How Weight Affects Fertility

In this excerpt from The Fertility Diet, by Jorge E. Chavarro, M.D. and Walter C. Willett, M.D., find out how being overweight or underweight can affect your fertility negatively and how to find the right weight to boost ovulation naturally and improve your changes of getting pregnant.

woman on dcotor's scale

Pregnancy takes energy, literally and figuratively. For our hunter-gatherer ancestors, the physical demands of pregnancy would have been easier to bear when food was plentiful and likely to remain that way. But when it was scarce, pregnancy would have been doubly difficult for mother and child, and maybe even for the whole family unit.

This is reflected in the exquisite lines of communication between the reproductive system and energy stores. A host of hormones provide feedback between the two that help determine fertility. Weight, or BMI, is an outward sign of this communication.


This is easiest to understand with undernutrition. As fat stores dwindle, menstruation and ovulation falter. At some point, they cease altogether. This makes sense from an evolutionary point of view — the body temporarily stops putting energy into reproduction and instead uses it for gathering food, staying warm and running the rest of the body.

A famine that affected the Netherlands near the end of World War II is one of many examples of this effect. Between October 1944 and January 1945, the average intake of food fell from 1500 calories a day to 700. Nine months after the famine started, the birth rate plummeted dramatically. As we describe in Chapter 11, you can see the effect today in women with anorexia, and sometimes in elite athletes, who stop menstruating when they become too light or too lean.


What about overnutrition? If stored energy is a good thing, then women with a lot of it should be extremely fertile. That's not the case. As weight climbs, so do levels of insulin and insulin-like growth factor-1, while sex hormone binding globulin declines. The net result of these changes is an increase in the amount of free and active testosterone and other male sex hormones in the ovaries and the bloodstream. This can dampen ovulation and can lead to delays in getting pregnant or block it altogether.

The body's storehouse for extra energy is adipose tissue, more commonly known as body fat. Long thought of as merely a passive depot, body fat is turning out to be an active and complex tissue. Like the pancreas or hypothalamus, adipose tissue generates a variety of hormones that influence appetite, activity, weight and reproduction. Adiponectin is the most abundant protein made by fat cells. It helps stimulate fat-burning processes, makes cells more sensitive to insulin, and may enhance ovulation.

The more weight you gain, though, the less adiponectin your fat cells make. This dropoff can contribute to insulin resistance and interfere with ovulation. It also elevates leptin and other hormones that disrupt ovulation. Extra body fat boosts levels of interleukin-6 and other cell-signaling molecules that interfere with the ability of a fertilized egg to implant itself in the lining of the uterus. The main hormones secreted by fat cells and their effects on reproduction are listed in Table 10.1.

Weight and fertility in the Nurses' Health Study

Weight is one bit of information that the participants of the Nurses' Health Study report every other year. By linking this information with their accounts of pregnancy, birth, miscarriage and difficulty getting pregnant, we were able to see a strong connection between weight and fertility. Women with the lowest and highest BMIs were more likely to have had trouble with ovulatory infertility than women in the middle, as shown in Figure 10.3.5. Infertility was least common among women with BMIs of 20 to 24, with an ideal around 21.

Keep in mind that this is a statistical model of probabilities that links weight and fertility. It doesn't mean you'll get pregnant only if you have a BMI between 20 and 24. Women with higher and lower BMIs than this get pregnant all the time without delay or any medical help. But it supports the idea that weighing too much or too little for your frame can get in the way of having a baby.

Fertility zone for weight

We call the range of BMIs from 20 to 24 the fertility zone. It isn't magic — nothing is for fertility — but having a weight in that range seems to be best for getting pregnant. If you aren't in or near the zone, don't despair. Working to move your BMI in that direction by gaining or losing some weight is almost as good. Relatively small changes are often enough to have the desired effects of healthy ovulation and improved fertility. If you are too lean, gaining five or 10 pounds can sometimes be enough to restart ovulation and menstrual periods. If you are overweight, losing 5 to 10 percent of your current weight is often enough to improve ovulation.

How to calculate your BMI >>

In the pages that follow, we focus on weighing too much, because it is a bigger problem here and around the world. Once considered a problem only in high-income countries like the United States, overweight and obesity are now on the rise in low- and middle-income countries, particularly in urban settings where they often paradoxically stand side by side with malnutrition.

The 7-1/2 Percent Solution

Things can look a bit bleak if you are overweight and a healthy weight is many pounds away. Take heart. Numerous studies have shown that overweight women can jump-start ovulation by losing a modest amount of weight — 5 to 10 percent of their starting weight — even when that loss doesn't get them into the healthy weight range. It may be a bit less for some, a bit more for others. We'll split the difference and take a middle ground, which we'll call the 7-1/2 percent solution.

Let's put this into practical terms. A 7-1/2 percent weight loss means 12 pounds for a woman starting at 160 pounds, 15 pounds for one starting at 200 pounds, and 19 pounds for one starting at 250 pounds. Although such goals are a challenge, they are far less daunting than aiming for a healthy weight, the advice usually offered by health care professionals.

Among women with polycystic ovary syndrome (PCOS), this amount of weight loss can not only restore ovulation and menstruation but can also clear the skin and curb excess facial and body hair. These improvements are probably the result of better sensitivity to insulin and a decrease in the amount of male hormones in circulation.

Among all overweight women, a modest 7-1/2 percent weight loss improves health across the board. Take prevention of diabetes as an example. A major trial, called the Diabetes Prevention Program, included more than 3,200 people who were at risk for developing type 2 diabetes. Those who lost just 7 percent of their weight and exercised about 30 minutes a day cut their risk of developing diabetes by nearly 60 percent. They decreased their harmful LDL cholesterol and triglyceride levels and increased their protective HDL, sometimes more than people in the study who took metformin, an antidiabetes drug. Losing a modest amount of weight can have positive effects on other silent conditions, like high blood pressure and kidney disease.

It can also make a difference in how you feel, from easing the sleep-stealing breathing problem known as sleep apnea to alleviating arthritis pain and giving you more energy.

Getting to 7-1/2

An extraordinarily simple equation describes how to shed pounds:

calories out > calories in = weight loss

In English: Burn more calories than you take in day after day, and you will absolutely, positively end up weighing less than when you started.

Of course, Einstein's e = mc2 is also a simple equation. It's too bad that the ease of explaining something doesn't necessarily indicate how easily it can be translated into action.

When it comes to losing weight, creating an imbalance between calories in and calories out isn't as simple as falling off a log. Appetite, metabolism and life get in the way, making weight loss a challenge for most people and seemingly impossible for some. But you can do it. You have a goal — having a baby — that is a powerful motivator. By following the strategies outlined in this chapter, you can lose weight in a way that is good for your health, fertility, peace of mind and relationship with food.

This is an excerpt from from The Fertility Diet (chapter 10, "The 7-1/2 Percent Solution") by authors Jorge E. Chavarro and Walter C. Willett.

Read more

How the pill can affect fertility
The male factor: The low-down on fertility sperm testing
Infertility myths

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