Big not always bad: Overweight and pregnant

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Impaired uterine function is not the cause of lower birth rates among overweight women using fertility technologies to become pregnant, according to a Mayo Clinic study published in the August 2003 issue of Fertility and Sterility.

Previous studies have shown that overweight women using fertility technologies such as in vitro fertilization have significantly lower birth rates. This fact has caused some researchers to question whether body mass somehow disrupts endometrial function -- the ability of the uterus to receive an embryo and carry a pregnancy to term.

The Mayo Clinic study results painted a more encouraging picture for overweight women using fertility technologies. "Our research shows that when we control variables such as hormone levels and embryo quality, the uterus works fine, regardless of body mass. The key is good embryos," says Ian Tummon, MD, a Mayo Clinic reproductive endocrinologist who is the study's principal investigator.

The study involved 96 women of childbearing age whose body mass indices ranged from below normal to obese. Each woman received hormone injections to prepare the uterus to receive embryos. Researchers made efforts to standardize the quality of embryos implanted. Embryo implantation rates for non-obese and obese subjects were nearly equal (27 percent and 29 percent, respectively). Live birth rates also were comparable, with 44 percent of non-obese and 42 percent of obese women giving birth.

Although the Mayo Clinic study suggests that uterine function is not the cause of lower birth rates among overweight women using fertility technologies, it also raises questions. "We still need to explore possible connections between weight and other factors such as hormone levels or embryo development, which can affect a woman's ability to become pregnant and carry a pregnancy to term," says Dr Tummon.

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