Oct. 29, 2003 -- Women who take longer than a year to get pregnant are more likely to have low birth weight babies or need a cesarean section.
A newly reported Danish study found that women who get pregnant after more than a year of trying were up to twice as likely to require a C-section or have babies born preterm or low birth weight.
The study is the largest ever to examine infertility and birth outcomes. It is also the first study to find an increase in risk among women who have trouble conceiving.
Researcher Olga Basso, PhD, and U.S. collaborator Donna Baird, PhD, of the National Institutes of Health, analyzed nearly 56,000 births from the Danish National Birth Cohort. The registry keeps detailed records on pregnancies and births, including the time it takes a woman to conceive. Roughly one in 10 women included in the analysis spent more than a year trying to get pregnant, which is the generally accepted definition of infertility.
The researchers found the problems even among couples who had not undergone infertility treatments.
The findings were not as consistent among women that used infertility treatment to conceive. While researchers found the same pregnancy and delivery problems in women who had infertility treatments, this was seen only in women who have had prior deliveries. Basso says women should not be overly alarmed by the findings, but she says they do deserve further study.
In this study researchers excluded women who were pregnant with more than one baby because multiple birth pregnancies greatly increases the risk of problems.
The findings are reported in the November issue of the journal Human Reproduction.
"We don't know if all types of infertility are associated with problem births, or whether this is limited to certain types of infertility," she tells WebMD. "But it is important to find out, because this is a common problem. As many as 10% of babies are born to infertile couples."
Preterm Births Increase Most
"Once we had adjusted the figures to account for factors such as smoking, body mass index, and age, the increase was lower, suggesting that these factors may explain some of the association," Basso says. "But significant associations with infertility still remained."
The most troubling increase was in preterm births, the researcher says. First-time mothers in the study were almost 40% more likely to have a preterm baby and the risk was nearly 80% higher for women who had given birth previously.
Who's at Risk?
In an earlier study, Baird reported a similar increase in the risk of preterm delivery among women who conceived without fertility treatments. The risk was 30% higher than normal for women who conceived after seven to 12 months of trying and 60% higher for those who conceived after trying for more than one year.
Other studies have documented an increase in delivery risk among women treated for infertility, but Baird tells WebMD that, as yet, no one has studied whether women with different types of infertility also have different risks.
"That is a logical next step," she says. "We need to know more about the specific factors in infertility that are associated with problem births so that we can identify the women who are really at high risk."