Second-trimester fetal head circumference in more than 350 000 pregnancies: Outcome and suggestion for sex-dependent cutoffs for small heads


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Abstract

ObjectiveTo explore the relationship between small fetal second-trimester head circumference (HC) and pregnancy outcome and identify a cutoff point for offering genetic testing.MethodData 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.ResultsWe 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.ConclusionSmall 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.What’s already known about this topic?Small fetal head size in second trimester is associated with adverse pregnancy outcome.Boys are on average bigger than girls at birth and in late pregnancy.What does this study add?Absolute risks for adverse outcome at a given HC, applicable for counsellingHC z-score is on average 0.52 larger in males than females at the second trimester scanIf small HC, the prediction of outcome is improved by sex-specific cutoffs

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