Which guidelines does this tool follow?

It uses the 2009 Institute of Medicine (IOM) / National Academies gestational weight gain guidelines, the standard reference used by OB-GYNs and midwives in the United States and many other countries.

Why does pre-pregnancy weight matter rather than current weight?

The IOM recommendations are based on your pre-pregnancy BMI because that reflects your nutritional status at conception. Using a weight measured after pregnancy begins would skew the category.

What are the IOM ranges for a single baby?

Underweight (BMI < 18.5): 28–40 lb (13–18 kg). Normal weight (18.5–24.9): 25–35 lb (11–16 kg). Overweight (25–29.9): 15–25 lb (7–11 kg). Obese (≥ 30): 11–20 lb (5–9 kg).

Twin pregnancies have higher targets. Normal weight: 37–54 lb (17–25 kg). Overweight: 31–50 lb (14–23 kg). Obese: 25–42 lb (11–19 kg). Underweight twins are not covered by the 2009 IOM guidelines due to insufficient data; consult your provider.

How is weight gain spread across trimesters?

The first trimester (weeks 1–13) typically accounts for only about 1–4.5 lb total regardless of BMI. In the second and third trimesters, gain is more linear — roughly 0.44 lb/week for obese women up to about 1 lb/week for underweight women.

Does this replace my doctor's advice?

No. This calculator is for educational reference. Every pregnancy is different, and your healthcare provider may set different targets based on your medical history, fetal growth, and other factors.

BMI During Pregnancy Calculator

Name: BMI During Pregnancy Calculator
Availability: InStock
Author: Nham Vu

Enter your pre-pregnancy height and weight to get your BMI category and IOM-recommended total and trimester-by-trimester weight gain targets.

Pre-Pregnancy Details

Unit System

Height

feet

inches

Pre-Pregnancy Weight (lb)

Pregnancy Type

Trimester-by-Trimester Guidance

Period	Weeks	Typical Total Gain	Rate / Week

Weekly rates apply to the 2nd and 3rd trimesters. 1st-trimester gain is relatively small and less predictable. Actual patterns vary — your provider may set different targets.

Enter your details on the left, then click Calculate.

Summary

Enter your pre-pregnancy height and weight to get your BMI category and IOM-recommended total and trimester-by-trimester weight gain targets.

How it works

Choose your preferred unit system — imperial (lb/in) or metric (kg/cm).
Enter your pre-pregnancy height and weight.
Select whether you are carrying one baby or twins.
Click "Calculate" to see your pre-pregnancy BMI and category.
Review the IOM-recommended total weight gain range for your category.
Check the trimester breakdown to understand the expected gain each period.

Use cases

Learn your pre-pregnancy BMI category before discussing weight goals with your OB-GYN.
Understand how much total weight gain is recommended for your BMI and pregnancy type.
Plan nutrition and activity for each trimester with gain-per-week targets.
Compare singleton versus twin pregnancy recommendations side by side.
Educate yourself on IOM guidelines before a prenatal appointment.
Reference a clean, ad-free tool without entering personal data into a pregnancy app.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu