# How much pregnancy weight gain?

Finds the recommended pregnancy weight gain for your pre-pregnancy BMI from the IOM 2009 guidelines, the range for your week of pregnancy, and whether your gain so far is within it.

- Page: https://www.acalculator.org/health/pregnancy-weight-gain-calculator
- JSON spec: https://www.acalculator.org/health/pregnancy-weight-gain-calculator.json
- Version: cb1ecb457c44

## Default answer

Example with the default inputs (Height 5 ft 5 in, Weight before pregnancy 140 lb, Weight now 152 lb, Week of pregnancy 20, Babies One): With a pre-pregnancy BMI of 23.3 (Normal weight), the IOM recommends a total gain of 25 to 35 lb (11.5 to 16 kg).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| height | Height | Your height. |
| before | Weight before pregnancy | Your weight before you were pregnant. |
| now | Weight now | Your weight today, to compare your gain so far with the range. |
| week | Week of pregnancy | Completed weeks of pregnancy, 0 to 42. |
| babies | Babies | One baby or twins. For twins the IOM gives total ranges only. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| total | Recommended total gain | The IOM 2009 total gain for the pre-pregnancy BMI (and twins), as the IOM table writes it. |
| bmi | Pre-pregnancy BMI | Weight before pregnancy ÷ height², to one decimal place. |
| category | BMI category | The IOM (WHO) category of the pre-pregnancy BMI. |
| rate | Weekly gain, second and third trimesters | The IOM weekly rate range for one baby after the first trimester. |
| soFarLow | Range this week: from | The low end of the recommended gain by the week entered. |
| soFarHigh | Range this week: up to | The high end of the recommended gain by the week entered. |
| gained | Gained so far | Weight now minus weight before pregnancy. |
| status | Your gain so far | Below, within or above the range for the week, compared in pounds to one decimal place. |

## Method

BMI = weight before ÷ height²; the IOM range for the BMI category; by week w: 0.5 × w ÷ 13 to 2 × w ÷ 13 kg up to week 13, then 0.5 + (w − 13) ÷ 27 × (total low − 0.5) to 2 + (w − 13) ÷ 27 × (total high − 2) kg up to week 40.

## Assumptions

- The ranges are the Institute of Medicine 2009 guidelines, by pre-pregnancy BMI category (WHO cut-offs), read from the BMI shown to one decimal place.
- The week-by-week range is a straight line from nothing at conception to the IOM first-trimester gain (0.5 to 2 kg) at 13 weeks, then a straight line to the IOM total at 40 weeks.
- For twins the IOM gives only provisional total ranges, and none for an underweight BMI.
- This is general guidance, not medical advice; your provider may set a different goal.

## Worked examples

1. height = 1.651, before = 63.502932, now = 68.94604, week = 20, babies = one gives bmi = 23.3, category = Normal weight, total = 25 to 35 lb (11.5 to 16 kg), rate = 0.8 to 1 lb (0.35 to 0.50 kg) a week, soFarLow = 3.351852, soFarHigh = 5.62963, gained = 5.443108, status = Within the range for week 20. Source: Institute of Medicine 2009, Table S-1 (normal weight, 11.5 to 16 kg; first trimester 0.5 to 2 kg).
2. height = 1.6, before = 80, now = 82, week = 10, babies = one gives bmi = 31.3, category = Obese, total = 11 to 20 lb (5 to 9 kg), soFarHigh = 1.538462, status = Above the range for week 10.
3. height = 1.7, before = 75, week = 30, babies = twins gives bmi = 26, category = Overweight, total = 31 to 50 lb (14 to 23 kg).

## FAQ

### How much weight should I gain during pregnancy?

It depends on your BMI before pregnancy. The IOM 2009 guidelines for one baby: underweight (BMI under 18.5) 28 to 40 lb; normal weight (18.5 to 24.9) 25 to 35 lb; overweight (25.0 to 29.9) 15 to 25 lb; obese (30.0 and over) 11 to 20 lb.

### How much should I gain each week?

After the first trimester, the IOM ranges per week are 1 to 1.3 lb (underweight), 0.8 to 1 lb (normal weight), 0.5 to 0.7 lb (overweight) and 0.4 to 0.6 lb (obese). The IOM assumes a gain of 1.1 to 4.4 lb (0.5 to 2 kg) in the first trimester.

### How is the range for my week worked out?

The IOM gives totals and weekly rates, not a table for each week. This calculator draws a straight line from no gain at the start to the IOM first-trimester gain (0.5 to 2 kg) at 13 weeks, then a straight line from there to the IOM total at 40 weeks. The chart shows both lines.

### What about twins?

The IOM gives provisional totals for twins: 37 to 54 lb for normal weight, 31 to 50 lb for overweight and 25 to 42 lb for obese. It gives no range for underweight women with twins, and no weekly ranges for twins.

### Why is my BMI category based on my weight before pregnancy?

The guidelines are set by pre-pregnancy BMI, because body size before pregnancy changes how much gain is healthy. The categories are the WHO BMI cut-offs, read from the BMI to one decimal place.

### I am above or below the range. What should I do?

Talk to your obstetric provider. Weight changes from week to week, and many things affect it. Your provider can look at your gain over time and set a goal for you.

## Sources

- Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington, DC: The National Academies Press; 2009. Summary, Table S-1, and the provisional guidelines for twins. https://doi.org/10.17226/12584
- Centers for Disease Control and Prevention. Weight Gain During Pregnancy (May 15, 2024). https://www.cdc.gov/maternal-infant-health/pregnancy-weight/index.html