The impact of reference growth standards on small- and large-for-gestational age outcomes among pregnancies conceived by fresh and frozen embryo transfers.

Objective: To describe differences in the frequency of small-for-gestational age (SGA) and large-for-gestational age (LGA) driven by different birth weight curves in assisted reproductive technology (ART)-conceived pregnancies.

Design: Retrospective cohort study.

Setting: Single academic medical center.

Patients: Singleton live births between the gestational ages of 36 weeks and 0 days and 42 weeks and 6 days from fresh or frozen embryo transfer (ET).

Interventions: None.

Main Outcome Measures: SGA (<10th percentile) and LGA (>90th percentile) classified by Fenton, INTERGROWTH-21, World Health Organization, Duryea, and Oken curves.

Results: The median birth weight and gestational age at birth among fresh ET pregnancies were 3,289g (interquartile range [IQR], 2,977-3,600g) and 39.4 (IQR, 38.6-40.3) weeks, respectively, and those among frozen ET pregnancies were 3,399g (IQR, 3,065-3,685g) and 39.4 (IQR, 38.7-40.1) weeks, respectively. The frequencies of SGA neonates using each birth weight standard ranged from 5.8% to 13.4% for fresh ET and from 3.5% to 8.7% for frozen ET. Those of LGA neonates ranged from 5.3% to 14.3% for fresh ET and from 6.6% to 21.2% for frozen ET.

Conclusion: The frequency of SGA and LGA neonates among ART-conceived gestations is partially driven by the birth weight standard. Selecting an appropriate standard that best reflects the patient population is critical to quantifying the risk of ART-conceived pregnancies.