Second‐trimester fetal head circumference in >350.000 pregnancies: Outcome and suggestion for sex‐dependent cutoffs for small heads

OBJECTIVE: To explore the relationship between small fetal second-trimester head circumference (HC) and pregnancy outcome and identify a cutoff point for offering genetic testing. METHOD: Data from second-trimester scans in Denmark were linked to national registers. Fetuses with anomalies diagnosed before this scan were excluded. Fetuses were grouped according to HC z-score. RESULTS: We included 352 515 singleton fetuses. The mean HC was significantly larger among males than among females with z-scores averaging 0.52 more in males. Small HC was associated with chromosomal anomaly, malformations of the CNS and heart, miscarriage/perinatal death, termination, preterm delivery, and intrauterine growth restriction (test for trend: P < .001 for all outcomes). Fetuses in the group with z-score less than -3 had the highest incidence of adverse outcome, irrespective of fetal sex. In the groups with z-scores between -3 and -2.5, and between -2.5 and -2, risk of adverse outcome was lower for females than males for all outcome categories. CONCLUSION: Small HC in second trimester is a prognostic marker for adverse outcome. The smaller the HC, the higher the risk of adverse outcome. We suggest an HC cutoff point of -2 SD for males and -2.5 SD for females for offering genetic testing.