Why does your pre-pregnancy weight matter?

AndieM.Ed., RD, LDN, CLC, RYT-200

What to Know

  • How to determine whether you are a healthy pre-pregnancy weight
  • Understanding body mass index (BMI)
  • Risks associated with pre-pregnancy underweight and overweight

Your pre-pregnancy weight is a critical factor in determining the recommended amount of weight you should gain once you become pregnant. Your healthy weight gain during pregnancy is associated with optimal outcomes for you and your baby – you’ll both be well nourished, your baby will be more likely to grow to an appropriate gestational size, and you’ll have an easier time carrying your baby to term and returning to your pre-pregnancy weight.

The recommendations for healthy weight gain during pregnancy stem from your pre-pregnancy body mass index (BMI), a calculation based on your weight and height.Calculate your BMI by dividing your pre-pregnancy weight in kilograms by your height in meters, squared (or just use an online BMI calculator like the one on the National Institute of Health website). If you are underweight, you will need to gain more weight to support your pregnancy than someone who is normal weight. Overweight and obese women need to gain less. Recommendations for prenatal weight gain are based on the Institute of Medicine’s (IOM) current guidelines as follows:

Pre-pregnancy weight category BMI Recommended total weight gain for one baby Recommended total weight gain for twins
Underweight Below 18.5 28-40 lbs
Normal Weight 18.5-24.9 25-35 lbs 37-54 lbs
Overweight 25-29.9 15-25 lbs 31-50 lbs
Obese Above 30 11-20 lbs 25-42 lbs

Achieving a normal weight before you become pregnant is important because being overweight, obese or underweight pre-pregnancy can lead to health risks for you and for your baby. Pre-pregnant obesity is associated with increased maternal risk for hypertension, gestational diabetes, thromboembolic disease, and cesarean delivery. Babies of obese moms are at higher risk for developing neural tube defects, macrosomia (large head), and being born preterm or stillbirth.

Underweight moms-to-be are at risk for anemia and may have difficulty conceiving due to lower fertility, while eating disorders specifically may increase the risk of miscarriage, obstetric complications, and postpartum depression. Babies of underweight moms are more likely to experience growth restriction and low birth weight.

What to Do

Achieve a healthy weight prior to trying to conceive

Use an online body mass index calculator or app to calculate your pre-pregnancy BMI to determine if you are underweight, normal weight, overweight or obese. If you are under or overweight, set nutritional and fitness goals to achieve and maintain a normal weight to planning a pregnancy. To help you do this, schedule a pre-conception appointment with your healthcare provider for an assessment. Be honest when sharing your history, especially with respect to diet, exercise, eating disorders, and stress level. You may wish to visit a nutrition counselor for extra support. Be aware that if you opt for bariatric surgery to manage obesity, you must wait a year prior to conceiving and should take multivitamin supplements both before and after conception to help compensate for decreased absorption of essential vitamins.

Remember what “eating for two” really means

Once you are pregnant, use your pre-pregnancy BMI to determine how much total weight you should gain during your pregnancy. An expecting mom of normal weight only needs an extra 340 calories per day during the second trimester and 450 calories per day during the second trimester. (See How much should I eat when I am pregnant? for example of snack ideas that fit within these caloric guidelines). Instead of “eating for two”, focus on “thinking for two”, and upgrade your dietary choices without overdoing your total dietary intake to support your baby’s growth and development (plus your own energy and nutrient needs!).

Follow instructions from your healthcare provider for special situations

Although obesity has been a primary focus in most discussions on pregnancy weight, nutritional deficits can be equally concerning. For example, strict vegans or vegetarians should check in with their healthcare provider or a nutritionist to ensure they are getting enough protein, omega 3 fatty acids, iron, zinc, iodine, calcium, vitamin D and vitamin B12. Women with eating disorders or those who engage in pica (ingestion of non-food such as clay, dirt, ice, and cornstarch) may require extra nutritional counseling and psychological assessment to help them manage their condition. Be sure to let your provider know if you have a special nutritional situation so you have appropriate support and can get started on prenatal vitamins as necessary.

Exercise is important during pregnancy and can help you with stay within your recommended calorie balance and healthy weight gain, among other things.

Talk to your healthcare practitioner about your individual exercise capabilities while pregnant.