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Fit to Be a Mom? How Exercise Affects Your Fertility

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The Ideal Weight to Conceive

The numbers on your scale can also be key to your ability to conceive. Exer?cise, of course, can help regulate your weight, but only if you've got a realistic grip on the numbers. According to a 2010 University of Texas Medical Branch at Galveston study, nearly 48 percent of underweight, 23 percent of overweight, and 16 percent of normal-weight reproductive-age women don't accurately assess their own body weights. Such a misperception could have an impact on your health habits, which could then affect your fertility.

Moreover, your ideal weight for hitting 5K PRs or fitting into your skinny jeans may not be the weight most conducive to conceiving. "You don't have to be a size 6 to have a baby," says lead study researcher and ob-gyn Abbey Berenson, MD. "This isn't about what looks good on a runway. It's about making your body healthy enough to carry a child." The sweet spot for many women translates to the normal BMI range (18.5 to 24.9), which is associated with optimal reproductive function. Research shows that 12 percent of infertility cases may result from being under that range and 25 percent from being over it. The two extremes tax the body in ways that disturb hormone production and ovulation, Dr. Brzyski says.

Even so, BMI is not always the best way to assess how weight will affect reproductive function. The measurement is based on height and weight and doesn't distinguish between fat and muscle -- and fit women have a lot of lean muscle mass. William Schoolcraft, MD, founder and medical director of the Colorado Center for Reproductive Medicine in Denver and author of If at First You Don't Conceive, often sends his patients to an exercise physiologist to measure their body fat percentage (through skinfold-caliper or buoyancy testing) instead. Ovulation is impaired if body fat is less than 12 percent or more than 30 to 35 percent, he notes. "Women take getting their periods as a sign they are at a healthy BMI and have normal fertility," Dr. Schoolcraft says. "However, you can have regular or somewhat regular periods and not ovulate, though it's unusual." If you menstruate every 26 to 34 days, you probably ovulate, but to make sure, pick up a basal body thermometer at a pharmacy. On waking, use the device at the same time each morning to measure your temperature, and track it on a basal body temperature chart (download one at to see if you're ovulating.

How Your Weight Affects Your Fertility

Though disrupted cycles and missed periods are commonly associated with elite athletes, Jamie Grifo, MD, PhD, director of the NYU Fertility Center in New York City, also sees his share of weekend warriors who overdo it. "I tell them to scale back," he says. "You want your body to be a fertility-promoting environment." More than an hour of vigorous exercise a day can lead to a decrease in the production of the hormones that stimulate ovary function, causing ovaries to become underactive and stop producing eggs and estrogen, in some women. The risk increases with exercise duration and intensity. What's more, Dr. Schoolcraft says, intense exercise sessions cause the body to break down the proteins in muscles, producing ammonia, a pregnancy-inhibiting chemical.

It seems counterintuitive that something that makes you feel good and has been proved to protect your body against myriad diseases and health problems can actually be bad for your fertility. Here's what happens: "Intense exercise lowers progesterone and throws off your hormone levels," says Sami David, MD, a reproductive endocrinologist in New York City and coauthor of Making Babies: A Proven 3-Month Program for Maximum Fertility. "Endorphins can suppress your FSH and LH, the hormones in your pituitary gland responsible for producing eggs, and the ovarian hormones estradiol and progesterone, making it harder for you to get pregnant or more likely to miscarry without knowing it." The bottom line: "The extremes of exercise -- too much or too little -- are never good," Dr. Grifo says. "You need to find a balance between the two; that's when your body functions optimally."

Michelle Jarc, 36, a teacher in Cleveland, got the same message from her doctor after she suffered a miscarriage and tried unsuccessfully for nine months to conceive again. "I'm a runner, and at that time I was racing in a 5K just about every weekend," Michelle says. Although her weight put her in the normal BMI range, she was having irregular menstrual cycles. Her doctor, who suspected that Michelle wasn't producing enough estrogen, put her on Clomid (a prescription drug that induces ovulation) and advised her to cut back on her workouts and, for good measure, gain a few pounds. "It was hard at first to listen to her advice. I was obsessed with being fit and maintaining my figure. But having a child became more of a priority," Michelle says. So she cut her twice-daily exercise routine to just one 30- to 45-minute-a-day workout and stopped worrying about what she ate. After that, conceiving was a cinch. Today Michelle has four kids -- a 5-year-old daughter, a 3-year-old son, and 14-month-old twin boys -- and is back to her prepregnancy weight and competing in 5Ks again.

Yet for sedentary women the subtle physiological changes that come from increasing exercise can benefit their odds of conceiving. Exercise improves metabolism and circulation, both of which contribute to better egg production. Regular activity also optimizes your reproductive system by stimulating the endocrine glands, which secrete hormones that help eggs grow. Plus, getting your sweat on is a known stress reliever -- a good thing, because stress significantly decreased the probability of conception in one study.

All those fertility-boosting benefits could help explain why some women find a bun in the oven shortly after stepping up their exercise routine. A doctor originally put the odds for Jennifer Marshall, 30, a marketing manager in Cincinnati with reproductive complications, to get pregnant at only 0.5 percent. Fast-forward through seven years of tests, surgeries, and many artificial insemination attempts: "I thought I would never get pregnant," Jennifer admits. Yet eight weeks into P90X -- a home DVD-based workout and nutrition program that she started because she was bored with her less intense walking and biking sessions -- she found herself staring at a plus sign on a pregnancy-test stick. Whether exercise was the ultimate catalyst, Jennifer's docs can't say. "They were just shocked I got pregnant," she says. But the new routine, which helped her lower her weight to 170 (at 5 feet 8 inches, she'd previously fluctuated between 175 and 210), was all that had changed recently. She gave birth to a healthy baby girl this past March.


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