In order to get pregnant, you must have a certain amount of body fat. Too little or too much can prevent ovulation. In our female ancestors, too little fat meant that
a woman would not be able to nourish an infant before and after birth. Too much fat also inhibited ovulation, as it does today.
Your body mass index (BMI) is one indicator of how easily you can get pregnant, as well as whether you are of normal weight, overweight or obese. It can help predict your fertility.
Source: MIMS Health Today