Weight affects fertility in a very large number of ways. Women who have high body mass index, who are either overweight or obese, have a higher likelihood of having irregular periods and having irregular periods. Which means that you are not ovulating or producing an egg every month. And obviously if you are not producing an egg, it’s harder to get pregnant. Even for women who are overweight or obese but they have regular cycles they still have a harder time getting pregnant. And data has also shown us that once women are pregnant, when they are overweight or obese they actually have a harder time staying pregnant.
There’s a higher risk of miscarriage as well in women who are either overweight or obese. Just to give you a rough idea, women who are obese will have about a 2 to 3 1/2 fold increase risk in terms of difficulty getting pregnant and it’s similar for miscarriage, as well in terms of the increase of risk for miscarrying compared to women of normal weight. So women who are underweight also have some difficulty getting pregnant. The difficulty getting pregnant is more subtle in that if you are slightly underweight there may not be a huge impact. It really focuses around the extent to which women are no longer ovulating.
So in the same way there’s an increased risk of not ovulating with significantly overweight and obese women who develop poly cystic ovary syndrome, there’s an increased risk of stopping having a period in women who are underweight. And in particular it’s not just women who are underweight, it’s women who consistently maintain a calorie-deficit. What that means is that they are consistently taking in less than they are utilizing. And what your body does is lower its basal metabolic rate in order to fix that gap and essentially make reproductive cycles a non-essential service. So it’s one of the services that the body will shut down if it’s seeing that there’s an energy deficit. And for that reason it can be difficult for those women to get pregnant.