When Ronda Kelly, A 5-foot-5-inch jewelry designer in Portland, Ore., began trying to get pregnant more than three years ago, she weighed 110 pounds and hadn't had a menstrual period in 15 years. Gaining just 8 additional pounds helped Kelly, then 34, start having regular periods again, yet it still took a total of a year of trying and, finally, intra-uterine insemination to conceive her daughter, Lauren, now 2 1/2.
Before she gained weight, Kelly's calculated body mass index (BMI) was 18.3. But the ideal BMI for conception is 20 to 25. Being either underweight, as Kelly was, or overweight can cause problems with ovulation as well as other conditions that make conceiving difficult. Recent studies indicate that if you're overweight, it can even be harder to get pregnant by means of high-tech methods such as in vitro fertilization.
"If body weight and body fat are either too high or too low, estrogen and progesterone concentrations in the body go down," explains Nancy Williams, Sc.D., an associate professor of kinesiology at the Pennsylvania State University in University Park. "An adequate estrogen level is needed to stimulate ovulation." Underweight women can have regular menstrual bleeding, but the cycles are likely to be anovulatory, which means ovulation doesn't occur.
Low progesterone levels can affect the luteal phase, the second half of your menstrual cycle. Having a luteal phase disturbance (LPD) means you might ovulate normally and fertilization occurs, but there's not enough time for the fertilized egg to properly implant in your uterus. Getting your period less than 10 days after you ovulate (as determined by an ovulation kit or a blood test) is a red flag for an LPD.
Not too big, not too small
Weighing too much can make conceiving as difficult as weighing too little. "Overweight and obesity exacerbate a host of things, such as causing anovulatory infertility and diminishing the response to fertility medications," says Richard S. Legro, M.D., a professor of obstetrics and gynecology at the Pennsylvania State University College of Medicine in Hershey.
One problem caused by excess weight is insulin resistance. Women with this condition have higher levels of free androgens (male hormones) and free estrogen that can adversely affect ovulation and embryo implantation. A certain level of hormones is needed to promote a healthy uterine lining, or endometrium. When there is more tissue (read: fat) to "soak up" those hormones, the endometrium is deprived of them.