How small is too small in a twin pregnancy?

The objective was to determine whether small twins had a survival advantage with respect to small singletons after controlling for other factors associated with adverse perinatal outcome.

A hospital-based cohort study included all births between 1980 and 1995 of babies born between 24 and 43 weeks' gestation. Logistic regression was used to estimate the perinatal mortality risks for monochorionic and dichorionic twins with growth restriction after adjusting for gestational age, maternal age, parity, method of delivery, and the presence or absence of congenital malformations.

The study sample included 1062 dichorionic twins, 354 monochorionic twins, and 59,873 singletons. Small monochorionic and dichorionic twins showed a similar overall risk of perinatal mortality (odds ratio 1.40, confidence interval 0.86 to 2.25). However, monochorionic twins with birth weights <10th percentile faced an increased risk of perinatal death compared with singletons (odds ratio 2.45, confidence interval 1.20 to 5.02). Dichorionic twins had no such increased risk (odds ratio 0.91, confidence interval 0.45 to 1.84).

Twins with growth restriction are not protected against perinatal loss, even after adjusting for congenital malformations. In fact, monochorionic twins are more than twice as likely to die in the perinatal period as are their singleton counterparts.