There is evidence that both female and male fertility are decreased by being either overweight or underweight. It is recommended that people aiming to conceive naturally or with ART maintain a healthy body weight (BMI between 20 and 25).
During pregnancy, overweight and obesity are associated with increased risk of adverse maternal and infant health outcomes such as increased risk of miscarriage, gestational diabetes, pre-eclampsia, premature births, stillbirths and perinatal mortality. The babies of overweight mothers are more likely to be obese and suffer cardiovascular disease and diabetes as adults.
Obesity in men is associated with infertility by causing impaired semen quality, decreased libido and erectile dysfunction. A 6-month group programme for overweight infertile women, which provided dietary and exercise advice alongside support to make healthy changes, was shown to be effective. The women had an average weight loss of 10.2kg/m2 and most of the anovular women in the group resumed spontaneous ovulation.
It is also of note that women with a BMI of more than 32 must lose weight in order to become eligible for publicly funded fertility treatment
Low maternal weight before pregnancy and poor weight gain during pregnancy are known to result in an increased chance of pre-term delivery and low birth weight in infants. Low body weight is also an important cause of anovulation. Low body weight can often also reflect poor nutritional status which can ultimately affect the health of the pregnancy and baby.