Body weight & fertility (high & low BMI)
Body weight is one of the few fertility factors we can, to some extent, influence. Both extremes — being overweight and being very underweight — can make conception more difficult.
The aim of this article is not to create guilt, but to explain, calmly, why weight matters and how much difference even a modest, realistic change can make.
How do we measure weight in relation to fertility?
The most common measure is the body mass index (BMI) — weight in kilograms divided by the square of height in metres. A healthy range is roughly 18.5–24.9. Below 18.5 we speak of low weight, while above 30 we speak of obesity.
BMI is a useful but not perfect tool; it does not distinguish muscle from fat. Even so, at the population level it gives a reliable indication of where someone stands in relation to fertility.
How does higher weight affect fertility?
Obesity disrupts hormonal balance and can lead to irregular or absent ovulation — a common cause of difficulty conceiving. Even in women who do ovulate, higher weight is associated with lower conception and live-birth rates after IVF.
In assisted reproduction, a high BMI often requires higher medication doses and is linked to a greater risk of cycle cancellation. The man is affected too: obesity is associated with poorer sperm quality.
What does higher weight mean for pregnancy?
Beyond conception, obesity increases obstetric risks: gestational diabetes, high blood pressure and pre-eclampsia, complications during delivery, and a higher rate of caesarean section. The risk of miscarriage also rises.
For this reason, addressing weight before pregnancy is not only about the chance of conceiving, but also about the safety of mother and baby during pregnancy.
How much weight loss is needed to make a difference?
The good news is that you do not have to reach an “ideal” weight. Even a modest loss of about 5–10% of body weight can restore ovulation in many women with anovulation and improve the chance of conceiving — naturally or with treatment.
The aim is a gradual, sustainable change through a balanced diet and regular, moderate exercise — not extreme diets that are not maintained and can be harmful.
And very low weight?
Very low weight matters just as much, although it is discussed less. When body fat falls too low — often because of restrictive eating, intense exercise or an eating disorder — the brain can “switch off” the signals that control the ovaries. The result is hypothalamic amenorrhoea: periods stop and ovulation ceases.
In these cases, restoring a healthy weight and an adequate energy intake is often enough for ovulation to return. The help of a dietitian, and psychological support where needed, can be invaluable.
What is the balanced approach?
The goal is not perfection or a number on the scale, but a healthy, stable weight within a reasonable range. For both extremes, changes should be gradual, realistic and supported, not a source of anxiety.
Weight is one of the many factors in fertility — important and modifiable, but not the only one. Improving it raises your chances; it does not guarantee them on its own, nor is it a reason for self-blame.
Summary
Both extremes of weight affect fertility. Obesity (BMI > 30) disrupts ovulation, lowers conception and birth rates, and increases obstetric risks; even a modest 5–10% weight loss helps substantially. Very low weight (BMI < 18.5) can stop ovulation through hypothalamic amenorrhoea, with restoring a healthy weight often being the answer. The goal is a healthy, stable weight through gradual, realistic changes — without guilt.
Sources: ASRM Practice Committee, “Obesity and reproduction: a committee opinion”; NICE, Fertility problems: assessment and treatment (CG156); ESHRE lifestyle recommendations.
⚕ Medically reviewed by Prof. Stratis Kolibianakis · Thessaloniki Medical Association Reg. No. 16340
ARTICLE 37 · LIFESTYLE & FERTILITY
