Infants exposed to antenatal corticosteroid therapy (ACT) to accelerate lung maturation have a clinically significant reduction in birth size, according to a new of study of 278,508 births published this week in PLOS Medicine by Alina Rodriguez of the University of Lincoln and Imperial College London, UK, and colleagues.

Guidelines currently recommend one dose, repeated over 24 hours, of ACT to accelerate lung maturation in cases of threatened preterm birth. However, many exposed infants end up born at term and are therefore exposed unnecessarily to any potential harms of ACT. In the new study, researchers studied all live-born singleton births in Finland from 2006 through 2010. De-identified data were available on ACT exposure, birth weight, birth length, head circumference, Apgar scores, and medical care of infants.

4,887 women (1.75%) were treated with ACT and, of those exposed, more than 44% (2173) of infants were born at term. Adjusted analyses showed significant differences in birth weight between exposed and unexposed infants, with an apparent reduction in birth weight of 61.54 grams for very preterm babies exposed to ACT (±SE 28.62, P<.03), 222.78 grams for preterm babies (±SE 19.64, P<.001), 159.25 grams for near term babies (±SE 19.14, P<.001), and 91.62 grams for term babies (±SE 11.86, P<.03). Associations were also seen for birth length and head circumference. There were no significant differences in Apgar scores, but ACT-exposed infants generally required greater medical care during the first seven days of life and beyond.

"These findings provide strong evidence indicating that ACT is associated with reduced fetal growth in humans and provide an agenda for further studies," the authors say. "Early care decisions need to identify high-risk patients and weigh benefits of ACT against potential harm of unnecessary exposure."