To our knowledge, this study is the first to report outcomes associated with oxytocin discontinuation after the active phase of labor that was induced because of FGR. Our findings suggest that the cesarean delivery rate does not change based on whether oxytocin is continued or discontinued. However, compared to the continuation group, the incidence of uterine tachysystole was lower in the discontinuation group. Discontinuation of oxytocin was not associated with a longer duration of the active phase of labor and the second stage of labor when compared to the equivalent metric in the continuation group. Oxytocin can be safely discontinued after the active phase of labor without increasing the risk of cesarean delivery or other unfavorable outcomes.
Previous studies have reported that because the delivery of newborns with FGR is associated with NRFS, the rate of cesarean delivery is high (14-40.9%) [18–23], particularly in cases with low CPR [23]. The indication for cesarean delivery was fetal distress in 9.5–29% of these cases [18–20]. In our study, because women who experienced cesarean delivery before the active phase of labor were excluded, the cesarean delivery rate was only 4% (5/127). However, after including women who have experienced cesarean delivery before the active phase of labor, the total cesarean delivery rate was 14.1% (23/163), which is consistent with previous reports. Regarding the decision to induce labor in cases of FGR, the choice to perform cesarean delivery due to NRFS is often made before the active phase of labor. In our study, the cesarean delivery rate was lower than expected for women who had entered the active phase labor, and the majority of women delivered vaginally. As a result, we could not find a significant difference in the cesarean delivery rates of the continuation group and discontinuation group. However, the frequency of uterine tachysystole was lower in the discontinuation group than in the continuation group. A previous meta-analysis of nine randomized controlled trials compared continuation and discontinuation of oxytocin after the active phase of labor and its effect on labor induction and labor augmentation [7–15, 24]. In that meta-analysis, oxytocin discontinuation after the active phase of labor significantly lowered the cesarean delivery rate (9.3% vs 14.7%) and uterine tachysystole rate (6.2% vs 13.1%) compared with oxytocin continuation until delivery [24]. When labor enters the active phase, further oxytocin administration does not seem to have any benefit except for shortening the labor length. However, it is associated with some adverse events; therefore, decreased use of oxytocin is encouraged [25, 26]. Because the cause of FGR is associated with placental dysfunction, and because FGR is a risk factor for fetal heat rate abnormalities, uterine tachysystole associated with the inappropriate or excessive use of oxytocin should be reduced.
Discontinuing oxytocin did not prolong the active phase of labor or second stage of labor. On the contrary, the duration of labor in cases of FGR was short compared to what is generally considered to be the induction time for AGA fetuses, and we had to use caution because of the rapid progression of labor. It has been reported that the rate of precipitous delivery, defined as delivery of the fetus within less than 3 hours of the commencement of regular contractions, of infants with low birthweight (less than 2500 g) is high (28.5%) [27]. Precipitous delivery is concerning because it is a known risk factor for fetal stress and respiratory distress in newborns [28], and insufficient time for newborn resuscitation is a possibility. A short delivery time is by no means an advantage in labor induction for FGR. In our study, although we could not reduce the length of labor and delivery, uterine tachysystole was reduced, thus leading to the possibility of less fetal distress. In 2014, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine recommended that the definition of active phase of labor should be revised to include cervical dilatation of 6 cm [29]. However, this recommendation does not distinguish between a fetus with FGR and a fetus with AGA. We identified one previous study that investigated labor progression in preterm deliveries [30] and reported that the median traverse time from 5 to 6 cm of cervical dilation was less than 30 minutes for nulliparous and multiparous women. After achieving 6 cm of dilatation, both groups rapidly progressed to 10 cm (median, 18 minutes). Therefore, the authors speculated that the true active phase of labor begins at 5 cm of cervical dilatation in preterm births. Regarding labor and delivery, it is possible to infer that the onset of the active phase of labor occurs earlier in cases of FGR at or near term than in cases of AGA at term. Therefore, the definition of the active phase of labor as cervical dilatation ≥ 4 cm was suitable for our study. After cervical dilatation ≥ 4 cm with labor induction for FGR, attention should be focused on the rapid progression of labor and clinicians should consider discontinuing or reducing oxytocin infusion to prevent uterine tachysystole.
There were some limitations associated with this study. First, this was a retrospective cohort study based on data from a single institution where a limited number of women fulfilled the inclusion criteria. The population included both parous and nulliparous women. Stratification according to parity would have been beneficial, but the study population was too small. Moreover, oxytocin could not be discontinued in some women assigned to the discontinuation group (19/51; 37%). Furthermore, oxytocin had to be discontinued in some women assigned to the continuation group due to NRFS or rapid progression of labor (11/74; 15%). Because of the rapid progression of labor with FGR and the need to be cautious because of the possibility of fetal heartrate abnormalities, fewer women were able to adhere to the protocol. When we evaluated the actual treatment received, the tachysystole rate and abnormal fetal heart rate incidence were significantly higher in the continuation group than in the discontinuation group. However, the cesarean delivery rate was not different between groups. Second, because oxytocin has a half-life of approximately 3 minutes, tachysystole is perhaps one of the easier tocodynametric issues to resolve; however, it remains unclear whether this method can reduce the cesarean delivery rate. Therefore, further large prospective studies adhering to the protocol involving discontinuation of oxytocin are required to determine whether this approach can reduce the cesarean delivery rate.