Impact of Gestational Age at Delivery on Fetal Heart Rate Monitoring During the Second Stage of Labor

Abstract

Objective: To clarify whether gestational age at delivery affects the interpretation level of fetal heart rate (FHR) tracings in the second stage of labor in primiparous and singleton pregnancies. Methods: The database at one tertiary hospital in Japan was retrospectively reviewed for women with singleton fetuses in cephalic presentation and vaginal labor at 37+0 gestational weeks between June 1, 2011, and March 31, 2013. Continuous FHR tracings in the second stage of labor were subdivided into 10-minute intervals, each of which we called a window, from the beginning through delivery, and were assessed according to the five-tier classification proposed by the Japan Society of Obstetrics and Gynecology, in which level 1 is normal, level 2 is subnormal, and levels 3-5 are abnormal patterns. Subjects were divided into two groups, including early term (37 0/7 to 38 6/7 weeks) and full term (39 0/7 to 41 6/7 weeks), according to the delivery age. Results: In total, 914 parturient women were eligible for the study protocol, including 228 and 686 women in early and full term, respectively. Vacuum extraction was more often observed in full term than in early term (P=0.007). Although the maximal level, number of level-5 windows, number of level-4 windows, and number of level-3 and level-4 windows were similar between the groups, the summation of level-4 windows 3 was significantly higher in full term than in early term (P=0.004). Conclusion: Full term delivery has a higher risk than does early term delivery from the standpoint of FHR monitoring.