The Effects of Weight Gain for Women

Similarly, a study from 2015, involving 1,950 women who were attempting pregnancy, found that changes in their body weight after age 18 affected their ability to get pregnant. (5) For every 11-pound increase in their body weight after age 18, it took them 5 percent longer to get pregnant; women who were underweight (with a BMI under 18.5) also experienced a 25 percent longer time to pregnancy than those whose weight was in the normal range.

“It’s kind of like Goldilocks and the three bears: You need the right amount of body fat for a successful pregnancy,” says Joseph A. Hill, MD, a reproductive endocrinologist and infertility specialist at the Fertility Centers of New England in Boston.

The Root Source of Weight-Related Fertility Problems

In order for a woman to become pregnant, the ovary needs to release an egg (ovulation) so that a man’s sperm can fertilize it. Without ovulation, fertilization just can’t happen. Even if fertilization does happen, your body needs to be able to support implantation and the fetus’s development in the womb (uterus). The right amounts of female hormones are essential for these processes to occur properly. (1)

Body Weight Influences Hormone Levels in Women

Being overweight or underweight can wreak havoc with your hormone levels, shutting down estrogen production in particular, and your ability to ovulate regularly. (1)

If you’re underweight and have too little body fat, you may have too little progesterone and higher than normal levels of the stress hormone cortisol, either of which could impair ovulation and implantation.

“If you’re underweight, your body senses this as stress and shuts off the reproductive system to focus on things that are essential to survival,” explains Brooke H. Wertz, MD, a reproductive endocrinologist and fertility specialist at the NYU Langone Fertility Center in New York City. “The normal pattern of brain signals to the ovary are disrupted, which leads to ovulatory dysfunction.” This is especially true if you have an eating disorder or hypothalamic amenorrhea (in which your period stops because you’re not eating enough or you’re exercising too much).

Fat Cells, Estrogen, and Irregular Menstrual Cycles

Meanwhile, being overweight can be equally if not more problematic. Because fat cells make estrogen, being overweight or obese can interfere with ovulation and lead to irregular menstrual cycles. “If you have ovulatory issues and you’re overweight, there’s a chance those two things are related,” says Mary Ellen Pavone, MD, an associate professor of obstetrics and gynecology at the Northwestern University Feinberg School of Medicine in Chicago.

What’s more, “the excess fatty tissue produces a weak estrogen that leads to increased levels of androgens [male hormones],” Dr. Hill explains. “The combination creates a toxic milieu in the uterus, so even if ovulation occurs, the chances of implantation failure or miscarriage are increased.”

What to Know About Body Weight and Fertility Treatments

Making matters worse, being overweight or obese can compromise your response to fertility treatments, particularly medication that induces ovulation, Dr. Wertz notes. Women who are obese have a lower success rate with in vitro fertilization (IVF), according to a 2011 study. (6)

In a meta-analysis from 2018, researchers reviewed 49 studies that investigated the association between women’s body weight and their success with assisted reproductive technology (ART). (7) Overweight and obese women had a 19 percent lower live birth rate after ART than normal-weight women did; obese women also had a 52 percent greater incidence of miscarriage after ART.

The Good News About Fertility and Body Weight

Losing excess weight often helps improve fertility in women who are overweight or obese — and you don’t necessarily have to reach an ideal weight (or a BMI under 25). “In women who are obese, losing 5 to 10 percent of their weight can bring significant changes in their ability to get pregnant by improving ovulation and egg quality,” Wertz explains. “It’s easier for them to get pregnant, and they often have a better overall pregnancy experience” with fewer complications, such as high blood pressure or diabetes during pregnancy.

Similarly, when obese men slim down, their sperm DNA fragmentation improves significantly, as does their percentage of normally shaped sperm, according to a 2018 study. (8) This is noteworthy because both sperm abnormalities contribute to subfertility.

Cutting Calories, Exercising Can Improve the Likelihood of Pregnancy

A meta-analysis of 40 studies, published in 2017, concluded that reduced-calorie diets and exercise interventions improved pregnancy outcome by 59 percent among overweight and obese women, as well as resulting in weight loss and ovulation improvement. (9)

Similarly, research from 2014 evaluated the effect of “meaningful” weight loss (10 percent of total body weight) on 52 overweight patients who were being treated for infertility. Those who lost 10 percent of their weight through diet and exercise modifications had significantly higher conception and live birth rates than those who didn’t lose that much. (10)

And a study published in 2017 found that after obese women lost an average of 12 pounds through an individualized diet and exercise program, they had a 3.5 times higher live birth rate after one cycle of IVF than those who didn’t shed excess weight. (11)

Ovulation, Conception Processes May Benefit From a Healthy Body Weight

So as far as baby-making goes, it’s worth making an effort to work with your healthcare team to achieve or maintain a healthy weight by following a nutritious, balanced diet with reduced calories, and exercising regularly. (Same goes for when you do conceive; getting your healthcare team’s clearance for any diet changes is critical for your and your baby’s health.)

In terms of getting pregnant, even if conceiving naturally isn’t happening after shedding excess pounds, weight loss can improve your odds of having a successful pregnancy with IVF, Hill says.

If you’re underweight, gaining some weight may help restore normal ovulation; if it doesn’t, you may be a candidate for ovulation induction agents (to increase production of follicle stimulating hormone) or progesterone supplementation, Hill says.