Routine 36-week scan: optimizing delivery timing of large for gestational age fetuses

Large for gestational age fetuses present an increased risk of labor and delivery complications, potentially preventable through timely childbirth.

To evaluate the risk for cesarean due to fetal compromise and/or failure to progress in pregnancies with large for gestational age detected at routine 36-week ultrasound scan and to identify optimal birth timing.

Analysis of prospectively collected data from 2 UK Fetal Medicine centers including ultrasonographic estimated fetal weight at 35 to 36 weeks' gestation and outcomes. Of 107,875 pregnancies, 84,397 were analyzed after excluding prelabor cesareans and estimated fetal weight <10th percentile. The final cohort included 7695 large for gestational age (estimated fetal weight >90th percentile), of which 3384 were inductions and 4311 were spontaneous labors. A competing risk model examined labor type (spontaneous vs induced) and cesarean for fetal compromise and/or failure to progress, assessing cumulative incidence and instantaneous hazard curves.

This study showed that: first, the hazard of cesarean delivery due to fetal compromise and/or failure to progress increases progressively with advancing gestational age with steeper rise after 40 weeks; second, at a given gestational age, the hazard is higher in nulliparous vs parous and in induction vs spontaneous labor; third, there is a gradient of hazard progression when estimated fetal weight increases from the 90th to above the 95th percentile; fourth, a policy of induction at 38 to 39 weeks for fetuses with estimated fetal weight >95th or 90th to 95th percentile approximates the hazard and cumulative incidence of large for gestational age fetuses delivering spontaneously at 40 weeks; and fifth, induction beyond 41 weeks is associated with a substantially increased risk.

The findings support induction of labor at 38 weeks in cases of estimated fetal weight above the 95th percentile and at 39 weeks for estimated fetal weight between the 90th and 95th percentile, as this strategy achieves a risk of cesarean delivery due to fetal compromise and/or failure to progress comparable to that of spontaneous labor at 40 weeks, while avoiding the progressive increase in risk observed beyond 40 weeks. Alternatively, conservative management until 41 weeks may be considered, followed by elective cesarean delivery if spontaneous labor has not occurred by then, to avoid potentially harmful effects of induction at that stage, associated with a marked increase in cesarean risk.