Pregnancy Weight Gain Calculator Formula
Understand the math behind the pregnancy weight gain calculator. Each variable explained with a worked example.
Formulas Used
Recommended Gain (Low)
recommended_low = total_gain_lowRecommended Gain (High)
recommended_high = total_gain_highExpected Gain by Now (Low)
expected_so_far_low = weeks_pregnant <= 13 ? total_gain_low * weeks_pregnant / 40 : total_gain_low * 0.25 + (total_gain_low * 0.75) * (weeks_pregnant - 13) / 27Pre-Pregnancy BMI
pre_bmi = bmi_preVariables
| Variable | Description | Default |
|---|---|---|
pre_weight_lbs | Pre-Pregnancy Weight(lbs) | 140 |
height_inches | Height(inches) | 65 |
weeks_pregnant | Weeks Pregnant(weeks) | 20 |
bmi_pre | Derived value= (pre_weight_lbs * 703) / (height_inches * height_inches) | calculated |
total_gain_low | Derived value= bmi_pre < 18.5 ? 28 : (bmi_pre < 25 ? 25 : (bmi_pre < 30 ? 15 : 11)) | calculated |
total_gain_high | Derived value= bmi_pre < 18.5 ? 40 : (bmi_pre < 25 ? 35 : (bmi_pre < 30 ? 25 : 20)) | calculated |
How It Works
IOM Pregnancy Weight Gain Guidelines
Most weight gain occurs in the second and third trimesters, with about 1-2 lbs per week being typical for normal-weight women.
Worked Example
A 140 lb, 65-inch tall woman at 20 weeks pregnant.
- 01Pre-pregnancy BMI = (140 x 703) / (65 x 65) = 23.3 (Normal)
- 02Recommended total gain = 25-35 lbs
- 03Expected by week 20 = 25 x 0.25 + (25 x 0.75) x 7/27 = 6.25 + 4.9 = 11.1 lbs (low end)
Frequently Asked Questions
How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI. Normal-weight women should gain 25-35 lbs. Overweight women 15-25 lbs. Underweight women 28-40 lbs.
When does most weight gain occur?
About 25% of total gain occurs in the first trimester. The remaining 75% occurs in the second and third trimesters.
Is it dangerous to gain too much weight?
Excessive weight gain increases risk of gestational diabetes, preeclampsia, cesarean delivery, and postpartum weight retention. Talk to your provider about your target.
Learn More
Guide
How to Calculate Pregnancy Due Date
Learn how to calculate your estimated due date using Naegele's Rule and ultrasound dating. Understand how conception date, LMP, and gestational age all relate to predicting your baby's arrival.
Ready to run the numbers?
Open Pregnancy Weight Gain Calculator