London Women who take more than a year to get pregnant have a slightly higher than normal chance of giving birth prematurely, new research indicates.
The reasons for premature birth are a mystery in nearly half of cases, and experts say the research, published this week in the journal Human Reproduction, indicates fertility troubles could explain part of the problem.
Premature birth -- when the baby is delivered before 37 weeks of pregnancy -- is a major public health concern because underdeveloped babies are at increased risk of death in the first year of life and are more likely to develop disorders if they survive.
The rate of premature births in the United States has jumped in the last 20 years from about 9.4 percent of all births to almost 12 percent. About a half-million premature infants are born annually in the United States.
The study is the largest ever conducted on infertility and birth outcomes.
Dr. Olga Basso of the University of Arhaus in Denmark and Dr. Donna Baird of the U.S. National Institute of Environmental Health Sciences analyzed nearly 56,000 singleton births in Denmark, where women had reported early in their pregnancies how long it took them to get pregnant.
About 11 percent of the women had to try for more than a year to get pregnant -- the common definition of infertility.
Among the women who had conceived their first baby within a year, 5.4 percent gave birth prematurely. That compared with 7.4 percent of the women who had trouble but eventually conceived naturally and 7.6 percent of women who had fertility treatment after at least a year of failing to get pregnant.
Once the results were adjusted for factors that influence pre-term birth, women who had trouble getting pregnant were about 40 percent more likely to give birth prematurely than those who conceived easily, the study found.