Identifying the risk of spontaneous preterm birth after term singleton cephalic birth, by mode of birth, cervical dilatation, and incision extensions at cesarean section: a cohort study.

Abstract

Increasing evidence links cesarean section, especially at full dilatation, to subsequent spontaneous preterm birth. However, many studies to date lack sufficient detail to determine the impact of advancing cervical dilatation in labor, the effect on spontaneous preterm birth at gestations under 32 weeks, and data to support a mechanism of effect. To confirm an independent association between mode of birth and subsequent spontaneous preterm birth at gestations <37, <32, and <28 weeks, including by cervical dilatation, and to explore any impact of unintended uterine incision extension at cesarean section. An observational cohort study from 2008 to 2021 from a single tertiary hospital in Auckland, New Zealand, including 20,499 consecutive live birthing pairs where the index pregnancy was a term cephalic singleton and the subsequent pregnancy a live birth from 20 weeks of gestation. The primary outcome was spontaneous preterm birth at thresholds 37, 32, and 28 weeks. Data were ascertained and extracted from an electronic maternity database and clinical records. Univariable and multivariable statistical techniques were employed with adjustments for ethnicity, nulliparity, birthweight and gestation at index birth, interpregnancy interval, and age and smoking at subsequent birth. Spontaneous preterm birth, dilatation at cesarean, and unintended uterine incision extension were identified from clinical records, where not available in routinely collected data sources. Participant index births included 10,309 unassisted vaginal births, 4132 assisted vaginal births, 2139 prelabor cesareans, and 3919 in-labor cesareans; 1302 at 0 to 4 cm, 943 at 5 to 7 cm, 743 at 8 to 9 cm, and 931 at full cervical dilatation. The spontaneous preterm birth rate among subsequent births was 3.1% (636/20,499). The risk of spontaneous preterm birth after in-labor cesarean section was elevated at all gestation thresholds compared to vaginal birth. Risk increased with advancing cervical dilatation at cesarean (<5 cm: relative risk, 1.0; 95% confidence interval, 0.7-1.4; 5-7 cm: relative risk, 1.8; 95% confidence interval, 1.3-2.5; 8-9 cm: relative risk, 1.8; 95% confidence interval, 1.3-2.6; full dilatation: relative risk, 2.0; 95% confidence interval, 1.5-2.7). Cesareans at full dilatation were particularly associated with greater risk of spontaneous preterm birth before 32 weeks (relative risk, 4.5; 95% confidence interval, 2.6-7.8) and 28 weeks (relative risk, 7.7; 95% confidence interval, 3.9-15.3). Cervical dilatation ≥5 cm remained independently associated with more than 2-fold increased risk of spontaneous preterm birth at <37 weeks, independent of uterine incision extensions and other confounders. Uterine incision extensions were identified in 387 (6.4%) cesareans and were not independently associated with spontaneous preterm birth (adjusted relative risk, 0.9; 95% confidence interval, 0.5-1.7) unless extending through the cervix and into the vagina (adjusted relative risk, 5.1; 95% confidence interval, 1.8-14.5). Cesarean section in labor (>5 cm cervical dilatation), and particularly at full dilatation, is a risk factor for subsequent spontaneous preterm birth. Uterine incision extension into the cervix and vagina during cesarean is an independent risk factor for subsequent spontaneous preterm birth.