454: Counsel confidently: Reliability of bedside EFW to predict survival at periviability

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Periviability counseling hinges on survival outcomes generated from the NICHD Extremely Preterm Birth Outcome Data calculator, which uses estimated fetal weight (EFW) as a proxy for birth weight (BW). Formal ultrasound (US) by certified sonographers is often considered superior to bedside US by housestaff but does not enable rapid decision making at presentation. We hypothesize that at periviability: 1) bedside EFW (bEFW) is as accurate as formal EFW (fEFW) when compared to BW, 2) EFW is a good proxy for BW in the NICHD calculator, and 3) survival statistics generated by bEFW are as accurate as those by fEFW. This retrospective cohort study included singleton deliveries 22-25 weeks GA with an EFW within 10 days of delivery. A fetal growth model by GA and growth percentile was developed from national birth weight data to adjust for interval weight gain between US and delivery. Mean percent accuracy of bEFW and fEFW were compared. A nested regression was performed to evaluate the effect of US, maternal and fetal variables. Absolute change in NICHD survival estimates based on EFW, BW and adjusted BW was calculated and mean difference in survival based on bEFW and fEFW were compared. Mean percent accuracy for bEFW was 9.40% (n=167, 95% CI 8.25-10.56%) based on BW and 9.43% (n=146, 95% CI 8.13-10.76%) based on adjusted BW. Mean percent accuracy for fEFW was 8.57% (n=140, 95% CI 7.48-9.65%) based on BW and 8.66% (n=108, 95% CI 7.31-10.01%) based on adjusted BW. There was no statistically significant difference comparing bEFW to fEFW based on BW (p=0.30) and adjusted BW (p=0.43). Nested regression modeling showed no differences after adjusting for sonographer level, days between US and birth, year of US, fetal sex, fetal number, maternal age, race, ethnicity or tobacco use (all p>0.05). In 85% of cases, survival statistics generated by EFW were within 10 percentage points of those using BW or adjusted BW, and 98% of cases were within 20 percentage points. There was no difference in survival statistics generated by bEFW versus fEFW compared to BW (p=0.34) and adjusted BW (p=0.43). Bedside EFW performs as well as fEFW at predicting BW and is a reliable proxy for BW. NICHD survival statistics calculated by bEFW are as accurate as those calculated by fEFW. Providers can confidently use bEFW to expeditiously and reliably counsel patients at risk of imminent periviable delivery.