A woman’s (and men's) weight before conception - either too much or too little - can be an important factor in determining a couple's chances of pregnancy success.
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The ideal BMI (body mass index) for optimal fertility is considered to be between 20-25. Having a BMI that is too low or too high can affect fertility by causing hormonal imbalances and problems with ovulation.
BMI is a screening tool that is used to evaluate your body size. It is not a percentage of body fat, but rather a generalised score that helps determine if your weight is healthy. BMI is calculated using your weight and height and does not take into account other factors such as age, gender or muscle mass.
BMI is still a useful guide, especially if you also take your waist circumference into consideration. A woman's waist circumference should be less than 80 cm and a man's waist should be less than 90 cm.
Body Mass Index
30 and over
Women who are overweight or obese have less chance of getting pregnant overall. They are also more likely than women of healthy weight to take more than a year to get pregnant.
Being overweight can cause hormonal imbalance, often leading to oestrogen dominance and problems with insulin or Type 2 Diabetes.
For women who have been diagnosed with poly cystic ovarian syndrome (PCOS) and are overweight, a reduction in weight of as little as 5% can greatly enhance fertility and pregnancy outcome.
If a mother is obese, it increases the risk of pregnancy complications and health problems for the baby. Complications associated with obesity in pregnancy include miscarriage, hypertension, pre-eclampsia , gestational diabetes, infection, blood clotting, need for induction of labour, Caesarean birth and stillbirth.
A low BMI (body mass index) may lead to fertility problems and studies have shown that up to 12% of infertility cases are due to being underweight or having a low BMI.
Not every woman having a low BMI will experience fertility problems. A low BMI becomes a problem only when it starts to affect the menstrual cycle and specifically ovulation.
Some women are naturally thin. If she follows a healthy, nutritious diet, a moderate exercise program and has an otherwise healthy lifestyle there is usually nothing to be concerned about. However, if a woman is thin because of poor eating and lifestyle choices or excessive exercise, her fertility and pregnancy success can be adversely affected.
Studies show that overweight and obese men have lowers perm counts and fewer sperm that can swim well than men of healthy weight. Being overweight or obese can also cause hormonal changes, such as higher than normal levels of oestrogen which can make a man less fertile and less interested in sex. Men who are very overweight are also more likely to have problems getting an erection..
Being underweight can reduce a man’s sperm quality and his fertility.
Assisted conception is much more likely to be successful if a woman’s BMI is healthy range. Not only are conception rates higher in women with healthy BMI, but miscarriage rates and the possibility of complications such as diabetes, pre-eclampsia and miscarriage is much higher in overweight women.