Being overweight or underweight can reduce fertility, so it is important to keep your body weight within the normal healthy range. Body Mass Index (BMI) is an indication of your body weight and can be calculated by dividing weight by height. You should aim for a BMI of between 20 and 25, as this will optimise your chances of conception.
A BMI below 19 in a woman in calorie deficit state the body senses famine and ovulation is switched off to prevent the risk of having a baby with malnutrition. Excessive exercise can reduce body fat and increase muscle mass to a point where periods cease for the same reason. Risk of miscarriage is also increased in low BMI women.
Having a BMI greater than 30 can reduce fertility by 50%. Pregnancy for women with a 30 plus BMI is often associated with problems such as maternal diabetes, high blood pressure, big babies and increased risk of caesarean section.
The good news is that as soon as you get back on track with your body weight, ovulation and fertility quickly return to normal. Even a minimal weight loss (less than 5% of body weight) can make a difference, therefore advice with weight and its management is an important part of fertility treatment. For men, being slightly overweight may also reduce sperm count, so men should also make an effort to keep their BMI within the normal range.