You might be thinking, “Hold up; I thought pregnancy meant the freedom to gain weight. Why measure my body fat?” Body-mass index (BMI), is calculated from a formula based on your height and weight and can be factor into fertility and healthy nutrition for supporting the growth of your fetus.
If your BMI is either too high or too low, your periods can be disrupted, affecting conception. Specifically, a BMI that is below the 10th percentile, meaning your body fat is less than 22 percent of the total, means you could experience irregular periods or amenorrhea. Eating fewer calories than you use during the day can disrupt your body’s ability to produce estrogen and progesterone, which regulate your menstrual cycle and by extension your ovulation and fertility. However, when your diet includes enough fuel to support your exercise routine and energy use, there will be no change in the hormones responsible for fertility.
If you’re having house guests, you buy more groceries. If you’re going to house a baby in your body, you need enough nutrition for both of you. Your kids won’t just raid the fridge as teenagers—they’ll do it in the womb, too.
On the whole, medical experts agree that there is no real evidence that exercise intensity threatens fertility—as long as you get balanced nutrition to maintain a healthy fat store. In fact, research shows that overweight women experience a higher rate of infertility than those who have lower BMIs, with 12 percent of cases attributable to being underweight and 25 percent to obesity.
What’s more, vigorous exercise actually lowers your risk for ovulatory infertility, as long as you maintain a healthy BMI. Every hour that you exercise each week is associated with a 5 to 7 percent lower risk of infertility related to ovulation.
If your doctor or midwife is concerned about your BMI being too low, you might be advised to decrease your workout intensity or training volume temporarily while you boost your calorie intake to meet your body’s demands (based on what you’re burning). If that’s the case, don’t fret about lost workouts. Consider replacing one calorie-sucking cardio workout per week with a yoga or dance class. You might not tap into your aerobic endorphins, but you can still achieve the delicious feel of a sweaty workout.
This is a great time to try a new fitness activity you haven’t been able to fit into your regular routine or specific training regimen. It could be your chance to try belly dancing and archery—just don’t do them together. Fitness isn’t a threat to pregnancy; it’s an asset. As long as your training or exercise program increases gradually in intensity and volume, it will be less likely to jeopardize your menstruation because the reproductive system adapts to changes in exertion and metabolism.5 Older studies that linked exercise and infertility sampled women who started very intense programs or who had other complicating factors, such as stress or nutritional deficits.6 More recent research that compares exercisers of various levels of intensity with nonexercisers finds that the rate of conception is similar between groups and that approximately 5 percent of women in both groups experienced infertility.
Check back soon for the next article: More on Exercise, BMI, and Getting Pregnant
Fit & Healthy Pregnancy is a friendly, comprehensive guide to exercising during pregnancy from athlete-moms Kristina Pinto and Rachel Kramer, MD.
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Please consult your doctor before embarking on an exercise program during pregnancy.