What to Know If You Are Underweight While Pregnant

Knowing Your Nutritional Needs During This Time

Gaining weight during pregnancy is important for the growth and development of your future baby. Weight gain is also a key component for storage of breastmilk after delivery. Being underweight and pregnant should not be a concern so long as you are receiving adequate nutrition during gestation and gaining enough weight throughout your pregnancy.

Being underweight before pregnancy may indicate the need to gain more weight than the average weight woman. Additionally, women with a history of eating disorders like anorexia or bulimia nervosa, who may have had nutritional deficiencies prior to conception, will need additional nutrients during pregnancy.

Working with your healthcare provider, such as a registered dietitian who specializes in pregnancy, can assist in making sure you have appropriate weight gain and adequate nutrition throughout your pregnancy.

Underweight while pregnant
Verywell / Emily Roberts

Gaining Weight

It can be difficult to imagine yourself gaining weight during pregnancy, but it's critical for growing a healthy baby. Understanding where the important weight goes can help you to understand the necessity of adequate weight gain. According to the American Pregnancy Association, weight gain is distributed as such:

  • Baby: 7 to 8 pounds
  • Placenta: 1 to 2 pounds
  •  Amniotic fluid: 2 pounds
  • Uterus: 2 pounds
  • Maternal breast tissue: 2 pounds
  • Maternal blood: 4 pounds
  • Fluids in maternal tissue: 4 pounds
  • Maternal fat and nutrient stores: 7 pounds

Most of this weight will come off once you’ve had the baby. Any additional weight gain will eventually come off, too, and varies from woman to woman depending on whether they are nursing, dieting, and exercising.

Gain Weight Before Pregnancy If Possible

Receiving adequate nutrition prior to pregnancy can be beneficial to you and your baby. Low body weight can often be a predictor of infertility and premature infants. If you are underweight, gaining a few pounds before getting pregnant can be helpful. Emily Mitchell, a registered dietitian and certified diabetes educator for the Center of Fetal Medicine, says, “If you are underweight and trying to get pregnant, aim to gain two to five pounds and if you are unable, focus on gaining two to five pounds during your first trimester.”

To gain this amount of weight, you’ll need to increase your calories by about 300 calories per day. (For women who are not underweight, they typically don’t need extra calories during their first trimester and need roughly 340 extra calories per day in the second trimester, and 450 extra calories per day during the third trimester.)

Women who are underweight may need additional calories throughout pregnancy.

Your registered dietitian can provide you with nutrient dense, high-calorie food options to help you achieve your needs.

If You Have a History of Eating Disorders

Being underweight (considered a BMI of less 18.5) before getting pregnant isn’t usually an issue unless you have a history of eating disorders such as anorexia nervosa or bulimia nervosa. Mitchell reports that when she is counseling a woman with a previous eating disorder, she may find that she needs to focus on certain key nutrients, such as folic acid, iron, protein, and fat.

She says, “When reviewing a woman’s food intake, I like to work on filling in the gaps for the key nutrients they are missing. And if a woman doesn’t have much fat storage, it is critical that they are getting adequate nutrition intake consistently and gaining weight in the second and third trimester.”

What Happens If I Have A Hard Time Gaining Weight?

Fatigue is a common symptom of inadequate weight gain. Additionally, Mitchell says, “women who have a difficult time gaining weight during pregnancy have a longer postpartum recovery and maybe putting their bone health at risk. They can also be at increased risk for nutritional deficiencies like anemia.” 

Total Weight to Gain

The amount of weight you should gain depends on your weight and BMI (body mass index) before pregnancy. You'll receive more specifics from your obstetrician. But, generally speaking, the Institute of Medicine recommends you should gain:

  • 25 to 35 pounds if you were a healthy weight before pregnancy, with a BMI of 18.5 to 24.9
  • 28 to 40 pounds if you were underweight before pregnancy, with a BMI of less than 18.5
  • 15 to 25 pounds if you were overweight before pregnancy, with a BMI of 25-29.9
  • 11 to 20 pounds if you were obese before pregnancy, with a BMI of over 30

Second and Third Trimesters

Some research suggests that inadequate weight gain, particularly in the second and third trimesters, can increase the risk of a pre-term baby or a cesarean delivery. Babies born prematurely have lower birth weights and research suggests that these babies have a higher risk of developing specific health issues later in life such as heart disease, high blood pressure, and diabetes. 

Therefore, the Institute of Medicine recommends that underweight women gain roughly one to 1.3 pounds of weight per week in the second and third trimesters. 

Helpful Tips

  • Eat small, frequent meals every few hours, especially if you are feeling nauseous.
  • Avoid meal skipping.
  • Snack on nutrient dense, high-calorie snacks, such as nuts, whole grain crackers with cheese, hummus, or avocado, sliced fruit spread with nut butter, sandwiches on whole grain bread, creamy soups, steel cut oats with fresh fruit, and chopped nuts.
  • Drink high-calorie beverages, such as smoothies made with full-fat milk or milk alternative with added protein powder.
  • Cook with extra fat such as oils, butter, and try adding cheese as a topping.
  • Most importantly, work with a registered dietitian to assure you are getting adequate amounts of macro and micronutrients.

What If I Start to Lose Weight?

Many women lose weight during their first trimester due to nausea, vomiting, and morning sickness. This can be normal and should be monitored especially if you were told to gain more weight. In addition, it is normal for weight to fluctuate from week to week, but if you lose weight suddenly, you should contact your healthcare provider right away, especially in your third trimester.

Should I Take Extra Vitamins?

All women are recommended to take a prenatal vitamin before and during pregnancy. Daily intake of certain nutrients such as folic acid, calcium, and iron needs increase during pregnancy. These nutrients are vital for the growth and development of the unborn baby.

It’s important to find a supplement that has 100 percent of the daily value, as well as one that does not have an excess of certain nutrients like folic acid (600 micrograms) and iron (27 milligrams). Special requirements will be made for women who are anemic—they may need more iron. Additionally, certain fat-soluble vitamins can be problematic if supplements far exceed the daily value. For example, “Increased vitamin A has been linked to birth defects,” says Mitchell.

The best way to receive adequate nutrition is to eat a variety of fruits, vegetables, whole grains, calcium-rich foods such as almond milk, Greek or plain yogurt, protein sources like beans, eggs, and chicken, and healthy fats like fatty fish, nuts, seeds, and oil. Mitchell says, “optimally women are getting nutrients from food first because they are absorbed better than supplement form.”

DHA

If you are not eating adequate amounts of oily low-mercury fish such as wild salmon, halibut, and sardines, or fortified eggs, you may need docosahexaenoic acid (DHA) supplementation. Mitchell recommends, a minimum of “200 mg of DHA to support baby’s brain, nerve, and eye development.” Alternative sources of vegan DHA include seaweed, but speak to your dietitian or obstetrician before adding that to your diet.

Vitamin D

Vitamin D is also an important nutrient during pregnancy and may play a role in preventing pre-term labor and infections. Since vitamin D isn’t found in a ton of different foods, getting enough during pregnancy can be tough, especially if dietary intake of eggs, salmon, and fortified foods such as milk are low. When this occurs, Mitchell will recommend “supplementation of 2,000-5000 IU [international units] daily.”

A Word From Verywell

If you are underweight and pregnant, consult with your doctor on how much weight to gain in order to promote a healthy pregnancy. Additionally, if you have a history of eating disorders or inadequate dietary intake, you may need additional supplementation. Working with a registered dietitian who specializes in maternal nutrition is an important step in assuring you have appropriate weight gain and nutrition throughout your pregnancy. Speaking with a therapist or support group specializing in eating disorders may also be helpful in making sure you're taking care of yourself and your future baby.

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Article Sources

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  • Sharma et.al. Rate of Second and Third Trimester Weight Gain and Preterm Delivery Among Underweight and Normal Weight Women. Matern Child Health J. 2016 Oct;20(10):2030-6. DOI: 10.1007/s10995-016-2032-y.

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