BMI is your Body Mass Index, which is a
ratio of height to weight. It is calculated as
follows: BMI = your weight in kg divided by
the square of your height in metres. For
instance if your weight is 70kg and your
height is 1.70m, the sum would be:
70÷(1.70x1.70) = 22.5. So you have a BMI
of 24.2.
If your BMI is under 20 you are considered
underweight; if it is between 20 and 25 you
are considered healthy; between 25 and 30
is overweight; between 30 and 40 is obese
and over 40 dangerously obese.
Your BMI plays a significant role in fertility
for both men and women.
For women, being over or underweight can
both cause fertility problems. You need to
be the correct weight in order to produce the
appropriate amount of hormones to regulate
ovulation and menstruation. Having a BMI
of under 20 is a problem for fertility and a
BMI of over 30 can affect your response to
IVF treatment and reduce the success rate
of a cycle.
It is not only having the right BMI that is
critical for fertility. The amount of body fat
you have is just as important. In healthy
adult women fat comprises about 28% of
body weight and if it drops below 22%
ovulation could stop.
Women with an
average or above average body weight, or
who exercise very rigorously, may have a
lower body fat and a higher muscle content,
which may lead to their periods becoming
irregular or stopping altogether. Sensible
advice for these women would be to reduce
their exercise until their body fat returns to
the normal range.
If your BMI is under 20 you may have
problems conceiving and the risk of
miscarriage is higher than normal. Your
body does not have enough fat stores to
sustain a pregnancy and so ovulation is
shut down. When you gain enough weight
your body senses that fat stores are more
plentiful and pregnancy is a viable option
and you become fertile again.
If your BMI is over 30 research has shown
that just losing even 10% of your body
weight can be all that’s needed to trigger
ovulation, make your periods more regular
and increase fertility. And this is particularly
important if you have polycystic ovary
syndrome (PCOS).
Being overweight can affect male fertility
too, reducing both the quality and quantity
of sperm. Having a BMI of over 25 for men
is associated with poor sperm motility.
Obese men also produce substantially less
sperm than average which contains higher
levels of abnormalities. Obviously when
both the man and woman are overweight
this can have a combined effect on
fertility. But the good news is that when
men lose weight it results in a significant
increase in not only the sperm count but
also the number of normal sperm.
If you need to reduce your BMI changing
your diet to one that is nutritious and
healthy and taking regular exercise is
often enough to kick-start weight loss.
Women & Children’s
Directorate
Your Body Mass Index
(BMI) Matters in Fertility
Treatment
Further information
Further information can be obtained from NHS Direct,
www.nhsdirect.nhs.uk
Author: Harriet Norman
Date: June 2016
Review Date: June 2019
Darent Valley Hospital
Darenth Wood Road
Dartford
Kent
DA2 8DA
Telephone: 01322 428100
www.dvh.nhs.uk