Having a healthy weight when you are planning to have a baby can boost your fertility and protect you and your baby from complications during pregnancy.
Nine percent of women across the world have a BMI of less than 18(1). These women are considered to be underweight and may have difficulty getting pregnant. Low energy intake and excessively high energy expenditure may suppress ovulation, causing periods to stop or infrequent periods. The reason for this is unclear, however, there are three possible ways that low body weight can affect a woman’s ability to have a baby. The first is the hormone responsible for fertility (oestrogen) is dependent on and directly related to having a minimum about of body fat(3). The second theory is that fat cells are not a useless mass but produces its own hormones, called adipokines(4), that helps to regulate menstruation(5). Finally, there is evidence that reproductive function is dependent on the availability of reserve energy. Low energy reserves can lead to low levels of the hormones that control the menstrual cycles, such as gonadotropin-releasing hormone(6) and luteinising hormones (7).
Twenty percent of women worldwide have a BMI over 25 (1). These women have excess weight and may also have difficulty conceiving. These women are more likely to have conditions that cause irregular or no menstruation such as polycystic ovarian syndrome (8), insulin resistance, and excess male hormone production (androgens). Excess amounts of fat cells can increase the level of inflammation in the body which can increase the risk of circulatory problems, heart complications and clotting. Overweight or obese women are at greater risk of problems during pregnancy, including diabetes, high blood pressure, and complications during pregnancy and delivery. The risks to their babies include pre-term deliveries and birth defects (9).
For these reasons, the British Fertility Society recommend that women should achieve and maintain a healthy weight between (BMI between 18 and 25 kg/m2) before planning to become pregnant or undergoing assisted fertility treatments. They report that women who are underweight or overweight have lower success rates for fertility treatments and are at greater risk of complications for mother and baby (10).
If you are planning to have a baby and you are either underweight or overweight, the good news is that even a 10% shift in your body weight can significantly improve your chance of a successful pregnancy and of having a healthy baby.