Maternal factors associated with small-for-dateness among twins.

The maternal risk factors that correlate with small-for-dateness among twins have been analyzed using a sample of 659 twins pairs and a matched sample of singletons. Non-marital status, job involvement, and the previous delivery of a low-birth weight (less than 2,500g) infant present a negative interaction with twinning, as low gestational age-adjusted birth weight does not correlate significantly with these risk factors among twin gestations, while it does among singleton gestations. On the other hand, the effects of parity, habitual maternal weight, smoking during pregnancy, and twinning are additive on gestational age-adjusted birth weight. Indeed, the decrease in adjusted birth weight associated with these risk factors is of the same magnitude among twins and singletons and is statistically significant in both cases. These findings suggest that exposure of twin pregnancies to these latter risk factors, and particularly to smoking during pregnancy, can lead to the delivery of newborns with extremely low birth weights.