General advice

Body mass index or BMI is calculated by your weight to height ratio and is a practical estimation, used by health professionals, of weight range.

At your booking visit with the midwife, your height and weight will be recorded and your BMI will be calculated.

Approximately 1 in 3 women start pregnancy with a BMI greater than 25kg/m2 and are considered overweight. Most women who are overweight have a straightforward pregnancy and birth and deliver healthy babies. However research shows that women with a raised BMI of more than 30kg/m2 at the start of pregnancy are at slightly increased risk of complications during pregnancy and labour and the higher your BMI, the greater the risk of complications.

One of the aims of care during pregnancy is to identify those women who are at increased risk, in order to offer the best care throughout their pregnancy, to suit their individual needs and reduce the risks where possible.

BMI Classification

You have a normal healthy range if your BMI is between 18.5 and 24.9. Between 25.0 and 29.9 is classed as overweight and 30 and over is classed as obese. Before you are pregnant you can work out your BMI using this calculator (but once you're pregnant it may not be accurate.)

Pregnancy risks associated with a raised BMI

A raised BMI >30kg/m2 increases the risk of the following:

Risks during labour and giving birth associated with a raised BMI

It is worth remembering that most women have straightforward births. However, we know that women who have a high BMI have more chance of having:

  • monitoring difficulties 
  • restricted mobilisation
  • difficulty carrying out procedures
  • increased risk of bleeding 
  • difficulties with epidural/spinal analgesia
  • bigger babies
  • assisted delivery 

Because of these increased risks, it is recommended that if you have a BMI of 35kg/m2 or above, you have your baby in hospital on the delivery unit and are not suitable for the Rosie birth centre.

If your BMI is between 30 and 34.9 and you would like to use the Rosie birth centre, please discuss this with your community midwife.

Key messages

  • the majority of overweight women have a straightforward pregnancy and birth
  • enjoy a normal healthy diet and keep your weight gain to a minimum
  • keep active and take regular, moderate intensity exercise
  • take a high dose folic acid supplement of 5mg (milligrams) daily for the first 12 weeks of pregnancy to reduce the risk of neural tube defects
  • take 10 micrograms of Vitamin D throughout pregnancy and during breastfeeding
  • choose the most appropriate place to give birth
  • women with a high BMI are at a higher risk of complications during pregnancy and labour
  • following delivery mobilise as soon as possible especially if you have a caesarean section
  • breastfeeding your baby can aid weight loss and is recommended to prevent the risk of obesity for the baby is later life
  • reducing your BMI by 5 will reduce your chances of gestational diabetes in your next pregnancy by 80%

Being underweight and pregnant

If you have a low BMI speak to a midwife or GP about how to build yourself up gradually over the coming months. Follow their advice to eat a variety of healthy, nutritious foods, so your baby will get the calories and essential vitamins and nutrients he or she needs to develop well.

You may be more likely to have a premature or low-birth-weight baby if your BMI is low. Babies who are born early and small sometimes need extra care when they are born.