Monitoring and timing of delivery in suspected late fetal growth restriction at term.

Background: Infants affected by late fetal growth restriction are at risk of adverse perinatal outcomes, either due to placental insufficiency or the consequences of late iatrogenic prematurity. There is no randomized trial reporting the optimal timing of delivery, and recommending delivery at 37 weeks can cause harm. Antenatal ultrasound parameters can be used to risk-stratify these pregnancies into low-risk cases, which are associated with a lower likelihood of adverse outcomes and are thus suitable for full-term delivery, and high-risk cases, which may benefit from early-term delivery.

Objective: This study aimed to prospectively evaluate the performance of an antenatal risk stratification and management protocol in reducing severe adverse perinatal outcomes in fetuses with late fetal growth restriction.

Study Design: This was a prospective study of singleton pregnancies with nonanomalous fetuses at ≥32 weeks of gestation with suspected late fetal growth restriction managed between 2018 and 2022 at University College London Hospital, United Kingdom. Pregnancies with unknown outcome or diagnosis of a structural, chromosomal, or genetic abnormality were excluded (but women with placenta previa were included). At 36 weeks, fetuses were classified as low-risk if the estimated fetal weight was between the third and 10th centiles with normal Doppler findings and no abdominal circumference drop, or if the estimated fetal weight was >10th centile with an abdominal circumference drop ≥50 centiles from previous scans and/or a cerebroplacental ratio below the fifth centile. The high-risk group included all fetuses with an estimated fetal weight below the third centile, fetuses of any size with an umbilical artery Doppler pulsatility index >95th centile, and fetuses with an estimated fetal weight between the third and 10th centiles with at least one of the following features: a mean maternal uterine artery Doppler pulsatility index >95th centile, an abdominal circumference drop ≥50 centiles, or a cerebroplacental ratio below the 5th centile. Delivery was advised at 40 to 41 weeks of gestation in the low-risk group or 37 to 38 weeks in the high-risk group. The primary outcome was a composite of severe adverse perinatal outcomes (perinatal mortality and severe morbidity including acidosis at birth, mechanical ventilation, encephalopathy, use of inotropes, and infections). Based on expert consensus and targeting a clinically meaningful 5% difference between the 2 groups, our sample size calculation determined that 372 patients were required in each group (alpha, 0.05; power, 85%). Adverse maternal outcome was defined as operative delivery due to abnormal intrapartum fetal heart rate. Multiple logistic regression analysis and modeling to estimate the risk of severe adverse perinatal outcome at different gestational ages were performed.

Results: After exclusions, 1002 of the original 1065 patients were included (594 in the low-risk and 408 in the high-risk groups). There was a significant difference in gestational age at delivery between the low- and high-risk groups: 39 (interquartile range, 38-40) vs 38 (37-38) weeks (P<.001). Neonates from pregnancies classified as low-risk were less likely to have severe adverse perinatal outcomes (3.2% vs 8.3%; adjusted odds ratio, 2.1; 95% confidence interval, 1.2-3.9; P=.02). There was no difference in adverse maternal outcome between the low- and high-risk groups (20.5% vs 24.8%; adjusted odds ratio, 1.2; 95% confidence interval, 0.9-1.6, P=.23). There were no cases of extended perinatal mortality (including stillbirth, neonatal death, and infant death up to 6 months) in the low-risk group (4 cases in the high-risk group).

Conclusion: Our study demonstrated that appropriate risk stratification of singleton nonanomalous fetuses with late fetal growth restriction enables conservative term management for pregnancies at low risk of adverse perinatal outcomes, with lower adverse perinatal outcomes compared with high-risk late fetal growth restriction cases, without an increase in perinatal mortality and maternal interventions.