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

Background

Large for Gestational Age (LGA) fetuses present increased risk of labour and delivery complications, potentially preventable through timely childbirth.

Objective

To evaluate the risk for caesarean due to fetal compromise and/or failure to progress in pregnancies with LGA detected at routine 36-weeks ultrasound scan and to identify optimal birth timing.

Study Design

Analysis of prospectively collected data from two UK Fetal Medicine centers including ultrasonographic estimated fetal weight (EFW) at 35-36 weeks’ gestation and outcomes. Of 107,875 pregnancies, 84,397 were analyzed after excluding prelabour caesareans and EFW 90th percentile): of which 3,384 were inductions and 4,311 were spontaneous labours. A competing risks model examined labour type (spontaneous vs. induced) and caesarean for fetal compromise and/or failure to progress, assessing cumulative incidence and instantaneous hazard curves.

Results

This study showed that: first, the hazard of caesarean 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 labour; third, there is a gradient of hazard progression when EFW increases from the 90th to above the 95th percentile; fourth, a policy of induction at 38-39 weeks for fetuses with EFW ˃95th or 90-95th percentile approximates the hazard and cumulative incidence of LGA fetuses delivering spontaneously at 40 weeks; and fifth, induction beyond 41 weeks is associated with substantially increased risk.

Conclusion

The findings support induction of labour at 38 weeks in cases of EFW above the 95th percentile and at 39 weeks for EFW between the 90th and 95th percentile, as this strategy achieves a risk of caesarean delivery due to fetal compromise and/or failure to progress comparable to that of spontaneous labour 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 caesarean 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.