Underweight and fertility when planning a pregnancy

If you have a low BMI, there are things you can do to improve your chances of getting pregnant and reduce the risk of pregnancy complications.

It’s important to try and maintain a healthy weight before you get pregnant. 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 gradually by following a healthy diet. There is help and support available, if you need it.  

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 and birthing people of any ethnicity, having a BMI of under 18.5 shows that are underweight. Calculate your BMI here.

There are many reasons for having a low BMI:

  • Some people are naturally thin and gaining weight may be difficult.  
  • Not eating enough food because of an eating disorder – to live healthily you should have around 2,000 calories a day through a healthy, balanced diet. Read more about eating disorders below.
  • Over-exercising and not having enough food to replace the energy used up.
  • Being unwell.  There might be a medical reason for your low weight, such as an overactive thyroid or type 2 diabetes. Speak to your GP if you are losing weight without changing your diet or exercise.
  • Loss of appetite due to worry or stress. It’s important to look after your emotional health as well as your physical health. Speak to your GP if you are concerned that your mood has changed.  
  • Lack of money for food. There are local charities and community services that may be able to help you if you don’t have enough money for food. You can find out more from your GP, local council or Citizens Advice. You can also find details of local food banks through the Trussell Trust.

Eating disorders 

An eating disorder is a mental health condition where people have an unhealthy relationship with food. People with eating disorders develop unhealthy eating behaviours, such as eating too much or too little. You may also worry about your weight or body shape.

It can be very hard to realise you need some help. But it is very important to talk to your GP as soon as you can. You can take a partner, family member or friend with you for support, if it helps.  

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 are not ready to speak to someone in person, you can talk in confidence to an adviser from eating disorders charity Beat.

With treatment, most people can recover from an eating disorder.

If you have an eating disorder and are feeling anxious about your body changing during pregnancy, you may also find it helpful to read more about eating disorders during pregnancy.  

Can a low BMI affect my fertility?

Having a low BMI can also cause your periods to become irregular or stop. This  can make it harder to get pregnant.

If you are not having periods, putting on weight to get to a healthy BMI may help regulate your menstrual cycle.

There may be other reasons you are having irregular periods.  

Can being underweight affect my pregnancy?

Having a low BMI can increase the risk of complications in pregnancy, including:

This can be difficult to read. If you are still underweight when you get pregnant, your midwife, doctor or specialist will provide extra care. This will help you and your baby stay as healthy and well as possible.  

Find out more about being underweight during your pregnancy.

How to put on weight safely

The best way to put weight on safely is to get help from a qualified healthcare professional. Your GP can check for any health conditions that may have caused weight loss and give you advice about making changes to your diet. They may also refer you to a dietitian.  

Try to gain weight gradually by eating a balanced diet, choosing from a variety of food groups, such as fruit and vegetables, starchy carbohydrates and dairy. Add protein to your meals with beans, pulses, fish, eggs and lean meat.

Try not to use sugary or high fat foods such as chocolate, cakes and sugary drinks to gain weight.

Try to build muscle with strength training or yoga. Exercise may also improve your appetite. Find out more about the benefits of exercise before getting pregnant.

More support

Beat can provide information and support for eating disorders.

Mind is a mental health charity providing information about eating disorders, including accessing treatment and support.  

Talk ED is a national charity supporting anyone affected by any eating disorder or eating distress.  

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 offer early intervention and crisis support to struggling families.  

NICE Guidelines (2010) Weight management before, during and after pregnancy. National Institute for Health and Care Excellence. https://www.nice.org.uk/guidance/ph27

NHS. BMI healthy weight calculator. https://www.nhs.uk/live-well/healthy-weight/bmi-calculator/ (Page last reviewed: 29 March 2023 Next review due: 21 April 2020 Next review due: 29 March 2026)

NHS. Unintentional weight loss. https://www.nhs.uk/conditions/unintentional-weight-loss/ (Page last reviewed: 16 February 2022 Next review due: 16 February 2025)

NHS. Eating disorders. https://www.nhs.uk/mental-health/feelings-symptoms-behaviours/behaviours/eating-disorders/overview/ (Page last reviewed: 12 February 2021 Next review due: 12 February 2024)

NHS. Irregular periods. https://www.nhs.uk/conditions/irregular-periods/ (Page last reviewed: 26 July 2022 Next review due: 26 July 2025)

Burnie, R, Golob, E, Clarke, S. Pregnancy in underweight women: implications, management and outcomes. The Obstetrician & Gynaecologist 2022; 24: 50– 57. https://doi.org/10.1111/tog.12792

NHS. Healthy ways to gain weight. https://www.nhs.uk/live-well/healthy-weight/managing-your-weight/healthy-ways-to-gain-weight/ (Page last reviewed: 28 March 2023 Next review due: 28 March 2026)

Review dates
Reviewed: 29 August 2023
Next review: 29 August 2026