Understanding BMI and fertility

Your Body Mass Index (BMI) is a figure that indicates whether you're at a healthy weight for your height.

To work out your BMI, you divide your weight (in kilograms) by your height (in meters) squared. BMI is mostly used by healthcare professionals to gauge if your weight is in a healthy range, or if

it's below or above this.

For example, if you weigh 65kg and you’re 5 feet 4 inches (=1.63m) tall, your BMI is 24.9kg/m².

The BMI weight categories as determined by the World Health Organization (WHO) are:

• below 18.5kg/m²: underweight

• between 18.5 and 24.9kg/m²: healthy weight

• between 25.0 and 29.9kg/m²: overweight

• above 30.0kg/m²: obese

BMI is by no means a perfect measure of someone’s health. It only measures someone’s weight and

doesn’t tell us anything about the differences between bone, fat and muscle. For example, very muscly people will often have a high BMI, but that doesn’t mean they are unhealthy. 

So, what has a person’s BMI got to do with their fertility? Research shows that if your BMI is too low or too high, it can make it tricky to get pregnant.

Having a low BMI: If you have a low BMI (under 18.5kg/m²) it can make it harder to have a baby. Your body might not have enough fat to keep your hormones balanced, which for women can disrupt your periods and make it harder to get pregnant.  

You also need to make sure that you consume enough calories to nourish both yourself and your baby, once pregnancy is established.

If you think this might be you, it's a good idea to talk to a doctor about how to get to a healthier weight.

Having a higher BMI: On the other hand, if you're BMI is over 25-30kg/m², it can also affect fertility and make it harder to get pregnant. Carrying more weight also disrupts hormones and cause problems with periods, ovulation, and sperm quality.  

If you are overweight, you may be around 16% less likely to get pregnant than someone with a healthy BMI and if you are obese, this increases to around 28%.

Even just bringing down weight by 5-10% can make a big difference to fertility and chances of pregnancy.

Things you can do to help:

1. Talk to a doctor: If you're worried about your weight and how it might be affecting your chances of having a baby, talk to a doctor. Our fertility specialists can help you figure out a plan to get to a healthier weight and improve your chances of getting pregnant.

2. Eat well: Make sure you're eating lots of healthy foods like fruits, vegetables, lean meats, and whole grains. These foods help keep your body healthy and can make it easier to have a baby.

3. Get moving: Try to do some exercise every day, like walking, dancing, or playing sports. Aim for 150mins/week of which 90mins should be moderate-high intensity aerobic (60-90% max HR). Exercise can help you lose weight if required and helps your body stay strong.

4. Prioritise sleep: Sleep is often overlooked yet can have a significant impact on our bodies. Getting a bad night's sleep has now been found to make us make worse health decisions next day such as poorer food choices, increased hunger and calorie intake, and less physical activity. All of these can contribute to weight management and hormone issues. Aim for 8-9 hours of sleep a night and try to go to bed and wake up at roughly the right times.

5. Take care of yourself:

   Trying to have a baby can be stressful, so make sure you're taking care of yourself emotionally too. Cortisol (the stress hormone) and stress related behaviours are the two main factors that make maintaining a healthy weight when stressed harder. Talk to friends or family if you're feeling stressed or consider talking to a counsellor who can help you cope with any worries you might have.

Trying to have a baby can be a long and tricky journey, but understanding how your weight affects your chances can help. By eating well, staying active, seeking appropriate medical advice, and taking care of yourself emotionally, you can improve your chances of having a baby and make the journey a little easier. So don’t lose hope – with a little help from Care Fertility, you can make your dream of having a baby come true!