Underweight during pregnancy

What does being underweight in pregnancy mean?

Your BMI is a measure that uses your height and weight to work out if your weight is in a healthy range. For pregnant women, your BMI calculation will be based on your weight before pregnancy.

Find out your BMI

Once you work out your BMI, this is the scale:

• Less than 18.5 = underweight
• 18.5 to 24.9 = healthy weight
• 25 to 29.9 = overweight
• 30 to 39.9 = obese
• 40 = severely obese

At your first antenatal visit, called the booking appointment, your midwife may measure your height and weigh you to work out your BMI.

How can being underweight in pregnancy affect me and my baby?

Most women who have a low BMI in pregnancy are fine, but there is an increased risk of:

• miscarriage
• premature birth  (when the baby is born before they are fully developed)
• the baby having a low birth weight
• gastroschisis (when the baby’s stomach doesn’t develop properly). 

This can be difficult to read but you will have support and there are things you can do to have a healthier pregnancy.

What will my midwife do if I am underweight?

If your BMI was less than 18.5 before you became pregnant, you may be advised to gain more weight than someone who is in the normal range.

Your midwife may refer you to the hospital antenatal clinic for extra scans to check on your baby's growth and development. You may also be referred to a dietitian to help you work on ways to eat well and gain a healthy amount of weight.

Your midwife or specialist will discuss with you what your target weight gain in pregnancy should be. This will be based on several factors, including your pre-pregnancy weight.

They might also ask to weigh you regularly during your pregnancy to check on your progress.

You may be given more specific advice based on why you have a low BMI.

There are many reasons for having a low BMI, including:

• not eating enough food because of an eating disorder (see below). To be healthy, women should have around 2,000 calories a day through a healthy, balanced diet
• over-exercising and not having enough food to replace the energy used in exercise
• being unwell.  There might be a medical reason for your low weight, such as an overactive thyroid. If you think this might be the case, talk to your midwife or GP
• loss of appetite perhaps due to worry or stress
• lack of money for food. There are local charities and community services that may be able to help you if you find that you do not have enough money for food every month. Ask at your GP or citizen’s advice bureau about what’s available locally. You may also be eligible for healthy start vouchers.

Eating disorders

An eating disorder is when you have an unhealthy relationship with food. This can take over your life and make you ill. If you have an eating disorder (or have had one in the past) putting on weight may be difficult for you. It can be hard, but it’s important to tell your doctor or midwife if this is the case.

You should have a named professional (usually your GP or midwife) to support you during your pregnancy and after you give birth who will:

• prepare you for the changes that will happen to your body
• regularly check how you are getting on
• offer you extra check-ups during your pregnancy
• talk to you about getting the right nutrition (having a good diet) to stay healthy for you and your baby.

Your mental health

Sometimes, depression or any other mental health condition can affect your weight. Up to 1 in 5 women develop mental health problems during pregnancy or in the first year after childbirth. Low mood, anxiety  and depression  are not unusual. If you have been feeling low for more than two weeks tell your doctor or midwife.

Your midwife may recommend a multivitamin supplement that's suitable for pregnancy, including folic acid, vitamin D and iron, and eating healthy extra calories to put on weight. 

What can I do for a healthy pregnancy weight gain?

If your diet is causing your low weight, try changing to a healthy, balanced diet  and aim to gain weight slowly.

Try to avoid high-calorie food and drinks full of saturated fat and sugar, such as cakes, sweets and sugary drinks. These foods have very little nutrition, which your baby needs to grow and develop well. If your diet has a lot of saturated fat and sugar during pregnancy, your children are more likely to suffer from high blood pressure and weight gain themselves later on in life.

When thinking about what to eat to gain weight, choose foods that are high in healthy fats, such as fish, chicken, nuts, avocados and unsaturated fats such as oils and spreads made from sunflower or olive oil.

Eat more pasta, beans and wholegrain cereals. Aim to eat three small meals and three healthy snacks every day.

Looking for more tips on what to eat in pregnancy? Find out more about balanced pregnancy diets and the best foods to eat when pregnant.

Exercise

If you are exercising a lot or you have a very busy schedule, try to cut down a bit. It's still important to be active in pregnancy, but you don't need to do more than the recommended amount. This is 30 minutes a day at least four days a week.

Remember that exercise doesn’t have to be strenuous to help you manage your weight and stay healthy in pregnancy. There are lots of gentler activities you could try, such as swimming, walking, yoga or pilates. 

More support and information

NHS Choices provides information on eating disorders.

Beat is a charity for people with eating disorders that provides information, support, local groups and an online chatroom.

National Centre for Eating Disorders provides information on eating disorders and available treatments, including counselling. 

Home-Start is a family support charity in the UK. Volunteers help families with young children deal with the challenges they face.

Family Lives is a charity may be able to help you to work out what social services are in your area that can help you.

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