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Body Mass Index (BMI) and Pregnancy

Unhealthy Weight and Pregnancy (For patients)
Reviewed By Medical Advisory Board     

Unhealthy Weight and Pregnancy

Unhealthy weight increases the risk of many health conditions including hypertension and diabetes, and avoiding the subject of unhealthy weight gain misses an opportunity to possibly prevent or treat many weight related illnesses [1-4]. The continued rise in obesity and related diseases is of global concern [5]. In 2010, 1 in 4  U.S. women of reproductive age (aged 18-44 years) were obese.

The American Congress of Obstetricians and Gynecologists (ACOG) recommends that the body mass index (BMI) be recorded for all women at their first prenatal visit, and that Information concerning the maternal and fetal risks of a very high BMI in pregnancy should be provided [6].

The Body Mass Index

The body mass index (BMI) is a measure of someone's weight in relation to their height. The BMI is equal to a person's weight divided by their height . It is calculated either as:

BMI = (Weight in pounds /  Height in inches 2 ) x 703

BMI = (Weight in kilograms  / Height in meters 2)


Body Mass Index Calculator

To calculate you BMI

1. Enter your weight and height 
Select "Calculate BMI" .

Input Weight kg lbs
Input Height cm in
Body mass index (kg/m2) =

The BMI is used to screen persons for weight categories that may lead to health problems [7].

BMI Category
= <18.5 Underweight
= 18.5-24.9 Normal weight
= 25-29.9 Overweight
30 -34.9 Obese- Class I
35 - 39.9 Obese- Class II
40 and above Obese- Class III

U.S. Obesity Trends 1985–2010

As shown by the maps below the number of persons with an unhealthy BMI has increased greatly over the last 25 years . The heaviest BMI groups have increased at the fastest rates.

There are many factors that may be related to the increased prevalence of obesity over the last 20 years. Among these factors are changes in behavior known to increase the risk for weight gain such as increased  energy intake, reduced rates of smoking,  decreased numbers of hours  slept , and environments that encourage physical inactivity. Some other factors that may be involved include increased pollutants in the environment that may affect hormones controlling body weight, increased use of medications that cause weight gain, and an aging population].

Obstetric Complications and BMI

 There is good evidence that women with a high body mass index (BMI > 30 ) before or during pregnancy are at greater risk for complications in pregnancy including :

  • Birth defects such as heart defects, neural tube defects, and other abnormalities.
  • Difficulty seeing all of the baby's organs and estimating the baby's weight with ultrasound.
  • Difficulty monitoring the  baby's heart rate tracing with  the fetal heart monitor
  • Gestational diabetes 
  • Pre-eclampsia
  • Problems having epidural and other anesthesia.
  • Delivery by emergency caesarian section.
  • Heavy bleeding after delivery.
  • Increased risk of forming abnormal blood clots.
  • Bladder and kidney infections.
  • Wound infection
  • Large babies with birthweight above the 90th percentile
  • Less likely to have successful vaginal birth after cesarean section
  • Increased risk of stillbirth.

The level of these risks increases as the mother's BMI increases [15-17, 31].

Risks of obstetric complications in the different BMI groups compared to normal

Click here to view table in a new window
Click here view table in same window

In view of the above risks the American Congress of Obstetricians and Gynecologists (ACOG) has  recommended [6]:

  • Preconception counseling for obese women who are planning a pregnancy.
  • BMI should be recorded for all women at the initial prenatal visit.
  • Information concerning the maternal and fetal risks of obesity in pregnancy should be provided
  • Overweight women (BMI 25-29.9) should be advised to gain no more than 15 to 25 pounds during pregnancy, and obese women (BMI = 30) no more than 15 pounds during pregnancy.
  • Obese women should be offered nutrition consultation, and they should be encouraged to follow an exercise program. This should be continued after delivery.
  • Screening for gestational diabetes at the initial prenatal visit with repeated screening later in pregnancy if the results are negative.
  • Women with a BMI of 35 or greater who have preexisting medical conditions, such as hypertension or diabetes, may benefit from a cardiac evaluation.
  • Consultation with Anesthesiology prior to delivery.
  • Use of compression stockings, hydration, and early mobilization during and after cesarean delivery should be encouraged.
  • Consultation with weight loss specialists before attempting another pregnancy

The Infectious Diseases Society of America recommends giving higher doses of prophylactic antibiotics to obese patients during surgery.  Overall, the type of skin incision does not appear to be associated with wound complications in the obese pregnant woman. However, vertical abdominal incisions have been associated with increased operative time, blood loss, and vertical hysterotomy. Wound complications in obese women undergoing cesarean delivery may be decreased by closing the subcutaneous tissue in women with at least 2 centimeters of subcutaneous adipose tissue.  Prophylactic use of subcutaneous drainage has not been shown to prevent significant wound complications after cesarean delivery.

Promoting A Healthy Lifestyle

The Institute of Medicine (IOM) Guidelines for healthy weight gain in pregnancy may help women avoid pregnancy complications such as gestational diabetes, preeclampsia, and cesarean delivery. Our Pregnancy Weight Gain Calculator and Prenatal Weight Gain Charts are based on the IOM guidelines , and may be useful in managing weight gain during pregnancy.

After delivery women at an unhealthy weight should be offered both nutrition consultation and an exercise program. Although physical activity alone produces only a modest reduction in weight compared with dieting alone [24-26] regular physical activity helps maintain weight loss [28] and results in many health related benefits in patients even if they lose no weight [27] .

According to the American College of Sports Medicine patients who have known coronary, metabolic, pulmonary disease, or signs or symptoms suggestive of coronary heart disease  may benefit from diagnostic exercise testing before beginning an exercise program. Most other patients who are obese or overweight can begin with activities such as walking starting with a 5 minute work out and gradually working up to their goal over many weeks [29,30].

Lastly, breast feeding can help patients to lose the weight gained during pregnancy,  and may also have a small protective effect against childhood obesity [32-34].

See also: Unhealthy Weight and Pregnancy (Information sheet for patients)

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Updated 12/17/2012


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