Outcomes of fetuses with small head circumference on second-trimester ultrasonography

We examined the outcomes of pregnancies in which the fetal head circumference (HC) was below the 5(th) centile at the routine second-trimester scan.

We retrospectively analysed outcomes of 18,377 women according to HC Z scores at second-trimester ultrasound examination between 2001 and 2008. We collected all major malformations, intrauterine deaths and other abnormal outcomes.

Six hundred seventy-four fetuses (3.7%) had an HC below the 5(th) centile. Twenty-one major malformations were noted, consisting mainly of neurological abnormalities (3.1%). There were seven intra uterine fetal death (1.3%). Of all the fetuses, 26% were lost to follow-up. Outcome and neurological development was normal in 467 cases, based on neonatal examination and/or parent or general practitioner reports. Major abnormalities were noted in respectively 26.2%, 3.0% and 1.1% of fetuses with Z scores < -2.5, -2.5 to -2.0, and -2 to -1.645, compared with 0.3% of fetuses with normal HC (p < 10(-4)).

A head circumference below the 5(th) centile at second-trimester scan is associated with various abnormalities, especially neurological disorders. The outcome was worse when the HC was smaller. An HC Z score below -2.5 was strongly associated with neurological and chromosomal abnormalities. Conversely, an HC Z score below -1.645 but above -2, excluding cases with prenatally diagnosed malformations, seems to be reassuring for favorable neonatal outcome.