Pregnancy Weight Gain: Week by Week Breakdown

Enter your pre-pregnancy height and weight, current weight, and week of pregnancy to get your pre-pregnancy BMI category, your IOM 2009 recommended total gain range, and your expected gain-to-date range.

Quick Facts

Guideline
Institute of Medicine, 2009
Total gain range and weekly pace both depend on pre-pregnancy BMI category. Assumes a singleton pregnancy.
First trimester (weeks 0-13)
1.1-4.4 lbs total
Same target for every BMI category.
2nd/3rd trimester pace
0.4-1.3 lbs/week
Lower BMI categories are assigned a faster recommended weekly rate.

Your Results

Calculated
Pre-pregnancy BMI
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BMI category
Total gain target
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IOM range for full pregnancy
Expected gain by this week
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IOM range at your current week
Your actual gain
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Current minus pre-pregnancy weight

Ready

Enter your height, pre-pregnancy weight, current weight, and week of pregnancy to see your expected gain range.

Understanding your pregnancy weight gain results

This calculator applies the Institute of Medicine's 2009 pregnancy weight gain guidelines, the standard reference used by most prenatal care providers in the United States. It first converts your pre-pregnancy height and weight into a BMI, sorts that BMI into one of four categories, and then reads off the total gain range and weekly gain pace assigned to that category. It then estimates how much gain is expected by your current week and compares that to what you have actually gained.

The formula, step by step

  • Pre-pregnancy BMI: BMI = 703 x weight (lbs) / height (inches)^2, calculated from your weight before pregnancy, not your current weight.
  • BMI category and totals: underweight (BMI under 18.5) targets 28-40 lbs total; normal weight (18.5-24.9) targets 25-35 lbs; overweight (25.0-29.9) targets 15-25 lbs; obese (30.0 and above) targets 11-20 lbs.
  • First trimester (weeks 0-13): every category shares the same target, about 1.1 to 4.4 lbs total, applied here in proportion to how far into those 13 weeks you are.
  • Second and third trimester pace: from week 14 onward, each category adds a weekly rate on top of the first-trimester amount - roughly 1.0 lb/week if underweight, 0.8-1.0 lb/week if normal weight, 0.5-0.7 lb/week if overweight, and 0.4-0.6 lb/week if obese.

Reading the comparison

The calculator shows your expected gain-to-date as a low-high range and your actual gain (current weight minus pre-pregnancy weight) next to it. Falling inside the range is described as "on track." Falling outside it by a small margin is flagged as slightly below or above pace; a larger gap is flagged as notably below or above pace. Being outside the range at one point in pregnancy is common and does not by itself indicate a problem - trends over several visits matter more than a single reading.

What this calculator assumes

The IOM guideline used here is built for singleton pregnancies at a normal starting BMI progression; it does not cover twins or higher-order multiples, whose recommended ranges are different and less firmly established. It also does not account for conditions such as gestational diabetes, hyperemesis, or multiple gestation, which change the appropriate target. Your prenatal care provider individualizes weight-gain goals using your full history, and their guidance should take priority over this general-purpose estimate.

Frequently Asked Questions

What formula does this pregnancy weight gain calculator use?
It uses the Institute of Medicine (2009) pregnancy weight gain guidelines for singleton pregnancies. Your pre-pregnancy BMI (weight in lbs times 703, divided by height in inches squared) sets your category - underweight, normal weight, overweight, or obese - and each category has its own total gain range and recommended weekly rate of gain for the second and third trimesters.
How is expected weight gain by week calculated?
For weeks 0-13 (first trimester), the IOM target of about 1.1 to 4.4 lbs total is applied in proportion to how far into the first trimester you are, the same for every BMI category. From week 14 onward, that first-trimester amount is added to your BMI category's weekly rate multiplied by the number of weeks past 13, giving a low-high expected range for your current week.
Is this result a medical diagnosis?
No - this calculator applies a population-based guideline, not a diagnosis or an individualized prescription. Gaining above or below the range is common and is not automatically a problem. Use the result as a discussion point with your prenatal care provider, who can factor in your full history.

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