genetic testing of embryos relies largely on invasive procedures. Recent evidence has identified embryonic DNA in the culture media of blastocysts, leading to rising interest in noninvasive PGT techniques. These require validation in future trials, but it is possible that by eliminating the need for embryo biopsy, noninvasive PGT may in time prove a cheaper and safer alternative (46Leaver M. Wells D. Non-invasive preimplantation genetic testing (niPGT): the next revolution in reproductive genetics?.Hum Reprod Update. 2020; 26: 16-42Crossref PubMed Scopus (84) Google Scholar). In addition to molecular-based techniques, artificial intelligence models on the basis of large morphokinetic datasets have been tested to determine their accuracy in the prediction of embryo ploidy status and miscarriage risk. However, there is heterogeneity between artificial intelligence models owing to the use of a variety of databases and annotation systems between clinics. This makes external validation studies difficult to perform, and to date, noninvasive ploidy prediction models remain experimental, with results requiring confirmation by invasive techniques in clinical practice (47Bamford T. Barrie A. Montgomery S. Dhillon-Smith R. Campbell A. Easter C. et al.Morphological and morphokinetic associations with aneuploidy: a systematic review and meta-analysis.Hum Reprod Update. 2022; 28: 656-686Crossref PubMed Scopus (16) Google Scholar). In the future, it is likely that gene-disease association studies will continue to unveil causal pathways through which genetic defects may contribute to disease phenotypes in which miscarriage is included, in isolation or alongside other manifestations (e.g., preeclampsia and thrombotic disease) (48Bortoletto P. Lucas E.S. Melo P. Gallos I.D. Devall A.J. Bourne T. et al.Miscarriage syndrome: linking early pregnancy loss to obstetric and age-related disorders.EBiomedicine. 2022; 81104134Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar). Furthermore, as the impact of different genetic variants becomes clearer, it may be possible to use polygenic risk scores to screen for individuals’ risk of sustaining miscarriage, whether sporadic or recurrent, although the use of such tools remains controversial (49Polyakov A. Amor D.J. Savulescu J. Gyngell C. Georgiou E.X. Ross V. et al.Polygenic risk score for embryo selection-not ready for prime time.Hum Reprod. 2022; 37: 2229-2236Crossref PubMed Scopus (5) Google Scholar). Although to date there has been no such thing as a “miscarriage gene,” the discovery of causal associations between genetic aberrations and pregnancy loss may ultimately allow for the correction of these anomalies through gene therapy, although presently such aspirations remain merely speculative. The use of genetic testing in reproductive medicine raises important ethical considerations. In a time of ever-growing reproductive inequalities, access to healthcare remains uneven across the world and often within nations. Few countries offer state-funded PGT-M and PGT-SR, and where this is available, stringent eligibility criteria exist (45Kay A.C. Wells J. Hallowell N. Goriely A. Providing recurrence risk counselling for parents after diagnosis of a serious genetic condition caused by an apparently de novo mutation in their child: a qualitative investigation of the PREGCARE strategy with UK clinical genetics practitioners.J Med Genet. 2023; 60: 925-931Crossref PubMed Scopus (3) Google Scholar). Unequal access to medically assisted procreation to prevent miscarriage for preventable genetic conditions may further exacerbate social and reproductive inequalities (17Quenby S. Gallos I.D. Dhillon-Smith R.K. Podesek M. Stephenson M.D. Fisher J. et al.Miscarriage matters: the epidemiological, physical, psychological, and economic costs of early pregnancy loss.Lancet. 2021; 397: 1658-1667Abstract Full Text Full Text PDF PubMed Scopus (372) Google Scholar). In addition, the line between embryo selection to prevent aneuploidy and using genetic testing to select the “best” possible child may be difficult to navigate, often bringing into conflict the principles of autonomy, beneficence, and nonmaleficence. This is apparent in cases where genetic conditions affect male and female children differently (e.g., Lynch syndrome). When, after PGT-M, all embryos test positive for the condition, parents may be tempted to request sex selection of the available embryos on the basis of their perception of how severe the phenotype may be in each sex. Although some defend that the precedence in such cases should be given to the principle of procreative beneficence, whereby parents could select an embryo that will give them the best chance of having a child who is expected to have the least probability of complications, the assembly of ethics committees by regulatory bodies is often necessary to guide clinical decision-making in these complex cases (49Polyakov A. Amor D.J. Savulescu J. Gyngell C. Georgiou E.X. Ross V. et al.Polygenic risk score for embryo selection-not ready for prime time.Hum Reprod. 2022; 37: 2229-2236Crossref PubMed Scopus (5) Google Scholar, 50Savulescu J. Procreative beneficence: why we should select the best children.Bioethics. 2001; 15: 413-426Crossref PubMed Scopus (524) Google Scholar). Miscarriage is caused most commonly by genetic abnormalities. In recent decades, the scientific knowledge of the genetic pathways underpinning pregnancy loss has increased, as has our ability to diagnose and prevent genetically linked miscarriages. Yet, there remains an urgent need for additional research into gene-disease associations, which may pave the way for targeted interventions in the future.