Being at a healthy weight increases your chances of conceiving naturally and having a healthy pregnancy and baby. The BMI (body mass index) calculation is a simple way to find out whether you are a healthy weight for your height. A BMI of 18.5–24.9 is considered healthy. A BMI of 25 or above is associated with risks for you and your baby. The higher your BMI, the greater the risks are.
Most people who are overweight have a straightforward pregnancy and birth and have healthy babies. Some of the risks with raised BMI include increased risk of thrombosis, gestational diabetes, high blood pressure, pre-eclampsia, induction of labour, caesarean birth, anaesthetic complications, and wound infections. A raised BMI also increases your risk of having a miscarriage, giving birth early (before 37 weeks), having a big baby or having a stillbirth. Being underweight can also increase your risk of miscarriage, low birth weight and giving birth early (before 37 weeks). The Royal College of Obstetricians and Gynaecologists (RCOG) information leaflet has all the current information and numbers relating to these risks: https://www.rcog.org.uk/en/patients/patient-leaflets/being-overweight-pregnancy-after-birth/
If your BMI is 30 or above, you are advised to take a higher dose of folic acid (5 mg per day).
Healthy eating and exercise can benefit you and your baby. Your GP can talk to you about healthy lifestyle when planning for pregnancy. If you are pregnant, trying to lose weight by dieting is not recommended. However, by making healthy changes to your diet, you may not gain any weight during pregnancy, and you may even lose a small amount. This is not harmful.