Impact of obesity on preterm birth in pregnancies prescribed vaginal progesterone for short cervix

Vaginal progesterone reduces the risk of spontaneous preterm birth among patients with short cervix. However, obesity has been shown to alter the absorption of vaginal progesterone in other patient populations. Despite the rising prevalence of obesity in pregnancy, the impact of obesity on response to vaginal progesterone for preterm birth prevention remains poorly characterized.

To evaluate whether obesity is associated with spontaneous preterm birth (sPTB) among patients treated with vaginal progesterone for short cervix.

We conducted a retrospective cohort study of 236 singleton pregnancies at a single academic medical center between 2015 and 2022. All patients had a cervical length ≤2.5 cm between 18 and 22 weeks' gestation and were prescribed 200 mg nightly vaginal progesterone. Patients were classified as having obesity (body mass index [BMI] ≥30) or not (BMI <30). The primary outcome was sPTB before 37 weeks. Secondary outcomes included sPTB before 34 and 28 weeks, neonatal intensive care unit (NICU) admission, preterm prelabor rupture of membranes, and cerclage placement for continued shortening. Multivariable and propensity score-adjusted logistic regression models were used to estimate associations between BMI and outcomes. BMI was also modeled as a continuous variable to assess a dose-response relationship.

Of the 236 patients, 60 (25%) had obesity and 176 (75%) did not. Compared to patients without obesity, those with obesity had higher odds of sPTB before 37 weeks (adjusted OR=2.23, 95% CI: 1.18-4.24), 34 weeks (OR=2.68, 95% CI: 1.27-5.65), and 28 weeks (OR=4.46, 95% CI: 1.50-13.25). Obesity was also associated with increased odds of NICU admission (adjusted OR=2.26, 95% CI: 1.17-4.36). When modeling BMI as a continuous variable, each 1-unit increase in BMI was associated with a 7% increase in the relative odds of sPTB <34 weeks (OR=1.07, 95% CI: 1.01-1.13) and a 10% increase in the relative odds of sPTB <28 weeks (OR=1.10, 95% CI: 1.02-1.18).

Among patients treated with vaginal progesterone for short cervix, obesity was associated with higher odds of sPTB and NICU admission. There appears to be a dose-response relationship, with increasing BMI associated with greater odds of earlier sPTB.