BMI and Fertility: Does Your Weight Matter to Conceive?

BMI and Fertility: Does Your Weight Matter to Conceive?

New evidence shows that both male & female weight matters when trying to conceive. High weight and low weight both influence fertility.

If you are an overweight female, will it take you longer to become pregnant? What if you are underweight? What if your partner is overweight or underweight? We now have new evidence that finally allows us to answer these questions.

bmi impacts fertility

Fertility doctors like me are gynecologists. That is we are trained as doctors for women. We tend to pay more attention to female factors that impact fertility. As a result, there have been a number of studies looking at the impact of a female's weight on fertility. But far fewer that focus on a man’s weight. There haven't been any good studies that have looked at the weight of both partners and a couple at the same time.

Today I am going to review the results of an enormous two-year study that looked at the impact of weight on how long it took couples to become pregnant.

What is BMI (Body mass index)?

First, a few facts you need to know. Fertility doctors don't look at weight by itself. We look at weight in relation to height. This is called BMI or body mass index.

Why? Taller people are going to weigh more because they are taller and aren't necessarily overweight. If you aren't sure what your BMI is and you know your height and weight, go to this website to calculate it.

BMI categories

Based on your BMI, we separate people into categories. These numbers might be a little different based on your ethnic background. 

  • a BMI is less than 18.5 you are considered underweight
  • BMI between 18.5 and less than 25 is considered normal
  • a BMI is 25 to less than 30 it falls within the overweight range
  • a BMI is 30 or higher it falls within the obese range

There are different categories for obese, but for today we're going to group those all together and just call them obese.

What is TTP (Time to pregnancy)?

The next term you need to understand is called time to pregnancy or TTP.

This is easy to understand. It is the length of time from when a couple starts trying to become pregnant until they become pregnant. It can be measured in days, weeks, months or years. Sometimes, it is measured by the number of menstrual cycles a woman has had.

We know of lots of factors that influence the time to pregnancy. For example, older women have reduced fertility and so they have a longer time to pregnancy compared to younger women.

pregnancy age bmi fertility

New study looked at BMI and fertility

Let's talk about this new study from China. When I say this was an enormous study, I mean it was absolutely enormous.

Researchers followed over 2.3 million couples who were just starting to try to become pregnant. They followed them closely for one year. 

At the start of the study that calculated the BMI for every couple. Both the men and the women. By the end of the study nearly 1.5 million couples had become pregnant.

Impact of female BMI on fertility

First let's look at the impact of women's BMI alone. The highest pregnancy rate was in the normal weight group. 64.14% of this group achieved pregnancy. 

Underweight women and overweight women were about 3% lower at 61%. Women who were obese however, had a pregnancy rate that was 9% lower: 55%.

Impact of male BMI on fertility

For men, the differences were much smaller:

  • 63.3% of couples became pregnant when the men were normal weight
  • Slightly lower for underweight men : 62.7%
  • Slightly higher for overweight men: 63.7% 
  • Slightly lower for obese men: 62.2% 

All those numbers were for analyzing each partner's BMI by itself.

Impact of the couple's BMI together on fertility

When the researchers looked at both partners together, there were some really interesting and unexpected results:

Compared to couples who were both in the normal weight category; if a couple were both underweight their pregnancy rate was reduced by 10%.

If both partners were obese, then their pregnancy rate was reduced by nearly 20%.

This next result is really interesting: if a woman was obese and the man was underweight, then their fertility was reduced by 30%.

Our bottom line:

This when trying to become pregnant, fertility is best when both partners have a normal BMI. If either partner is underweight or obese, or if a woman is obese and a man is underweight, it will take them longer to become pregnant.

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