Underweight during pregnancy

What does being underweight in pregnancy mean?  

At your first antenatal visit, which is called the booking appointment, your midwife will measure your height and weight to work out your BMI.

BMI stands for Body Mass Index. It is used to work out if your weight is in a healthy range. 

BMI is not a perfect way to measure your weight because it can’t tell the difference between fat, muscle or bone. But it is still the best way of assessing your weight.  

You will be categorised as being underweight if your BMI was under 18.5 either before or at the start of your pregnancy.

We understand that having your BMI calculated may feel uncomfortable. Remember, though, that health professionals are not judging you. They just want to be sure you and your baby are being looked after in the best way.  

We recommended that you are weighed by your healthcare professional during your pregnancy. But ultimately, like all antenatal care, it is your choice.  

If your BMI is under 18.5, you might get extra support and advice on how to increase what you eat and vary your diet.

Find out your BMI.

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

Most women who have a low BMI in pregnancy will have a healthy pregnancy and baby are healthy, but there is an increased risk of:

• miscarriage
• premature labour and birth (when your baby is born before they are fully developed)
• the baby having a low birth weight
• fetal growth restriction
• problems with your baby’s development that may affect their heart, diaphragm or stomach.

This can be difficult to read but try to remember that these risks are still small and there are things you can do to reduce them and have a healthier pregnancy.

What will my midwife do to support me if I am underweight?

If your BMI was less than 18.5 before you became pregnant you will be supported to gain more weight during pregnancy.

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 talk to you about what your target weight gain in pregnancy should be. This will be based on a few factors, including your pre-pregnancy weight.  

They might ask to weigh you during your pregnancy.  

They may also recommend you take a multivitamin supplement that's suitable for pregnancy and includes folic acid, vitamin D and iron.  

Helping you with the reason for your low BMI

Your midwife may give you more specific advice and help based on why you have a low BMI. This is so they can properly support you, depending on what the cause is.

They will understand that there are many reasons for having a low BMI, including:

• being naturally thin but healthy  
• having a medical reason for your low weight, such as an overactive thyroid or a gastrointestinal problem, such as inflammatory bowel disease or coeliac disease. If you think this might be the case, talk to your midwife or GP.  
• not eating enough food because of an eating disorder. To be healthy, you should have around 2,000 calories a day through a healthy balanced diet. 
• loss of appetite, perhaps due to depression, worry or stress
• over-exercising and not eating enough food to replace the energy used in exercise
• having severe morning sickness (hyperemesis gravidarum)
• lacking of money for food.

Eating disorders

An eating disorder is when you have an unhealthy relationship with food. This can take over your life, and make you hysically and mentally ill. If you have an eating disorder, or you have had one in the past, putting on weight may be mentally difficult for you.

It may be hard to talk to your doctor or midwife about this, but it is important. They can support you with what you are going through. Talking to them means they can suggest the best help and care for you.

You should have a named professional, usually your GP or midwife, to help you during your pregnancy and after you give birth. They will:

• prepare you for the changes that will happen to your body
• check in with you often, to see 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.

Find out more about eating disorders in pregnancy.

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 common. But there is support available.

Talk to your GP or midwife if you are worried about your feelings. They can help you find the support you need.

Find out more about your mental health and wellbeing during pregnancy.

Exercise

If you are exercising a lot, or you have a busy lifestyle, think about having more rest days. It is still important to be active in pregnancy, but you do not need to do more than the recommended amount, which is 30 minutes, 5 times a week. Find out more about how active to be during pregnancy.

Remember that exercise does not have to be strenuous to help you manage your weight and stay healthy in pregnancy. There are lots of gentler things you could try, such as swimming, walking, yoga or Pilates.

Lack of money

Talk to your midwife or GP if you do not have enough money for food. They may be able to put you in touch with places that can help you.  

You can find a food bank through the Trussell Trust, or through your Citizen’s Advice Bureau. And you may be eligible for the Healthy Start scheme in England, Northern Ireland and Wales, or the Best Start Food scheme in Scotland. Both give you a card with money on to buy healthy food, milk and vitamins.

There are also apps like Too Good to Go, that connect you to shops that sell food very cheaply as it nears its sell by date. The apps may have some food that is not very healthy but keep a look out for vegetable and fruit boxes from supermarkets, and other healthy options.

What can I do to make sure my pregnancy weight gain is healthy?

Try to follow a healthy, balanced diet and aim to gain weight slowly during pregnancy.

Avoid unhealthy 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. Instead, aim for regular healthy meals and occasional snacks.

Speak to your GP and midwife If you are not sure how to make healthy changes to your eating habits. They may be able to arrange extra support for you.  

More support and information

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

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.