The association between maternal parity and birth defects.

BACKGROUND: Previous studies observed that first birth is associated with an increased risk of some categories of birth defects. However, multiple statistical tests were conducted and it was unclear which of these associations would be replicated in a larger study. We used a large database to assess the association between maternal parity and 65 birth defects including birth defects that have not been previously studied.

METHODS: Using data from the Texas Birth Defects Registry for years 1999-2009, the risk of a birth defect occurring in a first, third, or fourth or higher birth was compared to the risk of a birth defect occurring in a second birth.

RESULTS: Women having their first birth had significantly increased odds of having an infant with 24 of 65 categories of birth defects when compared to women having their second birth. We also observed associations between first birth and an increased risk of five birth defects not previously reported (small penis, preaxial polydactyly, anomalies of the thoracic vertebrae, anomalies of the lumbar vertebrae, and sacroccygeal anomalies). Women having their third or fourth or higher birth had significantly increased odds of giving birth to infants with five of 65 birth defects when compared to second births.

CONCLUSIONS: Our observations regarding the categories of birth defects that were associated with first births were highly consistent with observations from two previous studies. Research into biological, behavioral, and environmental factors that may increase the risk of specific birth defects among first births is needed to further explore these associations.