Couples who conceive through certain types of infertility treatment may have a higher-than-normal likelihood of having a premature baby, a new study suggests.

Danish researchers found that among more than 20,000 women who gave birth at their hospital between 1989 and 2006, those who had conceived through in-vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) had a higher risk of preterm delivery.

Of the 730 babies born to women who underwent IVF or ICSI, nearly 8 percent were premature and 1.5 percent were very premature - born before the 32nd week of pregnancy. A normal pregnancy lasts 40 weeks.

In comparison, roughly 5 percent of babies born to fertile mothers were premature, and 0.6 percent were born very preterm, the researchers report in the journal Fertility and Sterility.

When the researchers accounted for factors like the mother’s age, weight and exposure to cigarette smoking, the IVF and ICSI procedures were still linked to a 53 percent greater risk of preterm delivery and a doubling in the odds of very premature birth.

Other forms of fertility treatment - namely, fertility drugs and insemination - were not related to the risk of preterm delivery.

Nor was the higher risk with IVF and ICSI explained by elevated rates of twin or higher-order births. The study included only singleton births.

Together, the researchers say, the findings suggest that something about the IVF and ICSI procedures themselves might raise the odds of preterm birth.

Both IVF and ICSI involve joining a woman’s egg and a man’s sperm in a lab dish, then - if fertilization is successful - transferring one or more embryos to the woman’s uterus. ICSI is typically used for male fertility problems, including a low sperm count or poor sperm quality. It involves isolating a single sperm and injecting it directly into the egg.

“The IVF/ICSI procedures include hormone stimulation and mechanical procedures. Both of these factors may influence the risk of preterm delivery,” lead researcher Dr. Kirsten Wisborg, of Aarhus University Hospital in Denmark, told Reuters Health in an email.

The fact that other forms of fertility treatment were not linked to preterm delivery suggests that infertility itself is not to blame, according to Wisborg. However, she pointed out, couples who undergo IVF or ICSI may have a different “reproductive pathology” than those who conceive via fertility drugs or insemination, as they frequently have been infertile for a longer period and have failed to conceive through those “low-tech” fertility treatments.

There may also be other factors, unmeasured in this study, that put women who undergo IVF or ICSI at greater risk of preterm delivery, Wisborg said. She and her colleagues accounted for the women’s age, body weight, smoking history, and caffeine and alcohol intake during pregnancy, but other variables could still be at work.

Another possibility, Wisborg said, has to do with the “vanishing twin” phenomenon. Some of the singleton births to women who underwent IVF or ICSI may have begun as a twin pregnancy, with only one fetus surviving beyond the early stages. Research suggests that these surviving fetuses are at increased risk of preterm delivery and low birth weight.

“However, we believe that the theory about the vanishing twin is not the only explanation of our finding,” Wisborg said.

The most important factor in reducing preterm birth risk with IVF or ICSI is to avoid higher-order pregnancies, according to Wisborg. But women can also lower the risk, she said, by not smoking and avoiding alcohol during pregnancy.

SOURCE: Fertility and Sterility, online February 26, 2010.