Pregnancy and body mass index (BMI)

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Your body mass index (BMI) is your height to weight ratio, and is a useful way to determine whether a person is underweight, overweight or obese.

Your BMI is recorded on your pregnancy notes and is a useful measurement for pregnancy.

  • An underweight person has a BMI less than 18.5
  • A person of a healthy weight has a BMI 18.5 to 24.9
  • A mildly overweight person has a BMI 25 to 30
  • A moderately overweight person has a BMI 30 to 35
  • A seriously overweight person has a BMI over 35

Raised BMI and pregnancy

Research is showing that women who have a high BMI at the start of their pregnancy are at a higher risk of complications during their pregnancy and labour.


The risk of developing diabetes in pregnancy is related to your body size and your BMI. The higher the BMI, the greater the risk of developing diabetes. Women who weigh over 100kg (or have a BMI over 30) will be offered a special test for diabetes (oral glucose tolerance test) when they are 26 weeks pregnant.

High blood pressure

Women who have a high BMI are at a greater risk of developing high blood pressure. Your doctor will be assessing your health at each antenatal visit and will check your blood pressure regularly. Developing high blood pressure in pregnancy can lead to a more serious condition called pre-eclampsia.

Urinary tract infections

Some women with a high BMI may be more prone to urinary tract infections.

Blood clots

A raised BMI is linked to an increased risk of blood clots. This is especially true if you have had a blood clot before or a first degree relative has had one before they were 50 (mother, father, brother, sister or any other children).

Physicians need to be aggressively counseling women about the importance of starting pregnancy at a healthy weight.

In a recent analysis, it was found that obesity, increasingly common in pregnant women, raises the risks to pregnant woman and baby. Overweight and obese pregnant women are at higher risk of C-Section, and less likely to breastfeed, while their children are at higher risk of high birth weight and childhood obesity.

It is recommended that both pre-pregnancy weight and weight gain during pregnancy need to be controlled. Maternal obesity is a well-known risk factor for obesity and chronic disease in childhood and starting pregnancy at ideal body weight will have far reaching benefits.

The researchers offer the following recommendations:

  • Body mass index should be measured as part of vital signs at routine annual check-ups and all women of child bearing age should be counseled to achieve and maintain optimal BMI.
  • Preconception counseling programs should include education regarding the poor maternal and perinatal outcomes among the obese and overweight.
  • Women with high BMI planning a pregnancy should be counseled to participate in intensive nutrition programs aimed to achieve optimum BMI prior to conception.
  • For underweight women with a BMI of less than 19.8, a weight gain during pregnancy of 28-40 pounds is recommended.
  • For normal weight women with a BMI between 19.8 and 26 the recommended weight gain during pregnancy is 25-35 pounds.
  • For overweight women with a BMI between 26.1 and 29 the recommended weight gain during pregnancy is 15-25 pounds.
  • For obese women with a BMI greater than 29 the recommended weight gain during pregnancy is about 15 pounds.