Underweight In Pregnancy: Risks, Causes And Nutrient Intake

A body mass index (BMI) of 18.5 kg/m2 or less is considered underweight in pregnancy. Maternal weight gain in pregnancy is important for the optimal growth and development of the unborn baby. The additional weight is stored as fat deposits in pregnancy to meet the calorie needs of breastfeeding in postpartum. However, some women may not gain weight during pregnancy, and it can cause concern. Underweight mothers also have an increased risk of having a baby with low birth weight and other growth issues.

Read on to learn more about how maternal underweight can affect a mother’s and baby’s health and ways to gain weight.

What Does Being Underweight In Pregnancy Mean?

Body mass index or BMI is often used as a screening tool for assigning weight categories — underweight, normal or healthy weight, overweight, and obesity.

If you are in the underweight category, do not panic. It may not always be a cause of concern. However, the lack of weight gain can be related to your weight before pregnancy or inadequate nutrition intake. This can also indicate that your body is not storing enough fat .

Some women may find it difficult to gain weight during the initial phase of pregnancy. A few women may even lose weight during early pregnancy due to morning sickness. However, it is not a bad sign as the baby during this time may not need as many calories or nutrients as in the later part of pregnancy.

Also, women with a history of eating disorders like anorexia or bulimia nervosa may find it difficult to gain or maintain a healthy weight range (1).

How Much Weight Should You Gain During Pregnancy If You Are Underweight?

There is no one-style-fits-all solution in this case. You need to consider your BMI, body weight before pregnancy, daily activities, dietary intake, etc.

As per the US Institute of Medicine’s (IOM) general recommendations, here is the ideal weight gain target for pregnant women based on their weight categories (2).

If you are expecting twins, the weight gain targets recommended by IOM are as follows:

Note: Your doctor may recommend a different weight gain regime suited to your body. In such a case, follow the weight gain guidelines recommended by your doctor.

According to the US IoM recommendations, women who are in their second and third trimesters and have a low BMI (<18.5) should try to gain between 1 – 1.3lb each week (2).

Discuss the appropriate weight gain target and diet & exercise routine with your healthcare provider not only during your initial visits but also throughout your pregnancy. You may work out a diet plan with your dietitian to fulfill your nutritional needs for a steady weight gain during all trimesters.

Does Being Underweight Pose Any Risks To You Or Your Baby?

It is important to gain steady weight during pregnancy, especially during the second and third trimesters.

• Staying underweight might have negative effects on the unborn baby. Some studies have shown that if a woman is underweight during pregnancy, then complications such as low birth weight (less than 2.5kg or 5.5lb) or preterm deliveries (before 37 weeks) may occur (3) (4).

• Babies born to underweight mothers (BMI< 18.5) could be small for their gestational age (SGA) (5), i.e., such babies are smaller than usual for the number of weeks of pregnancy.

Can Being Underweight Cause A Miscarriage?

Although being underweight during pregnancy has been associated with an increased risk of miscarriage, the results from several studies have been equivocal or ambiguous (6) (7) (8).

How To Make Sure You Receive Adequate Nutrition?

You can receive adequate nutrition by observing the food and prenatal vitamin recommendations made by your doctor or dietitian.

A woman’s body goes through physical and hormonal changes throughout pregnancy. Therefore, it is essential to fulfill your and your unborn baby’s nutritional needs with a healthy and balanced diet.

Know about your approximate daily calorific requirements during the first, second, and third trimesters based on your BMI. The US Centers for Disease Control and Prevention (CDC), makes these recommendations for women with underweight (9).

• In general, the first trimester does not require any extra calories.
• An additional 400 calories per day during the second trimester
• An additional 400-600 calories per day during the third trimester.

The recommended calorie limits provide an approximate estimate. Each woman has different nutritional needs. Therefore, your doctor is the best person to suggest the appropriate calories you need during pregnancy.

Nutrient Intake During Pregnancy

Your diet should contain adequate amount of both macronutrients and micronutrients (9) such as proteins, carbohydrates, essential/healthy fats such as omega 3s (DHA), vitamins and minerals (folate, iron, vitamin D, calcium, etc.)

• The World Health Organization (WHO) recommends 30mg to 60mg of elemental iron and 0.4mg of folic acid per day for pregnant women (10).

• If the pregnant woman is deficient in calcium, then a daily calcium supplementation of 1.5-2g is recommended (10). 

• You may obtain nutrients through dietary sources like fruits, vegetables, meat, and dairy products.

• Supplements may not be required for every woman. You may take them if your doctor recommends.

How to Prevent Being Underweight During Pregnancy?

Your ideal goal should be to have a healthy BMI before you plan your pregnancy. However, if this doesn’t work out, your goal should be to gain steady weight throughout your pregnancy. Underweight women usually tend to eat low calorie and low protein diet. Therefore, the American Pregnancy Association recommends the following guidelines (11).

• Do not skip a meal.
• Breakfast is the most important meal of the day. You can have an added protein boost in this meal by adding peanut butter, etc.
• Eat healthy snacks such as fresh fruit smoothies and dry fruits in between your meals.
• Include foods that are high in good fats such as nuts and avocados.
• Add extras such as honey, string cheese, and yogurt.
• Have fresh fruits or sugar-free fresh juices made of oranges, carrots, and apricots to have adequate vitamins.
• Avoid junk food.
• Do not forget to take your prenatal vitamins and supplements as recommended by your healthcare practitioner.

You may maintain a weekly chart to track your weight gain through your pregnancy. This will help you to know when you are going off track.

Tips to Gain Weight During Pregnancy

Gaining weight during pregnancy can depend on several factors, including daily diet, pre-pregnancy weight, metabolism, level of physical activity, or any underlying medical condition.

In addition to following the measures mentioned in the previous section, the below tips might come in handy when you are trying to gain those extra healthy pounds:

• Make your healthy food enjoyable by including some seasonings, toppings etc. For instance, instead of eating boiled vegetables or puree, you can add cream of spinach or broccoli soup in your meal.

• Eating small yet calorie-laden meals can help you in your weight gain regime. Focus on nutrition and not quantity. Eating a boiled egg, a slice of pasteurized cheese, or yogurt can give you those extra calories and also help you in tackling pregnancy-related issues such as nausea and heartburn.

• Pack a snack. If you are travelling for work or leisure, make sure to pack healthy snacks such as a banana, orange, or apple along with dry fruits or nuts to boost your nutrient intake.

• If you find it difficult to eat food, you can drink those calories. You can make smoothies with yogurt or milk and add protein powder (as recommended by your healthcare practitioner). You can use vegetables as well as fruits to make these smoothies.

• Indulge yourself in your favorite foods but occasionally.

• Keep yourself active and do light aerobic exercises after consulting your healthcare practitioner. According to the CDC, up to 150 minutes of aerobic activities (for example, brisk walking) in a week works well for most women. Make sure to check if you have any physical restrictions (10).

References

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