How does your weight affect fertility?

Body fat, as such, is not bad for us. The key is having a healthy amount of it. Fat acts as energy storage. But more importantly for fertility, it helps to regulate reproductive hormones and ovulation.

Girls at puberty need to reach 17% of body fat to start menstruating. However, 17% of body fat is insufficient to keep the periods regular. For the reproductive axis to run smoothly BMI has to increase and fat has to reach 22% of body weight. 

Underweight

Being underweight, reduces uterine receptivity, increases time to conception and increases the risk of miscarriage. 

Reduced uterine receptivity

Women make oestrogen in both their fat cells and ovaries. To ovulate and have healthy periods, healthy amounts of oestrogen from both sources are required. That’s why being underweight, or with less than 17% body fat, can compromise ovulation and stop periods altogether. 

As little as a 2% reduction from healthy fat levels can be enough for your cycles to be irregular.

Takes longer to fall pregnant

It takes four times longer to fall pregnant when you are underweight. As discussed, some of the reasons for this may be low levels of hormones, fewer ovulatory cycles, and lower uterine receptivity. 

Miscarriage

Very lean women make weaker forms of oestrogen. This, in turn, affects the uterus’ ability to host the embryo. This may be one of the reasons why women with a low BMI are 72% more likely to suffer a miscarriage in the first three months.

Overweight

Excess weight may lead to compromised ovulation, reduced uterine receptivity, and miscarriage. Furthermore, it may affect the chances of getting pregnant with IVF. Pregnancy complications are also more common. This includes high blood pressure, diabetes, low/high birth weight, and more complicated labours. 

Increased inflammation

Unhealthy amounts of fat, especially around internal organs, may create low-grade inflammation in the body. The biggest danger occurs when fat cells balloon in size and start to ooze inflammatory proteins—known as cytokines—into your bloodstream. Cytokines reach your ovaries and uterus, spreading inflammation. Inflammation can affect ovulation, hormone production, and uterine receptivity, reducing the chances to conceive. 

Anovulation

Excessive body fat may interfere with the mechanism of ovulation. For example, women with a BMI above 27 are three times more likely to have non-ovulatory cycles. Less frequent ovulation equals fewer chances to fall pregnant.

Miscarriage

Obesity may cause chronic inflammation. Increased inflammatory markers have been shown to affect ovarian health. In particular, healthy egg development. Obesity can also compromise endometrial receptivity. These could be the reasons why women who are overweight are more likely to miscarry.

Reduced success rate with IVF

Unfortunately, IVF can’t undo the damage caused by excess weight. On average, obese women’s chance of having success with IVF reduces by 20% (live birth rate). And being overweight reduces the chance by 9%. 

High BMI may lower IVF success rates because increased body fat can disregulate hormones and disrupt the health of the endometrium. These changes affect all aspects of fertility from egg quality, fertilisation to implantation and the risk of miscarriage. 

Excess weight will affect pregnancy and the health of future children

Maintaining a healthy weight is equally important during pregnancy. Excess weight during pregnancy may affect your child’s ability to have babies. Additionally, it increases their risk of developing diabetes and heart disease. Researchers found that for every kilo of excess weight women had pre-pregnancy, it increased the risk by 4.5% of their child becoming obese.

The health of the mother during pregnancy affects not only the child but even future generations. A good example of this is a study published in 2020

Researchers discovered, that the grandchildren of women who had no excess gestational weight gain were less likely to be overweight or obese in adolescence and when they were adults.

Not to mention that being overweight during pregnancy increases risks for gestational diabetes, high blood pressure, and preeclampsia.

The good news: overweight women who follow reduced-calorie diets and do exercise are more likely to lose weight, start ovulating, and fall pregnant. Some studies show that weight loss increases the chances of natural conception pre-IVF.