# Pregnancy weight gain calculator

> Recommended pregnancy weight gain by pre-pregnancy BMI (IOM 2009), for singletons or twins, with the weekly rate and a week-by-week chart of your gain.

Interactive version: https://www.calcopenly.com/health/pregnancy-weight-gain-calculator
Subject: Health and fitness calculators

The recommended total weight gain in pregnancy depends on BMI before pregnancy, using the 2009 Institute of Medicine (now National Academies) guidelines: 12.5–18 kg (28–40 lb) if underweight, 11.5–16 kg (25–35 lb) at a normal weight, 7–11.5 kg (15–25 lb) if overweight and 5–9 kg (11–20 lb) with obesity. In the second and third trimesters the report sets a weekly rate for each group, and it assumes 0.5–2 kg (1.1–4.4 lb) is gained in the first trimester. Twin pregnancies have provisional, higher ranges.

People use it to see whether their gain so far is on track. At the defaults, 60 kg before pregnancy at 165 cm (BMI 22.0) and 64.5 kg at week 20, the 4.5 kg gained sits inside the 3.4–5.6 kg band for that week.

The week-by-week band joins the first-trimester assumption at week 13 to the total range at week 40 with straight lines; the IOM itself publishes only the totals and the weekly rates.

## Inputs

- **Height**: Accepts 5'11" or 5 ft 11 in
- **Weight before pregnancy**
- **Current weight**
- **Week of pregnancy**: Completed weeks since the first day of the last period.
- **Pregnancy** (options: One baby, Twins)
- **Show gains in** (options: kg, lb)

## Results

- Recommended total gain — main result
- Pre-pregnancy BMI (kg/m²)
- BMI category (IOM 2009)
- Gain so far
- Your gain so far
- Recommended gain by this week, from
- Recommended gain by this week, to
- Weekly gain after week 13, from
- Weekly gain after week 13, to
- Weight at term, from
- Weight at term, to

## Formula

$$
\text{gain}(w) = \begin{cases} F\,\dfrac{w}{13} & w \le 13 \\[4pt] F + (T - F)\,\dfrac{w - 13}{27} & 13 < w \le 40 \end{cases}\qquad F = 0.5\text{–}2\ \text{kg},\ T = \text{IOM total}
$$

## Worked examples

### 165 cm, 60 kg, week 20, now 64.5 kg

- Height: 165 cm
- Weight before pregnancy: 60 kg
- Current weight: 64.5 kg
- Week of pregnancy: 20 weeks
- Pregnancy: One baby
- Show gains in: kg
- **Pre-pregnancy BMI: 22.0 kg/m²**
- **BMI category (IOM 2009): Normal weight (18.5–24.9)**
- **Recommended total gain: 11.5–16 kg**
- **Gain so far: 4.5 kg**
- **Recommended gain by this week, from: 3.4 kg**
- **Recommended gain by this week, to: 5.6 kg**
- **Your gain so far: Within the range**
- **Weekly gain after week 13, from: 0.35 kg/week**
- **Weekly gain after week 13, to: 0.50 kg/week**
- **Weight at term, from: 71.5 kg**
- **Weight at term, to: 76.0 kg**
- Checked against: IOM 2009 Table S-1 (normal weight 11.5–16 kg, 0.35–0.50 kg/week). Python 3.8 fractions: 60/1.65² = 22.04; 0.5 + 11 × 7/27 = 3.3519; 2 + 14 × 7/27 = 5.6296

### 5 ft 4 in, 150 lb, week 30, now 165 lb, in pounds

- Height: 64 in
- Weight before pregnancy: 150 lb
- Current weight: 165 lb
- Week of pregnancy: 30 weeks
- Pregnancy: One baby
- Show gains in: lb
- **Pre-pregnancy BMI: 25.7 kg/m²**
- **BMI category (IOM 2009): Overweight (25.0–29.9)**
- **Recommended total gain: 15–25 lb**
- **Gain so far: 15.0 lb**
- **Recommended gain by this week, from: 9.9 lb**
- **Recommended gain by this week, to: 17.4 lb**
- **Your gain so far: Within the range**
- **Weekly gain after week 13, from: 0.50 lb/week**
- **Weekly gain after week 13, to: 0.70 lb/week**
- Checked against: IOM 2009 Table S-1 (overweight 15–25 lb, 0.5–0.7 lb/week). Python 3.8 decimal: 68.0389 kg / 1.6256² m = 25.75; 1.1 + 13.9 × 17/27 = 9.8519; 4.4 + 20.6 × 17/27 = 17.3704

### Obese, week 10 of the first trimester

- Height: 160 cm
- Weight before pregnancy: 85 kg
- Current weight: 86 kg
- Week of pregnancy: 10 weeks
- Pregnancy: One baby
- Show gains in: kg
- **Pre-pregnancy BMI: 33.2 kg/m²**
- **Recommended total gain: 5–9 kg**
- **Recommended gain by this week, from: 0.4 kg**
- **Recommended gain by this week, to: 1.5 kg**
- **Your gain so far: Within the range**
- **Weekly gain after week 13, from: 0.17 kg/week**
- **Weekly gain after week 13, to: 0.27 kg/week**
- Checked against: IOM 2009 Table S-1 (obese 5–9 kg, 0.17–0.27 kg/week). Python 3.8 fractions: 85/1.6² = 33.20; 0.5 × 10/13 = 0.3846; 2 × 10/13 = 1.5385

### Underweight at 40 weeks

- Height: 170 cm
- Weight before pregnancy: 50 kg
- Current weight: 65 kg
- Week of pregnancy: 40 weeks
- Pregnancy: One baby
- Show gains in: kg
- **Pre-pregnancy BMI: 17.3 kg/m²**
- **BMI category (IOM 2009): Underweight (under 18.5)**
- **Recommended total gain: 12.5–18 kg**
- **Recommended gain by this week, from: 12.5 kg**
- **Recommended gain by this week, to: 18.0 kg**
- **Your gain so far: Within the range**
- **Weekly gain after week 13, from: 0.44 kg/week**
- **Weekly gain after week 13, to: 0.58 kg/week**
- Checked against: IOM 2009 Table S-1 (underweight 12.5–18 kg, 0.44–0.58 kg/week); 50/1.7² = 17.30

### Twins, normal weight

- Height: 165 cm
- Weight before pregnancy: 60 kg
- Current weight: 66 kg
- Week of pregnancy: 20 weeks
- Pregnancy: Twins
- Show gains in: kg
- **Recommended total gain: 17–25 kg**
- **Recommended gain by this week, from: not applicable**
- **Weekly gain after week 13, from: not applicable**
- **Weight at term, from: 77.0 kg**
- **Weight at term, to: 85.0 kg**
- Checked against: IOM 2009 Summary: provisional twin range 17–25 kg (37–54 lb) at a normal weight; no weekly rates for twins

### Boundary: BMI 25.0 is overweight

- Height: 160 cm
- Weight before pregnancy: 64 kg
- Current weight: 66 kg
- Week of pregnancy: 16 weeks
- Pregnancy: One baby
- Show gains in: kg
- **Pre-pregnancy BMI: 25.0 kg/m²**
- **BMI category (IOM 2009): Overweight (25.0–29.9)**
- **Recommended total gain: 7–11.5 kg**
- **Recommended gain by this week, from: 1.2 kg**
- **Recommended gain by this week, to: 3.1 kg**
- **Your gain so far: Within the range**
- Checked against: 64/1.6² = 25.000; IOM uses WHO cut-offs, overweight 25.0–29.9. Python 3.8 fractions: 0.5 + 6.5 × 3/27 = 1.2222; 2 + 9.5 × 3/27 = 3.0556

## Questions

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

It depends on your BMI before pregnancy. The 2009 IOM guidelines recommend 28–40 lb (12.5–18 kg) for a BMI under 18.5, 25–35 lb (11.5–16 kg) for 18.5–24.9, 15–25 lb (7–11.5 kg) for 25.0–29.9 and 11–20 lb (5–9 kg) for 30 or more. CDC's pregnancy weight gain trackers use the same ranges. A woman 165 cm tall who weighed 60 kg before pregnancy has a BMI of 22.0 and a target of 11.5–16 kg.

### How much weight should you gain per week in pregnancy?

Little in the first trimester, about 0.5–2 kg (1.1–4.4 lb) in total, then a steady weekly amount. From week 14 the IOM rates are 0.44–0.58 kg (1–1.3 lb) a week if underweight, 0.35–0.50 kg (0.8–1 lb) at a normal weight, 0.23–0.33 kg (0.5–0.7 lb) if overweight and 0.17–0.27 kg (0.4–0.6 lb) with obesity.

### How much weight should you gain with twins?

The IOM's provisional twin ranges, for a pregnancy reaching term at 37–42 weeks, are 37–54 lb (17–25 kg) at a normal pre-pregnancy weight, 31–50 lb (14–23 kg) if overweight and 25–42 lb (11–19 kg) with obesity. The committee found too little data to set a twin range for women who were underweight, and it gave no weekly rates for twins.

### How much weight should you gain in the first trimester?

About 0.5–2 kg (1.1–4.4 lb) in total by the end of week 13, whatever the pre-pregnancy BMI. That is the assumption behind every total in the 2009 IOM table; the weekly rates only start in the second trimester. This calculator draws that gain as a straight line from week 0, which puts the band at 0.3–1.2 kg by week 8 and 0.5–2 kg by week 13.

### How accurate is the pregnancy weight gain calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “165 cm, 60 kg, week 20, now 64.5 kg” is checked against IOM 2009 Table S-1 (normal weight 11.5–16 kg, 0.35–0.50 kg/week). Python 3.8 fractions: 60/1.65² = 22.04; 0.5 + 11 × 7/27 = 3.3519; 2 + 14 × 7/27 = 5.6296.

### Where does the method come from?

Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. National Academies Press, 2009 (Summary, Table S-1); CDC — Weight gain during pregnancy; Gilmore LA, Redman LM. Weight gain in pregnancy and application of the 2009 IOM guidelines: toward a uniform approach. Obesity 2015;23:507–511.

## Sources

- [Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. National Academies Press, 2009 (Summary, Table S-1)](https://nap.nationalacademies.org/read/12584/chapter/2)
- [CDC — Weight gain during pregnancy](https://www.cdc.gov/maternal-infant-health/pregnancy-weight/index.html)
- [Gilmore LA, Redman LM. Weight gain in pregnancy and application of the 2009 IOM guidelines: toward a uniform approach. Obesity 2015;23:507–511](https://pmc.ncbi.nlm.nih.gov/articles/PMC4340812/)

_Note: medical information, not professional advice._