Nulliparity, reductions tied to risk in ART twins. (Slowed Fetal Growth, Preterm Birth).

SAN FRANCISCO -- Women who conceive twins through assisted reproductive technologies are at increased risk for complications or adverse outcomes, compared with those carrying spontaneously conceived twins, but the difference in risk is not caused by the technologies themselves.

That conclusion emerged from an ongoing study of 3,106 twin pregnancies. According to the study, factors that were more common in twin pregnancies achieved through assisted reproductive technologies (ART)--specifically, nulliparity and fetal reductions--accounted for the higher risks of complications relative to women who conceived twins spontaneously.

The data came from an ongoing collaborative study at seven U.S. medical centers, study investigator Barbara Luke, Sc.D., said at the annual meeting of the Society for Maternal-Fetal Medicine.

Previous data showing a higher rate of complications and bad outcomes in multiple pregnancies from ART come mainly from small trials or trials that lacked a heterogeneous population. The current study avoided both limitations, said Dr. Luke, professor of epidemiology and public health at the University of Miami.

The current data came from 2,553 spontaneous twin pregnancies and 553 twin pregnancies conceived through ART. A total of 89 women in the ART group underwent twin-to-singleton fetal reduction.

Compared with outcomes in all other pregnancies, fetal reduction was associated with an increased risk for slowed fetal growth (less than 90 g per week) between 20 and 28 weeks' gestation and for birth before 32 weeks. Reduction also was linked to a tripling of the risk of birth before 30 weeks and the risk of having a very low-birth-weight infant, Dr. Luke said.

In the 3,017 nonreduced twin pregnancies, ART ...