February 21, 2023

Underweight in Pregnancy

Starting pregnancy while underweight can affect your baby's health or cause preterm labor. You are underweight if your BMI (Body Mass Index) is under 18.5. 

As previously explored in our last blog post, Obesity in Pregnancy, you can determine your BMI by taking your last known pre-pregnancy weight in pounds and multiply it by 703. Then, divide that sum by your height in inches, and divide the new sum by your height in inches again. Round to the nearest tenth. Here’s an example: 

Jane Doe weighs 112 pounds and is 5’6”.

  • First, we multiply 112 by 703 and get 78,736.
  • Next, we divide 78,736 by 66 (height converted into inches) and get 1,192.96.
  • Divide 1,192.96 by 66 and our new sum is 18.07.
  • Round to the nearest tenth and Jane’s BMI is 18.1 and is considered underweight. 

You can also use a BMI calculator to find out your BMI.

BMI Weight Categories

After finding out your BMI, you can use these guidelines to discover which weight category you fit into. 

Special Consideration

BMI is only a tool for measuring the estimated amount of body fat based on your height and weight. It is only one step in determining the overall health and wellness of a patient. BMI does not take into consideration the amount of fat mass versus lean muscle mass, so you should always speak with your midwives about any weight concerns you have. 


Trying to conceive while underweight may prove to be difficult. Having a low BMI may cause periods to become irregular or stop altogether. For women with irregular periods, it may be harder to determine when ovulation occurs. Women whose periods have stopped have also stopped ovulating, thus pregnancy becomes incredibly difficult until they can get enough nourishment to encourage menstruation again.

Schedule a preconception appointment with our midwives to help you work on getting to a healthy weight prior to pregnancy. During your appointment, the midwives will try to determine the underlying cause of being underweight and help you come up with a plan to gain weight. 


Underweight women should aim to gain 28-40 pounds during their pregnancy. Of course, for some, gaining weight is easier said than done. Try these tips to gain weight:

  • Eat small meals frequently, this may also help ward off nausea
  • Never skip meals, especially breakfast
  • Drink high-calorie drinks such as smoothies and protein shakes
  • Snack on nutrient-dense foods such as granola bars 
  • Focus on eating high-calorie, high-fat foods like peanut butter and avocados
  • Remember to take your prenatal vitamins and routinely go to your check-ups

Eating more is not always better, however. If your daily diet consists of mostly fast food or “junk,” food, there is little nutritional value to support a growing fetus. Aim to introduce key nutrients to your diet every day for a healthy pregnancy. 

Risks of Being Underweight in Pregnancy

It’s not uncommon for some women to lose weight during the first trimester due to morning sickness and loss of appetite, but a major concern for women who are already underweight is malnourishment. Malnutrition can lead to risks such as: 

  • Miscarriage
  • Preterm birth
  • Fatigue and nausea
  • Increased risk of nutritional deficiencies like anemia

Risks to the Fetus

  • Low birth weight
  • Growth and developmental delays

Underweight mothers may struggle to breastfeed after giving birth due to a lack of nutrition.


Women who are underweight should eat a high-fat, high-calorie, nutrient-dense diet to encourage weight gain. It’s important to keep up with regular check-ups to monitor weight and remember to take your prenatal vitamins every day. 

For more information about being underweight during pregnancy, contact us today!

Dakota Collins
Dakota Collins is a freelance writer and stay-at-home mother of two precious little girls who is passionate about freedom in pregnancy and childbirth.
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