Natural history of fetal position during pregnancy and risk of nonvertex delivery

To examine the natural history of fetal position throughout pregnancy and the likelihood for nonvertex delivery.We examined fetal positions at 15-22 weeks, at 31-35 weeks, and at delivery using data from the Routine Antenatal Diagnostic Imaging with Ultrasound trial. Characteristics of women with nonvertex and vertex presentation at delivery were compared. Multivariable logistic regression analysis was performed to determine risk factors for nonvertex presentation at delivery, and odds ratios were calculated for those risk factors found to be statistically significantly associated with nonvertex presentation.Data for 7,045 women who underwent routine prenatal ultrasound examinations were analyzed. We found no association between nonvertex presentation at 15-22 and at 31-35 weeks of gestation. A nonvertex fetus at 35 weeks had a 45% chance of spontaneous version by delivery. Multivariable logistic regression analysis found that multiparous women had half of the risk of nonvertex presentation as nulliparous women. Smoking during pregnancy (odds ratio [OR] 1.47, 95% confidence interval [CI] 1.10-1.96), low volume of amniotic fluid at 31-35 weeks (OR 3.74, 95% CI 1.85-7.53), and fundal position of the placenta at late ultrasound examination (OR 1.85, 95% CI 1.23-2.78) were all associated with significant increases in the risk of nonvertex position at delivery.Spontaneous version of a nonvertex fetus at 35 weeks of gestation is still likely.