Association of borderline fetal growth with progression to fetal growth restriction.

Objective: To evaluate if an estimated fetal weight (EFW) between the 10-15th percentiles at time of anatomy ultrasound, referred to as borderline fetal growth, is associated with progression to fetal growth restriction (FGR) on subsequent ultrasound, delivery of a SGA neonate or neonatal intensive care (NICU) admission.

Study design: We performed a secondary analysis using the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be data (NuMom2b). The exposures were normotensive pregnancies with non-anomalous singleton gestations with normal growth, defined as EFW >15th percentile at the anatomy scan compared to borderline fetal growth fetuses defined as those with an EFW in the 10-15th percentile. The primary outcome was FGR at subsequent ultrasound, defined as EFW or AC <10%. The secondary outcomes were NICU admission and small for gestational age (SGA) neonate. Univariable analyses were performed comparing maternal baseline demographic and clinical characteristics. Multivariable analysis was performed for the primary outcome with variables adjusted a priori for body mass index, smoking status, race/ethnicity, insurance status, and drug use.

Results: 4883 patients met inclusion criteria with 114 in the borderline fetal growth group and 4769 in the normal growth group. There were no significant differences in maternal demographic or medical characteristics (Table 1). In adjusted multivariable analysis, patients with borderline growth had significantly higher odds of being diagnosed with FGR at their subsequent scan (aOR 6.68, CI 3.98-11.20) compared to those with normal growth. For secondary outcomes, patients with borderline fetal growth were significantly more likely to have SGA neonates (6.14% vs. 2.67%, p= 0.025). There was no difference in admissions to the NICU between groups.

Conclusion: Diagnosis of borderline fetal growth at time of anatomy scan was associated with a significantly increased odds of progression to FGR at subsequent scan and delivery of a SGA neonate.