Short answer: Both overweight and underweight can affect fertility, mainly by disrupting ovulation. Research shows that a BMI significantly above or below normal can prolong time to conception and impair IVF outcomes. The good news: even a small weight change (2–5%) can improve ovulation and the chance of getting pregnant.
Why does weight affect fertility?
Adipose tissue is hormonally active and affects the balance of sex hormones. Both too much and too little body fat can therefore disrupt ovulation and the menstrual cycle.
Overweight
With overweight and obesity, the risk of anovulation increases, and the connection to PCOS is common. In IVF, overweight and obesity have been linked in studies to lower rates of pregnancies and live births, as well as more miscarriages compared to normal weight.
Underweight
A low BMI (below approximately 18.5) can lead to irregular or absent periods, as too little body fat can suppress reproductive hormones (so-called hypothalamic amenorrhea). Underweight can therefore also prolong time to conception.
What can be done?
Even a moderate weight change can make a difference – studies show that a reduction of 2–5% of body weight can restore ovulation in overweight individuals. Focus on sustainable habits rather than quick fixes, and seek help from healthcare if needed.
Frequently asked questions
Do I need to reach a certain BMI to get pregnant?
No, but a BMI close to normal facilitates ovulation. Even small changes can help.
Does this only apply to women?
No – a man's weight can also affect sperm quality.
This article provides general information and does not constitute medical advice. Please consult healthcare for individual support.
Sources
- The impact of female obesity on the outcome of fertility treatment (review article). PMC2970793.