Underweight and fertility when planning a pregnancy

If you are underweight it may affect your fertility and cause health problems during pregnancy. Depending on the reasons for having a low BMI, it may help you to put on weight slowly with a healthy diet.

If you are overweight/have a high BMI, read more here.

How do I know if I am underweight?

Your BMI (Body Mass Index) is a measure that uses your height and weight to work out if your weight is in a healthy range. 

For most women, having a BMI of under 18.5 shows that are underweight. You can calculate your BMI here (link to calculate your BMI).

There are many reasons for having a low BMI.

• Not eating enough food because of an eating disorder (see below). To live healthily 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 GP.
• Loss of appetite, perhaps from worry or stress. If you have ongoing worry or stress talk to your GP about it as you may have a mental health condition.
• 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 which community services are there that might to help you.

Eating disorders

If you are not eating enough food because you feel anxious or worried when you think about food and how you look you may have an eating disorder. It can be very hard to realise you may need help because eating disorders are linked to your mental health. But it is important to look for help now, before you get pregnant, so that you start pregnancy by having enough food to keep you and the growing baby healthy. If you do get the support you can get better.

Talk to your GP. You can take a partner, family member or friend with you.

You will probably be referred to an eating disorder specialist or team of specialists. Your treatment will depend on your symptoms but will usually involve talking therapy (counselling).

If you have an eating disorder and are feeling anxious about your body changing during pregnancy, you may also find it helpful to find out how your condition may be managed during pregnancy. 

Can a low BMI affect my fertility?

Yes. Having a BMI below 18.5 is classed as underweight, which can make it harder to get pregnant.

Having a low BMI can also cause your periods to become irregular or stop. This can be a sign that you are not ovulating (releasing an egg from your ovary each month), which is needed to get pregnant. If you are not having periods, putting on weight to get to a healthy BMI may help this. Read more about your fertility and ovulation disorders here.

Can being underweight affect pregnancy?

Most women who have a low BMI in pregnancy have healthy babies, but you will be at higher risk of the following:

• miscarriage
• preterm 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). 

Find out more about being underweight during your pregnancy.

How to put on weight

If lack of food is the reason for your low weight, try changing to a healthy, balanced diet and aim to gain weight slowly. Try not to use sugary or high-fat foods such as chocolate, cakes and sugary drinks. Instead aim for regular meals with healthy snacks that are high in unsaturated fats, such as nuts.

There is more information here about a healthy diet that helps conception and pregnancy.

Your GP can also give you help and advice if you think you are underweight.

More support

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. Helpline: 0845 634 1414

National Centre for Eating Disorders provides information on eating disorders and available treatments, including counselling. Helpline: 0845 838 2040

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 that may be able to help you to work out what social services are in your area that can help you. Helpline: 0808 800 2222

Are you ready to conceive? Use our tool to find out.