Pregnancy Weight Gain for Underweight Women — IOM Guidelines

Why Pre-Pregnancy BMI Matters

Starting pregnancy with a BMI below 18.5 places both mother and baby at elevated risk. Research from the Institute of Medicine and subsequent studies shows that underweight pregnancies have a roughly 40% higher risk of preterm birth (before 37 weeks), a 30 to 50% higher risk of low birth weight (below 2500 grams), and an increased likelihood of the newborn being small for gestational age. Maternal risks include anaemia, bone density loss, and prolonged postpartum recovery. The IOM set a higher weight-gain range specifically for this group to offset these risks and support adequate fetal growth through additional maternal fat reserves and nutrient stores.

IOM Weight-Gain Recommendation for Underweight

For individuals with a pre-pregnancy BMI below 18.5, the IOM recommends a total singleton pregnancy weight gain of 12.7 to 18.1 kg (28 to 40 lb). This is the widest range among all BMI categories. In the first trimester, gain of 1.0 to 2.0 kg is typical. In the second and third trimesters, the recommended weekly rate is 0.51 kg (1.0 to 1.3 lb) per week, which totals about 10.5 to 14.0 kg across the remaining 27 weeks. Staying within this range is associated with the lowest rates of preterm delivery and the highest proportion of appropriate-for-gestational-age births among underweight mothers.

Nutrient-Dense Food Focus

Underweight individuals need to maximize the nutrient content of every calorie consumed. Prioritise lean protein (poultry, fish, eggs, legumes, Greek yogurt) at 1.1 to 1.3 grams per kilogram of pre-pregnancy body weight per day. Include at least 2 servings of fatty fish weekly for omega-3 DHA, which supports fetal brain development. Iron needs increase to 27 mg per day to support the 50% expansion in maternal blood volume. Folic acid intake of at least 400 mcg daily reduces neural tube defect risk by up to 70%. Calcium at 1000 mg daily supports fetal bone mineralization. A prenatal vitamin is recommended alongside food to cover micronutrient gaps.

Calorie Boosters That Work

Building healthy weight requires a caloric surplus of about 340 calories per day in the second trimester and about 450 calories per day in the third trimester beyond pre-pregnancy needs. Nutrient-dense calorie boosters include: 2 tablespoons of natural peanut or almond butter (190 kcal, 14 g protein), one medium avocado (240 kcal, 10 g fiber), full-fat Greek yogurt (150 kcal per 170 g, 15 g protein), and smoothies with whole milk, banana, and spinach (300 to 400 kcal). Adding olive oil to vegetables, eating nuts as snacks, and choosing whole-grain bread and pasta over refined versions all add calories and nutrients without excessive volume. Eating 5 to 6 smaller meals per day helps if nausea or small appetite is a challenge.

When to Consult Your Prenatal Care Team

Contact your prenatal care team if your weight gain is tracking below the lower end of the IOM range by more than 2 kg at any prenatal visit, if morning sickness prevents adequate food or fluid intake for more than 24 hours, or if you are losing weight in the second or third trimester. Your care team may recommend dietary counseling, a registered dietitian referral, or more frequent prenatal visits to monitor fetal growth via ultrasound. They may also screen for underlying causes of underweight status such as hyperthyroidism, malabsorption conditions, or eating disorders, all of which require specialized management during pregnancy.