What to Know If You Are Underweight While Pregnant

Gaining weight during pregnancy is important for the growth and development of your future baby. Weight gain is also a key component of energy storage for making breastmilk after delivery.

The good news is that the risks associated with being underweight or not gaining enough weight can be offset by starting to put on weight. As a result, being underweight and pregnant is usually not a big concern, as long as you are receiving adequate nutrition during gestation and adding enough weight throughout your pregnancy—especially in the second and third trimesters.

Overview

Being underweight before becoming pregnant may indicate the need to gain more weight during pregnancy than a woman at an average or above weight. 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 and possibly a specialist, such as a registered dietitian who specializes in pregnancy, can help you achieve appropriate weight gain and nutrition throughout your pregnancy.

Underweight while pregnant
Verywell / Emily Roberts

Why Pregnancy Weight Gain Matters

It can be difficult for some women to gain 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. The pounds put on during pregnancy are distributed approximately as follows:

  • Baby: 7 to 8 pounds
  • Placenta: 1 to 2 pounds
  • Amniotic fluid: 2 pounds
  • Uterus: 2 pounds
  • Maternal breast tissue: 1 to 3 pounds
  • Maternal blood volume: 3 to 4 pounds
  • Fluids in maternal tissue: 2 to 4 pounds
  • Maternal fat and nutrient stores: 6 to 8 pounds

Most of this weight will come off relatively quickly after delivery and the postpartum period. Any additional weight gain can be lost, too, but this usually takes more intentional effort and will vary from woman to woman. Losing additional weight is usually aided by extended breastfeeding, regular exercise, and eating a well-balanced diet.

Weight Gain Before Pregnancy

If you are underweight, gaining a few pounds before getting pregnant can be helpful. This is particularly important if your pre-pregnancy diet was not adequate nutritionally. Receiving adequate nutrition prior to pregnancy is vital to your health and the health of your baby.

Low body weight can be a risk factor for infertility and preterm birth. 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 2 to 5 pounds and if you are unable, focus on gaining 2 to 5 pounds during your first trimester.”

To gain this amount of weight, you’ll need to increase your calories by about 300 calories per day. (Women who are not underweight don’t typically 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.)

A registered dietitian can suggest nutrient-dense, high-calorie food options to help you achieve your weight goals.

History of Eating Disorders

Being underweight (defined as having a BMI of less 18.5) before getting pregnant isn’t usually an issue unless you are extremely underweight, malnourished, and/or have a history of eating disorders.

Mitchell reports that when she counsels a woman with a previous eating disorder, she may find that she needs to focus on adding certain key nutrients, such as folate (vitamin B9), iron, protein, and fat, rather than on gaining weight.

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.”

Impact of Inadequate Weight Gain

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 may be putting their bone health at risk. They can also be at increased risk for nutritional deficiencies like anemia.” 

First Trimester

Many women lose a few pounds in the first trimester due to morning sickness. This is normal. Unless you are severely underweight or are losing weight rapidly (more than just a pound or two), it's not too concerning as the growing baby has limited nutritional needs at this early stage.

The key is to make sure you are getting the key nutrients, even if your weight is stable or you lose a few pounds. A high-quality prenatal vitamin is especially important when diet is negatively impacted by nausea, vomiting, or food aversions. During the rest of the pregnancy, however, gaining weight becomes much more important to your baby's healthy development.

Second and Third Trimesters

Some research suggests that inadequate weight gain, particularly in the second and third trimesters, can increase the risk of a preterm 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. 

Optimal Weight Gain

The amount of weight you should gain during pregnancy depends on your weight and body mass index (BMI) before pregnancy. You'll receive more specifics from your obstetrician. But, generally speaking, the Centers for Disease Control and Prevention (CDC) makes the following recommendations for pregnancy weight gan.

Pre-Pregnancy BMI Recommended Pregnancy Weight Gain
Underweight: Less than 18.5 28 to 40 pounds
Normal: 18.5 to 24.9 25 to 35 pounds
Overweight: 25 to 29.9 15 to 25 pounds
Obese: Over 30 11 to 20 pounds

BMI is calculated by dividing your weight in kilograms by the square of your height in meters. The resulting number puts you in one of the above categories. Be aware that BMI is a rough measurement of your body fat relative to your height and weight.

BMI not a perfect or definitive assessment of the healthiness of your weight for your body—it's simply a guideline. Your doctor can individualize your results and let you know specifically if your weight is a concern and how many pounds you should aim to gain.

Helpful Tips

Strategies for encouraging steady weight gain while pregnant include the following:

  • Eat small, frequent meals every few hours, especially if you are feeling nauseous.
  • Avoid meal skipping.
  • Always eat breakfast.
  • Snack on nutrient-dense, high-calorie snacks, such as nuts; whole-grain crackers with cheese, nut butter, 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 and butter, and try adding cheese as a topping.
  • Most importantly, if you have any questions or concerns, work with a registered dietitian to assure you are getting adequate amounts of macro and micronutrients.

According to the CDC, underweight women should aim to add approximately 400 calories per day in the second trimester and 400 to 600 in the third trimester.

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 but should be monitored, especially if you were advised to gain more weight. In addition, it is expected 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.

If you are having trouble gaining weight, while it's not recommended to wholeheartedly turn to less nutritious, empty-calorie foods, it is permissible to begin supplementing your diet with some additional higher-calorie, higher-fat foods, such as ice cream, peanut butter, cheese, and extra butter, to get your weight gain on track.

Taking Vitamins

It's recommended for all pregnant women to take a prenatal vitamin before and during pregnancy. Daily intake of certain nutrients such as folate (folic acid), calcium, and iron needs increase during pregnancy. These nutrients are vital for the healthy growth and development of your baby.

It’s important to find a supplement that has 100% 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.

However, the best way to receive adequate nutrition is to eat a variety of fruits, vegetables, and whole grains; calcium-rich foods, such as milk, almond milk, Greek or plain yogurt; protein sources like beans, eggs, fish, 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.”

Getting Essential Nutrients

Below are some of the essential nutrients all pregnant women need, which might require additional supplementation beyond the typical prenatal vitamin.

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 “200mg 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 high quantities in a variety of different foods, getting enough during pregnancy can be tough, especially if dietary intake of eggs, salmon, and fortified foods, such as milk, is low.

When this occurs, Mitchell will recommend “supplementation of 2,000–5,000 IU [international units] daily.”

A Word From Verywell

If you are underweight and pregnant, consult your doctor on how much weight you need 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 and support to achieve ideal nutrition and weight gain.

Working with a registered dietitian who specializes in maternal nutrition can assist you in reaching these nutritional goals. Speaking with a therapist or support group specializing in eating disorders may also be helpful.

If you are underweight but eat a diet with more than adequate nutrition, then you have less to worry about, as long as you are putting on weight. Most importantly, focus on putting on enough pounds and getting proper nutrition during the critical second and third trimesters in order to optimize the health of your baby.

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Article Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Weight, Fertility, and Pregnancy. U.S. Department of Health and Human Services Office on Women's Health. Updated December 27, 2018.

  2. Centers for Disease Control and Prevention. Tracking Your Weight For Women Who Begin Pregnancy Underweight.

  3. El Rafei R, Abbas HA, Charafeddine L, et al. Association of pre-pregnancy body mass index and gestational weight gain with preterm births and fetal size: An observational study from Lebanon. Paediatr Perinat Epidemiol. 2016;30(1):38-45. doi:10.1111/ppe.12249

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