Unisex vs sex-specific estimated fetal weight charts for fetal growth monitoring: a population-based study.

BACKGROUND

In contrast with birthweight or other growth charts, a feature of most intrauterine charts is that they are not differentiated by sex. Differences in weight by sex during pregnancy are considered to be relatively minor; however, small systematic differences may affect the sensitivity and specificity of screening for fetuses with growth restriction.

OBJECTIVE

To assess differences between unisex and sex-specific estimated fetal weight charts at the third-trimester ultrasound with regard to the sex ratio of fetuses detected with an estimated fetal weight <10th percentile and subsequent detection of small-for-gestational-age newborns with morbidity at birth.

STUDY DESIGN

The study included 9940 singleton live births from a French population-based study in 2016. Main outcomes were an estimated fetal weight <10th percentile at the routine third-trimester ultrasound between 30 and 35 weeks of gestation, and small for gestational age infants (birthweight <10th percentile) with neonatal morbidity (Apgar score <7 at 5 minutes and/or resuscitation in delivery room and/or admission to a neonatal unit). We used 2 charts with unisex and sex-specific options: the World Health Organization international standard chart and a customized chart for fetal sex based on Gardosi's gestation-related optimal weight model adapted to the French population (Epopé). Hadlock's unisex chart, commonly used in clinical care and research, was also included to provide an external reference. We compared the proportions of female and male fetuses with an estimated fetal weight <10th percentile and the sensitivity and specificity of such estimated fetal weight for predicting small-for-gestational-age newborns with morbidity when using unisex vs sex-specific charts, overall and by sex.

RESULTS

Among all singleton births, there were 51.6% males and 48.4% females. Males faced higher risks of being small-for-gestational-age with morbidity at birth (2.4% vs 1.8%; P=.031). Using the World Health Organization unisex chart, 6.9% of males and 9.9% of females had an estimated fetal weight <10th percentile vs 9.9% of males and 7.1% of females with the sex-specific chart; these proportions were 3.5% and 4.6% and 4.3% and 2.7%, respectively, for the Epopé. Proportions of estimated fetal weight <10th percentile using Hadlock's chart were slightly higher than those obtained using the unisex World Health Organization chart (7.5% of males and 10.6% of females), but the difference of about 3% was the same. The sensitivity of an estimated fetal weight <10th percentile for identifying small-for-gestational-age newborns with morbidity differed for males and females by type of chart; unisex charts detected more small-for-gestational-age females with morbidity and sex-specific charts detected more small-for-gestational-age males with morbidity, but the overall sensitivity was the same (49.1% for the World Health Organization chart and Hadlock's chart and 34.9% for the Epopé chart).

CONCLUSION

This study suggests that the use of sex-specific charts instead of unisex charts would reduce sex bias in intrauterine growth screening during the third trimester of pregnancy. Prospective studies are needed to assess the effects of using sex-specific charts rather than unisex charts on obstetrical management and outcomes.

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