Similar results were found when analyzing gestational age: the malformed infants had on average 4 -7 days shorter gestations than control infants of same birth order. The risk of preterm birth was 2 - 4 times higher for infants with a birth defect than for infants without.

Their non-malformed siblings, on the other hand, had mean gestational ages in the same range as their counterparts in unaffected families, as was their risk of preterm birth.