Calculate healthy pregnancy weight gain targets using IOM (Institute of Medicine) guidelines based on your pre-pregnancy BMI. Shows total recommended gain, trimester breakdown, weekly rate, and tracks current weight vs ideal. Includes twin pregnancy guidelines.
Enter your height and pre-pregnancy weight (the weight before you became pregnant, or your first-trimester weight before significant gain). Select metric (kg/cm) or imperial (lbs/inches). Select singleton or twin pregnancy.
Enter your current weight and current week of pregnancy (1–40). The calculator compares your actual weight gain to the IOM recommended gain for your gestational week.
Your pre-pregnancy BMI category is shown. The recommended total gain range, trimester-by-trimester targets, and weekly rate are displayed. A visual progress bar shows where your actual gain sits within the recommended range.
The 2009 Institute of Medicine guidelines (endorsed by ACOG and WHO): Underweight (BMI <18.5): 12.5–18 kg (28–40 lb). Normal weight (BMI 18.5–24.9): 11.5–16 kg (25–35 lb). Overweight (BMI 25–29.9): 7–11.5 kg (15–25 lb). Obese (BMI ≥30): 5–9 kg (11–20 lb). Twin pregnancy normal weight: 17–25 kg (37–54 lb). These guidelines were established from analysis of 120,000+ births and are designed to optimise outcomes for both mother and baby.
First trimester (weeks 1–13): 0.5–2 kg (1–4 lb) total, regardless of BMI category. Very little weight gain occurs early in pregnancy. Second trimester (weeks 14–27): approximately 0.3–0.5 kg (0.5–1 lb) per week for normal BMI. Third trimester (weeks 28–40): continued 0.3–0.5 kg per week. Most weight gain occurs after week 13. Total first-trimester gain is minimal compared to later trimesters.
Excessive gain increases risks of: gestational diabetes, pre-eclampsia, caesarean delivery, large-for-gestational-age baby, and difficulty losing weight postpartum. Insufficient gain risks: preterm birth, small-for-gestational-age baby, and developmental issues. However, these are statistical associations — individual variation is significant. The most important action is to discuss your specific situation with your midwife or obstetrician.
Approximate distribution of 13 kg (28 lb) gain for a normal-BMI woman: Baby: 3.2–3.6 kg (7–8 lb). Placenta: 0.7 kg (1.5 lb). Amniotic fluid: 0.9 kg (2 lb). Uterus growth: 0.9 kg (2 lb). Breast tissue: 0.9–1.4 kg (2–3 lb). Blood volume increase: 1.4–1.8 kg (3–4 lb). Body fluids: 1.4–1.8 kg (3–4 lb). Maternal fat/nutrient stores: 2.7–3.6 kg (6–8 lb). Most of this reverses naturally after birth and breastfeeding.
Generally no — calorie restriction during pregnancy can deprive the baby of nutrients needed for development. Even obese women are recommended to gain at least 5–9 kg (11–20 lb). The goal is to gain at the lower end of the recommended range, not to lose weight. Focus on nutrient-dense food quality rather than quantity restriction. Excessive nausea and vomiting (hyperemesis gravidarum) may cause weight loss — always consult your healthcare provider if this occurs.
BMI has known limitations: it does not distinguish between muscle and fat, and can misclassify fit or muscular women. However, it remains the best practical tool for population-level pregnancy weight gain guidance. The IOM guidelines are based on pre-pregnancy BMI and have been validated in large studies. If you believe your BMI misrepresents your body composition (e.g. you are an athlete), discuss with your healthcare provider for personalised recommendations.
