Weight and fertility: what the evidence supports, and what it doesn’t

This is a subject where accuracy matters more than usual, because the people reading it are often already anxious and have often been told something blunt in a short appointment.

Ovulation

The clearest effect of weight on female fertility runs through ovulation, and it is most pronounced in PMOS — the condition renamed in 2026 from polycystic ovary syndrome.

The most useful UK data come from the BAMBINI trial. In women with PMOS, obesity and absent ovulation, each 1% of body weight lost was associated with a 5.6% increase in the odds of ovulatory recovery, with no threshold below which nothing happened ^[1].

That is a more encouraging framing than “lose 5% first”. Recovery rates were highest in the 10–15% weight-loss band, but the relationship was continuous throughout.

At the other end, being significantly underweight also disrupts ovulation. NICE advises that women with a BMI under 19 and irregular or absent periods are likely to benefit from gaining weight ^[4].

IVF: the claim that is overstated

A systematic review of 21 cohort studies covering 682,532 cycles found ^[2]:

• Obesity (BMI ≥30) vs normal weight: live birth relative risk 0.85 — about a 15% relative reduction
• Overweight (BMI 25–29.9) vs normal weight: RR 0.94

The crucial subgroup finding: the effect was driven almost entirely by women with PCOS/PMOS. In that group, RR 0.78. In women without it, RR 0.92 — not statistically significant.

So “obesity halves your IVF chances” is wrong, and in women without PMOS the pooled effect did not reach significance at all. That does not mean weight is irrelevant to fertility treatment — anaesthetic risk, egg collection technical difficulty and pregnancy complications all matter — but the live-birth figure is not what it is often presented as.

The NHS funding question

This is where most of the real-world pressure comes from, and it is worth being precise.

NICE does not set a BMI cap for NHS-funded IVF. CG156 advises informing women with a BMI of 30 or above that they are likely to take longer to conceive and that weight loss increases the chance ^[4]. That is clinical advice, not a funding rule.

The caps are set by integrated care boards. The Department of Health and Social Care’s own comparison states that most ICBs require a BMI between about 19 and 30 for the person conceiving ^[5]. Some use 30, some 35, and some apply criteria to partners too.

If you are up against a threshold, find your own ICB’s fertility policy rather than relying on a national figure. The variation is real, and so is the deadline pressure it creates.

Male fertility

Less discussed and genuinely relevant — roughly half of fertility problems involve a male factor.

Obesity is associated with lower total testosterone, and with poorer sperm concentration, motility and DNA integrity. A meta-analysis of 12 studies in men with a mean BMI of about 45 found that weight loss produced ^[3]:

• Increased sperm concentration
• Increased progressive motility
• Reduced sperm DNA fragmentation

Those men had severe obesity, so the findings should not be stretched to a BMI of 28. But the direction is consistent, and weight loss reliably raises testosterone in men with obesity.

Other male factors worth addressing at the same time: smoking, alcohol, anabolic steroid use — which suppresses sperm production, sometimes for many months — and heat exposure.

What to do, practically

Do not crash diet while trying to conceive. Very low calorie approaches are not appropriate in pregnancy, and pregnancy can happen sooner than expected once ovulation returns.

Do take folic acid, 400 micrograms daily before conception and for the first 12 weeks; a higher dose is advised in some circumstances, including a higher BMI in some guidance — check with your GP.

Note the medicine timings. If you take a weight-loss medicine, semaglutide should be stopped at least two months and tirzepatide at least one month before trying to conceive. Tirzepatide also reduces the absorption of oral contraception, so if you are not trying to conceive, additional cover is advised for four weeks after starting and after each dose increase.

Do not wait for a perfect number. Age is usually the bigger variable, and the ovulation data show benefit from the first kilogram rather than at a threshold.

When to speak to a clinician

Seek advice if you:

• have been trying to conceive for 12 months, or 6 months if you are over 35
• have irregular or absent periods
• have PMOS, or symptoms suggesting it
• have a BMI under 19 or over 30 and are planning a pregnancy
• take a weight-loss medicine and are planning a pregnancy
• are a man with low libido, erectile difficulties or a history of anabolic steroid use alongside fertility concerns

Both partners should be assessed. Investigating only one is a common and avoidable delay.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.