Are Body Weight and Fertility Related?

If you have been trying for a baby without success, you may be wondering whether your weight is affecting your chances. Body weight can influence reproductive health, but it is only one factor. Age, ovulation, sperm quality, medical conditions and other circumstances also matter.

Being above or below a healthy weight can sometimes affect hormone levels and menstrual cycles. However, many people outside the standard BMI range conceive naturally, while people within that range can experience fertility problems. A number on the scales cannot explain an individual’s fertility on its own.

How to Tell If You Are Underweight or Overweight

Body mass index (BMI) compares your weight with your height. For most adults, the standard categories are underweight (below 18.5), healthy weight (18.5 to 24.9), overweight (25 to 29.9) and obesity (30 or above).

BMI is a useful starting point, but it is not a direct measure of body fat, nutritional health or fertility. It does not account fully for muscle mass, body composition or individual differences. Your GP can consider your BMI alongside your medical history, menstrual cycle and other relevant factors. You can also use the NHS adult BMI calculator.

Effects on Fertility When You Are Overweight

Having a higher body weight may affect fertility in some people, particularly when it is associated with irregular ovulation or conditions such as polycystic ovary syndrome (PCOS). Changes in insulin sensitivity and reproductive hormones can make ovulation less predictable, although not everyone with a higher BMI has these difficulties.

Higher BMI is also associated with an increased risk of some pregnancy complications, including gestational diabetes and pre-eclampsia. These are changes in risk, not inevitable outcomes. Individual health and pregnancy care are more informative than BMI alone.

For some people, supported changes in nutrition and activity can improve metabolic health and help restore regular ovulation. However, weight loss does not guarantee conception or a live birth, and fertility investigations should not automatically be postponed while someone tries to lose weight.

Effects on Fertility When You Are Underweight

Low body weight can also affect fertility. When the body does not have enough available energy, reproductive hormone signals may change, causing periods to become irregular or stop altogether. This can happen with inadequate food intake, excessive exercise or certain medical conditions.

NICE’s 2026 fertility guidance advises that people with a BMI below 18.5 who have irregular or absent periods may improve their chance of conception by increasing body weight. Where very high exercise levels contribute to absent ovulation, moderating activity may also help.

Not everyone who is underweight will experience fertility problems. There is no reliable rule that an underweight person will take a fixed number of extra months to conceive. If your periods have stopped, or you are losing weight without intending to, speak to your GP rather than attempting major dietary changes without advice.

BMI and Fertility Treatment

The relationship between BMI and access to fertility treatment is debated. BMI can be one factor in discussing the safety and likely outcomes of in vitro fertilisation (IVF), but it is not a complete assessment of someone’s health or fertility.

Eligibility for NHS-funded IVF in England depends on national guidance and local commissioning policies. Some services apply BMI criteria, so access may vary depending on where you live. If you have been told that your BMI affects eligibility, ask your GP or fertility clinic for the written policy, the reasons for the decision and the options available to you.

The NHS IVF guide explains how factors including age, BMI and the cause of fertility problems may influence treatment. NICE’s fertility guideline NG257, published in March 2026, provides further recommendations about assessment and treatment.

Keeping a Healthy Body Weight

Looking after your general health is worthwhile when planning a pregnancy, whatever your starting weight. A balanced diet, regular enjoyable activity, sufficient rest and support for any underlying medical conditions can all be part of preparation for conception.

Rather than focusing on a particular number on the scales, consider changes that are sustainable and appropriate for you:

• Eat regularly and well: Include a variety of vegetables, fruit, wholegrains, protein foods and sources of healthy fats. If you are underweight, seek advice on increasing your energy intake safely.
• Stay active: Moderate physical activity supports general health. If you have very low body weight, absent periods or unusually high exercise levels, ask a healthcare professional whether your routine needs adjusting.
• Avoid crash diets: Restrictive diets and rapid weight changes may be unhelpful, especially when nutritional intake is already inadequate.
• Seek individual advice: A GP or registered dietitian can help if you have PCOS, an eating disorder, an existing health condition or concerns about your weight.
• Prepare for pregnancy: Follow NHS guidance on folic acid, smoking, alcohol and any medicines you take when trying to conceive.

Sometimes a modest change in weight can improve ovulation in people whose cycles are affected by weight-related hormonal changes. That is not the same as proving that losing a particular percentage of body weight will lead to pregnancy. The best approach depends on the person, their health and the reason for any fertility difficulty.

It is also important not to overlook sperm health. Body weight and some associated health conditions may be linked with changes in semen quality, but male fertility cannot be judged by BMI alone. When conception is taking longer than expected, assessment of both partners may be appropriate.

When to Seek Fertility Advice

If you have been trying to conceive for a year without success, speak to your GP about a fertility assessment. Seek advice earlier if you are aged 36 or over, have irregular or absent periods, have a known condition that may affect fertility, or have another reason to be concerned.

A fertility assessment may include a discussion of menstrual cycles and medical history, checks for ovulation and semen analysis where appropriate. Weight may be considered, but it should not replace an assessment of other possible causes.

Conclusion

Body weight and fertility are related, but the relationship is more complicated than simply being above or below a BMI threshold. Weight can affect ovulation, reproductive health and some pregnancy risks, yet it does not determine whether someone will conceive.

Concentrate on your overall health and seek personalised advice if your periods are irregular, your weight is changing unexpectedly or pregnancy is taking longer than expected. Fertility difficulties deserve proper assessment, not assumptions based on weight alone.

Further Reading

• NICE: Fertility problems, assessment and treatment (NG257, 2026)
• NHS: Trying to get pregnant
• NHS: IVF

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