MRI of caval compression during left-lateral tilt in singleton and twin pregnancies

BACKGROUND It is unclear how different degrees of left-lateral tilt affect the volumes of the abdominal aorta and inferior vena cava (IVC) in pregnancy. OBJECTIVE To use magnetic resonance images to assess the volumes of the abdominal aorta and IVC in women with twin or singleton pregnancies in different degrees of left-lateral tilt. DESIGN Prospective cohort study. SETTING A single-centre university hospital. PATIENTS Women with singleton pregnancies (13) and twin pregnancies (13) at 32 to 38 weeks’ gestation. MAIN OUTCOME MEASURES Comparison of abdominal aortic and IVC volumes measured by MRI in women with singleton and twin pregnancies while in the supine or left-lateral tilt position at 15°, 30° and 45°. RESULTS Supine, the mean aortic and IVC volumes were not significantly different between the women with singleton and twin pregnancies. In a left-lateral tilt position of 15 o compared with supine, the mean IVC volume was not increased in either group (singletons: 6.3 ± 6.6 ml, 95% CI, −2.4 to 0.4; P = 0.174; twins: 3.9 ± 2.4 ml, 95% CI, −2.6 to 0.4; P = 0.138). At tilt angles of 30° or 45°, the mean IVC volume significantly increased (singletons 30°: 9.7 ± 5.8 ml, 95% CI, −6.1 to −2.7; P < .001; singleton 45°:13.8 ± 5.0 ml, 95% CI, −11.3 to −5.7; P < .001; twins 30°: 5.7 ± 2.1 ml, 95% CI, −4.0 to −1.4; P < .001; twins 45°: 12.8 ± 9.4 ml, 95% CI, −17.2 to −2.6; P = 0.003). Aortic volume was not significantly increased in either group at any of the examined tilt angles compared with the supine. CONCLUSION IVC volume is significantly increased by 30° and 45° left-lateral tilt positions compared with supine in women with singleton and twin pregnancies. TRIAL REGISTRATION This study was registered in the University Hospital Medical Information Network (UMIN) clinical trial registration (# UMIN000031273).