Weight and Fertility: Focus on Health More Than the Scale

When weight comes up while planning a baby, many women can feel judged. This article is not here to judge you, but to empower you with information. Body weight is only one of many factors that affect fertility, and it is possible to take healthy steps in every body. Let's look together at what the guidelines say.

Why does weight matter? The UK's National Health Service (NHS) lists body weight among the factors that can affect fertility. The most important reason is ovulation: according to the NHS, both excess weight and being very underweight can affect ovulation in women. Body mass index (BMI) is a starting point, but it does not define your health on its own. Your doctor will look at whether you have regular periods, your overall health and your age together.

What happens with low weight? The American Society for Reproductive Medicine (ASRM) notes that a BMI of 18.5 or lower often leads to irregular periods and can stop ovulation completely. The UK's NICE fertility guideline, updated in 2026, says that for women with a BMI under 18.5 whose periods are irregular or absent, gaining weight will probably increase the chance of getting pregnant. Being thin may not be your choice, so talking about this with your doctor and, if needed, a dietitian is the best approach.

What happens with high weight? According to NICE, women with a BMI of 30 or higher may take longer to get pregnant. If ovulation is not regular, losing weight probably increases the chance of getting pregnant. ASRM reminds us that many people with low or high weight have no trouble getting pregnant at all. When there is difficulty, the most common cause is ovulation problems. But according to ASRM, pregnancy rates may be lower in those with a BMI in the obese range even when they ovulate regularly, so ovulation is not the only factor. That is why a general health checkup also matters.

With PCOS, small changes can make a big difference: According to the Office on Women's Health (OWH) of the US Department of Health and Human Services, polycystic ovary syndrome (PCOS) is one of the most common, but treatable, causes of infertility in women. OWH notes that with PCOS, losing weight can help lower blood sugar, improve the way the body uses insulin and help hormones return to normal levels. According to the same source, losing even 10% of body weight can make periods more regular and increase the chance of pregnancy. For example, for someone who weighs 80 kg (about 176 lb), that is about 8 kg (about 18 lb). Healthy eating and regular movement also help ease PCOS symptoms.

The male partner is part of this journey too: Weight is not only a woman's concern. The NHS lists a BMI of 30 or higher among factors that reduce fertility, for women and men alike. NICE also says that men with a BMI of 30 or higher have a higher risk of reduced fertility. According to ASRM, a low sperm count and reduced sperm movement may be more common in men with excess weight. You can find other aspects of male fertility in our Male Partner Fertility article.

Sustainable steps, not crash diets: NICE notes that group programs combining exercise and nutrition advice lead to more pregnancies than weight loss advice alone. So the goal is not a quick number, but habits you can keep up for life. Instead of starving yourself, you might consider:

• Making a plan that suits you with your doctor or a dietitian
• Adding movement you enjoy, such as walking, swimming or dancing, to your routine together with your partner
• Filling your plate with vegetables, legumes and whole grains (details in our Fertility-Boosting Nutrition article)
• Making room for sleep and stress management too

Tell your doctor about medicines and treatments: NICE recommends that people trying to get pregnant be asked about the medicines they take and that possible effects be discussed, including weight loss medicines such as GLP-1 agonists. If you are using a weight loss injection or thinking of starting one, be sure to share your pregnancy plans with your doctor. ASRM also notes that for those with a BMI over 40, the most effective weight loss method is bariatric surgery. These decisions are personal and should be made together with your doctor.

When should you see a doctor?

• Your periods are irregular or do not come at all
• You have not gotten pregnant after a year of trying (the NHS recommends going sooner if you are 36 or older or have a known fertility problem)
• You are struggling with weight changes, eating habits or body image
• You are considering weight loss medicine or surgery, to discuss timing before pregnancy

Dear mom-to-be, your body is so much more than a number. Small changes that feel good to you support both your fertility and your future pregnancy. Treat yourself with patience and love; you are not alone on this path.