274: Microcephaly: a misnomer for small head circumference

Fetal microcephaly is diagnosed by ultrasound when the fetal head circumference (HC) is 3SD below the mean. The diagnosis has garnered interest since the association of Zika virus and fetal microcephaly was reported. However, there is little data on outcomes of patients with small HC identified on ultrasound. We sought to determine the outcomes of fetuses identified on ultrasound with HC < 3SD below the mean. Retrospective database review of obstetric ultrasound examinations of live fetuses performed at our ultrasound unit from 12/1/2005 to 12/31/2015 and follow-up of newborns. Data were included when the examination was performed between 18 and 40 weeks gestation, a HC was measured, the gestational age was well documented, and birth records were available. A small HC was defined as a HC <3SD from our population mean. Newborn findings were based on NICU clinical evaluation. The study population included 27,679 HC measurements. We identified 34 pregnancies with a HC< 3SD, 5 diagnosed between 18 and 19 weeks. Of these 34 pregnancies there were 18 preterm and 16 term births. Of the 34 newborns, 15 (44.1%) had a newborn birth HC < 3%tile, 9 (26.5%) were between the 3rd and the 10th%tile, and 10 (29.4%) were above the 10th%tile. The birth diagnoses were: 7 IUGR, 11 SGA, 12 AGA, 2 aneuploid, 1 with metabolic disorder, and 1 with choanal atresia. For the 5 fetuses with HC< 3SD prior to 20 weeks, none had a HC below the 3rd%tile at birth, 2 were between 3 and 10%, 3 had a HC above the 10th%tile. None of the 5 newborns with prenatally identified HC < 3 SD between 18 and 19 weeks required NICU admission or followup for microcephaly. Our study demonstrates that the majority of newborns prenatally diagnosed with a small HC did not have primary neurologic issues. We question the routine use of the term “microcephaly” when a fetus is identified by ultrasound with an HC 3SD below the mean. An ultrasound HC < 3SD may represent normal variation, placental insufficiency, misdiagnosis or more rarely genetic conditions or serious anomalies. The majority of newborns with a HC < 3SD below the mean had good outcomes. Notably, all fetuses with small HC prior to 20 weeks did well. In the absence of Zika virus, using a less stringent cutoff for diagnosis of microcephaly may cause alarm with no additional benefit. We recommend that the fetus with a HC < 3SD below the mean be diagnosed with a small head circumference instead of “microcephaly”.