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.

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, having a big baby or having a stillbirth.

Healthy eating and exercise can benefit you and your baby.
If your BMI is 30 or above, you are advised to take a higher dose of folic acid (5 mg per day).

Risks to you: 

-Thrombosis is a blood clot in your legs (venous thrombosis) or in your lungs (pulmonary embolism), which can be life-threatening. Pregnancy itself increases your risk of developing thrombosis. If you are overweight, the risk of developing thrombosis is further increased. Your risk for thrombosis will be assessed at your first antenatal appointment and will be monitored during your pregnancy. You may be offered injections of a medication called low- molecular-weight heparin to reduce your risk of thrombosis. This is safe to take during pregnancy. 

– Diabetes that is first diagnosed in pregnancy is known as gestational diabetes. If your BMI is 30 or above, you are three times more likely to develop gestational diabetes compared with women with a BMI under 25. You will be offered a test for gestational diabetes between 24 and 28 weeks. If the test shows that you have gestational diabetes, your doctor will closely monitor you throughout your pregnancy. 

– Being overweight increases your risk of developing high blood pressure and preeclampsia If you have a BMI of 30 or above, your risk of pre-eclampsia is 2–4 times higher compared with those with a BMI under 25. Your blood pressure and urine will be monitored at each of your appointments. Your risk of pre- eclampsia may be further increased if:

• you are over 40 years old
• you have had pre-eclampsia in a previous pregnancy
• your blood pressure was already high before pregnancy 

If you have these or other risk factors, your doctor  may recommend a low dose of aspirin to reduce the risk of you developing pre-eclampsia.

– All pregnant women are asked some questions about their mental health at their first antenatal (booking) appointment. Being overweight slightly increases your risk of developing mental health problems in pregnancy and after birth. 

Risks to your baby: 

-The overall likelihood of a miscarriage in early pregnancy is 1 in 5 (20%), but if you have a BMI of 30 or above, your risk increases to 1 in 4 (25%).

-If you are overweight before pregnancy or in early pregnancy, this can affect the way your baby develops in the uterus (womb). Overall, around 1 in 1000 babies in the UK are born with neural tube defects (problems with the development of the baby’s skull and spine), but if your BMI is 30 or above, this risk is nearly doubled (2 in 1000).

-If you are overweight, you are more likely to have a baby weighing more than 4 kg, which increases the risk of complications for you and your baby during birth. If your BMI is 30 or above, your risk is doubled from 7 in 100 to 14 in 100 compared with women with a BMI of between 20 and 30.

-The overall likelihood of stillbirth in the UK is 1 in every 200 births. If you have a BMI of 30 or above, this risk increases to 1 in every 100 births.

-All women in the UK are offered an ultrasound scan at around 20 weeks to look for

structural problems that your baby may have. This scan is less accurate at picking up problems if your BMI is raised. 

You can reduce your risk: 

– A healthy diet will benefit both you and your baby during pregnancy and after birth. You may be referred to a dietician for specialist advice about healthy eating.

Trying to lose weight by dieting during pregnancy 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. 

– Physical activity will benefit both you and your baby. If you have not previously exercised routinely, you should begin with about 15 minutes of continuous exercise, three times per week, increasing gradually to 30 minutes sessions every day. Some examples of healthy exercise include swimming, walking and pregnancy yoga. 

– Folic acid helps to reduce the risk of your baby having a neural tube defect. If your BMI is 30 or above, a daily dose of 5 mg of folic acid is recommended. This is higher than the usual pregnancy dose and is only available on prescription. Ideally, you should start taking this a month before you conceive and continue to take it until you reach your 13th week of pregnancy. However, if you have not started taking it early, there is still a benefit from taking it when you find out that you are pregnant. 

Hope you found this information helpful.