Late preterm delivery has a distinctive second-stage duration and characteristics

Late preterm neonates born between 34.0 and 36.6 weeks' gestation are at increased risk for short- and long-term morbidity and mortality when compared with their term counterparts. Currently, no separate labor curve is available for late preterm births, and this group's optimal duration of the second stage of labor has never been defined separately.

This study aimed to compare the second stage duration between late preterm and term births.

This was a retrospective study from May 2014 until May 2021. Eligible were women with a singleton pregnancy, vertex presentation, spontaneous or induced onset of labor, and those who delivered vaginally beyond 34.0 weeks of gestation. The primary outcome of our study was to compare and characterize the second stage of labor duration between late preterm and term births.

We analyzed 962 late preterm and 9476 term vaginal deliveries. Women who delivered during the late preterm period were more likely to be multiparous (52.4% vs 45.2%; P<.001) and fewer required oxytocin during labor (41.2% vs 54.4%; P<.001) or used epidural analgesia (75.2% vs 83.6%; P<.001). The overall mean duration of the second stage of labor was significantly shorter in the late preterm period than at term (1.08±1.09 hours vs 1.49±1.22 hours; P<.001). This was even more pronounced for nulliparous women (1.05±1.00 hours vs 2.10±1.17 hours; P<.001). Among multiparous women, epidural use significantly affected the duration of the second stage of labor, and the second stage was relatively longer during the late preterm period than at term in this subgroup (1.16 vs 0.5 hours; P<.001). Using a multivariate Cox regression, variables such as maternal age (hazard ratio, 1.02; 95% confidence interval, 1.01-1.04), parity (hazard ratio, 4.11; 95% confidence interval, 3.65-4.63), preterm birth (hazard ratio, 2.08; 95% confidence interval, 1.4-3.10), and birthweight at delivery (hazard ratio, 1.15; 95% confidence interval, 1.01-1.30) shortened the second stage, whereas induction of labor (hazard ratio, 0.75; 95% confidence interval, 0.66-0.86) and epidural use (hazard ratio, 0.68; 95% confidence interval, 0.64-0.86) extended its total duration. Regardless of parity, lower rates of operative vaginal deliveries were observed in the late preterm period than at term (3.7% vs 15.5%; P<.001). This period was also associated with lower rates of third- and fourth-degree perineal lacerations (0.2% vs 2.2%; P<.001) but higher rates of chorioamnionitis (1.7% vs 0.1%; P<.001), Apgar score at 5 minutes <7 (1.0% vs 0.2%; P<.001), and admission to the neonatal intensive care unit (19.3% vs 1.0%; P<.001).

Women who delivered vaginally during the late preterm period had a distinctive second-stage duration. Primarily, it was shown to be significantly shorter for nulliparous and multiparous women. Future studies should further clarify the optimal duration of this stage in relation to neonatal outcomes at such a vulnerable period.