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Body weight and fertility

Does my weight affect my fertility and chances of having a baby?

Your weight, whether too high or too low, can affect your chance of getting pregnant, which is why being a healthy weight is important. Being overweight or underweight can also reduce the efficacy of fertility treatments and can increase the risk of pregnancy complications.

This information is designed to help you have the best chance of getting pregnant and having a healthy baby. Your healthcare professionals will not judge you for being overweight or underweight and will support you in optimising your weight.

What is BMI?

Your body mass index (BMI) is used to calculate whether your weight is healthy. It is calculated by dividing your weight in kg by height in meters squared. A healthy BMI is 19 to

<25. For example, a BMI for 20 means 20kg/m2.

If your BMI is less than 19, it can be harder for you to get pregnant.

If you have a BMI of 30 or more (classified as obese), it can take longer to get pregnant and there is also a higher risk of pregnancy complications in both mum and baby. The best way of protecting your health and your baby’s health is to lose weight before getting pregnant.

How does weight affect my fertility?

For pregnancy to occur, the ovaries must release an egg for a sperm to fertilise.

Some women who are underweight or obese will have problems getting pregnant and often this is due to the ovaries not releasing an egg (ovulation issues).

Body mass index in the obese range can lead to irregular periods (menstrual cycles) and lack of ovulation. However, even obese women with regular periods have lower pregnancy rates compared to women with a healthy BMI.

A visit to a healthcare professional before becoming pregnant can help identify other health problems related to obesity that can impact pregnancy such as polycystic ovary syndrome (PCOS), thyroid disease and diabetes.

What if I am underweight?

A BMI of <19 (underweight) often causes irregular menstrual cycles and may cause ovulation to stop altogether. This is especially true if you are losing weight because you are not eating enough or because you are exercising too much. Women with a low BMI should work with

their doctor and other healthcare professionals to understand the cause of their low BMI and find ways of optimising it.


Does obesity affect my chances of getting pregnant with treatment?

There is good evidence that obesity lowers the chance of success in vitro fertilization (IVF). There are many reasons for this. Being overweight can have a negative affect on the way your body responds to the fertility drugs. A smaller number of eggs can be produced, compared to women with a healthy BMI.

Obesity can also make procedures that are part of the fertility treatment, such as the egg collection, more challenging and less efficient, with fewer eggs of lower quality being collected. Lastly, it can reduce the chance of the embryo (the fertilized egg) being implanted in the womb and increase the chance of an early pregnancy loss.

Does being overweight increase my chances of having a miscarriage?

Obese women who achieve pregnancy, are more likely to experience pregnancy loss. Higher miscarriage rates are seen following natural conception, ovulation induction and assisted conception. The overall risk, for all women, of a miscarriage under 12 weeks is 1 in 5 (20%), but if you have a BMI over 30, your risk increases to 1 in 4 (25%). If you are underweight (BMI less than 19) it can also increase your chance of having a miscarriage.

Could reaching a healthy weight help me get pregnant?

Yes. If you have a BMI of 30 or above, whether you are planning your first pregnancy or are between pregnancies, it is advisable to lose weight. If you lose weight, you:

  • increase your chance of getting pregnant
  • reduce the additional risks to you and your baby during pregnancy
  • reduce your risk of developing diabetes in further pregnancies and in later life.

You must have a BMI between 19 and < 25 for at least 6 months to qualify for NHS funded IVF treatment.

If overweight, your healthcare professional should offer you a structured weight loss programme or you can contact Live Well Stay Well (see useful contacts below) for guidance and additional services available. You should aim to lose weight gradually (up to 1 kg or 2 lbs a week) as crash dieting is not good for your health. Remember, even a small loss in weight can have significant benefits to your health and fertility.

You may be offered a referral to a dietician or an appropriately trained health professional. If you are not yet ready to lose weight, you should be given contact details for support for when you are.

If you have a BMI of 30 or more, you should take 5 mg of folic acid daily for at least a month before you conceive (ideally taken for 3 months before becoming pregnant). Continue taking this until you reach your 13th week of pregnancy.

If you are underweight (BMI less than 19) we will also advise you to gain weight gradually and make recommendations on how to do it safely.

Further information

‘Why your weight matters during pregnancy and after birth’, information leaflet provided by the Royal College of Obstetricians and Gynecologists (RCOG)

Food Fact Sheets on other topics including Weight loss, Glycemic index, Pregnancy, Healthy Eating and Food and Mood are available at

Live Well Stay Well   01628 857311

Useful Contact Numbers

Stoke Mandeville Hospital, Consultant Obstetricians: 01296 316239/6548
Wycombe Hospital, Consultant Obstetricians: 01494 425009/425724