Twins conceived by test tube or “in vitro fertilization” (IVF) are more likely than twins conceived through sexual intercourse to be born prematurely and to be delivered by Cesarean section, a review of previous studies suggests.

IVF has led to an increase in the number of preterm births and low birth weight infants, but the mothers of these infants are typically older and at higher risk of both outcomes, explain Dr. Sarah McDonald from University of Ottawa, Ontario, Canada and colleagues in the American Journal of Obstetrics and Gynecology.

They therefore compared the rate of adverse birth outcomes for 2303 IVF twins and 2326 spontaneously conceived twins born to women of similar age.

IVF twins were around 50 percent more likely than spontaneously conceived twins to be born preterm, the investigators report, but there were no differences in the likelihood of having a low birth weight.

IVF twins were also twice as likely to be admitted to the intensive care unit and 33 percent more likely to undergo Cesarean section than were spontaneously conceived twins, the report indicates.

The two groups of twins did not differ in the rates of stillbirth, very low birth weight, or major complications of delivery, the researchers note, and there was no difference in the rate of birth defects.

“IVF twins have worse perinatal outcomes than spontaneously conceived twins who are matched for maternal age,” the team concludes.

Possible reasons for the increase in preterm birth “include a factor inherent to the IVF technology, a history of infertility itself, or physician or patient anxiety,” the investigators speculate.

SOURCE: American Journal of Obstetrics and Gynecology, July 2005.