Prenatal screening for microcephaly: an update after three decades

Background: Due to the recent outbreak of Zika virus, there has been a newfound interest in fetal and neonatal microcephaly. In 1984, Chervenak et al. proposed criteria for the prenatal ultrasound diagnosis of microcephaly as <= 3 standard deviations (SD) from the mean. Despite improvements in medicine these criteria have not been reevaluated in 30 years. Objective: To examine how the original 1984-Chervenak et al. criteria for the diagnosis of fetal microcephaly apply to a current population utilizing modern ultrasound equipment and techniques. Study design: Retrospective database review of 27,697 ultrasound exams between 18 and 40 weeks' gestation. Mean and SDs were calculated for each week of gestation from 18 to 40 completed weeks and these were compared to the 1984 data. Results: There is no statistically significant difference in gestational age-specific mean head circumference (HC) between the two studied populations. Because the current dataset is larger the SD differ. Conclusions: The 1984 ultrasound criteria for microcephaly remain valid. Physicians today have two alternatives: either use the 3SD cutoff as recommended by Chervenak et al. and endorsed by the Society for Maternal-Fetal Medicine (SMFM) or develop a new dataset for one's population with statistical validation.