Age at first birth is associated with the likelihood of frailty in middle-aged and older women: A population-based analysis from NHANES 1999–2018

Objectives This study examined whether age at first birth (AFB) is associated with the prevalence of frailty in middle-aged and older women. Methods The study included 10,828 women (age ≥ 45 years) from the National Health and Nutrition Examination Survey (NHANES) (1999–2018) in the United States. AFB data were collected using a standardized reproductive health questionnaire. Frailty was measured using a 53-item frailty index and was diagnosed if the score on that index was over 0.21. Survey-weighted logistic regression models were used to assess the association between AFB and the prevalence of frailty. A survey-weighted restricted cubic spline (RCS) model was used to determine the dose–response relationship between AFB and frailty. Mediation analyses were performed to estimate the mediated effects of education levels, family poverty income ratio, and parity on the association between AFB and the likelihood of frailty. Finally, sensitivity and subgroup analyses were conducted to validate the robustness of our findings. Results Among the 10,828 women, 3828 (35.4 %) had frailty. The RCS depicted a U-shaped association between AFB and frailty. Compared with the women in the reference group (AFB: 33–35 years), women in the other groups (AFB: < 18, 18–20, 21–23, and 24–26 years) had a higher likelihood of frailty, with respective odds ratios (95 % confidence intervals) of 3.02 (1.89–4.83), 2.32 (1.54–3.50), 1.83 (1.19–2.81), and 1.64 (1.07–2.53). However, no statistically significant differences were detected for women with AFB of 27–29, 30–32, or > 35 years compared with the reference group. Education levels, family poverty income ratio, and parity significantly mediated the approximately linear negative association between AFB and frailty in the subset of women with AFB of ≤32 years and the mediation proportions were 23.4 %, 32.4 %, and 18.3 %, respectively (all p < 0.001). Conclusions Based on our results, we conclude that early AFB is associated with a higher likelihood of frailty in middle-aged and older women.