# Pregnancy Weight Gain Calculator

Last reviewed: 2026-11-11

Enter your height, your weight before pregnancy and how many weeks along you are. You get the recommended total gain for your BMI, the range for this week, and how many extra calories this trimester usually needs.

## Quick Answer: How much weight should I gain during pregnancy?

It depends on your BMI before pregnancy. For one baby, the Institute of Medicine recommends 28–40 lb (12.5–18 kg) if you were underweight, 25–35 lb (11.5–16 kg) at a healthy weight, 15–25 lb (7–11.5 kg) if overweight and 11–20 lb (5–9 kg) with obesity. Most of it comes after the first trimester, at about 0.8–1 lb (0.35–0.5 kg) a week for a healthy BMI.

- First trimester: about 1–4.5 lb (0.5–2 kg) in total for most people.
- Second and third trimesters: a steady weekly gain that depends on your starting BMI.
- Twins: higher provisional ranges, such as 37–54 lb (17–25 kg) at a healthy BMI.
- Calories: about 0 extra in the first trimester, 340 in the second and 450 in the third.

Weight gain in pregnancy is not a single number. The healthy range depends on where you started, and it builds slowly through the first trimester before picking up from about week 14.

This calculator uses the Institute of Medicine (2009) guidelines that ACOG and the CDC follow, so the range it shows is the one your provider is likely to use as a starting point.

Use the calculator: https://www.slimai.ai/calculators/pregnancy-weight-gain-calculator

## Recommended Total Gain for One Baby

Institute of Medicine (2009) ranges by pre-pregnancy BMI, used by ACOG and the CDC.

### BMI below 18.5 — 28–40 lb (12.5–18 kg)

Underweight before pregnancy. About 1–1.3 lb (0.44–0.58 kg) a week in the second and third trimesters.

### BMI 18.5–24.9 — 25–35 lb (11.5–16 kg)

Healthy weight before pregnancy. About 0.8–1 lb (0.35–0.5 kg) a week in the second and third trimesters.

### BMI 25–29.9 — 15–25 lb (7–11.5 kg)

Overweight before pregnancy. About 0.5–0.7 lb (0.23–0.33 kg) a week in the second and third trimesters.

### BMI 30 and above — 11–20 lb (5–9 kg)

Obesity before pregnancy. About 0.4–0.6 lb (0.17–0.27 kg) a week in the second and third trimesters.

## Why your pre-pregnancy BMI sets the range

Gaining too little in pregnancy is linked with a higher chance of preterm birth and a baby who is small for gestational age. Gaining too much is linked with gestational diabetes, high blood pressure, a large baby, caesarean birth and weight that stays on after the baby arrives.

Where those risks balance out depends on how much energy and tissue you started with, which is why the Institute of Medicine ties each range to pre-pregnancy BMI rather than giving everyone the same number.

## How much weight to gain by week

The first trimester adds very little: the guidelines assume about 0.5–2 kg (1–4.5 lb) by week 13, and nausea means some people gain nothing or lose a little. From week 14 the gain becomes steady, at a weekly rate that depends on your BMI.

Real weigh-ins are not a straight line. Water, digestion, clothes and the time of day move the scale by a kilo or more, so compare your trend over three or four weeks with the range rather than reacting to one reading.

## How many extra calories do you need?

For one baby, most people need no extra calories in the first trimester, about 340 extra a day in the second and about 450 extra a day in the third. That is roughly a Greek yogurt with fruit and nuts in the second trimester, or a sandwich and a glass of milk in the third.

Start from your pre-pregnancy maintenance calories — the TDEE calculator gives an estimate — and add the trimester amount. Our guide to how many calories to eat during pregnancy explains what to focus on beyond the number.

## Twins and other special cases

With twins the Institute of Medicine gives provisional total ranges: 37–54 lb (17–25 kg) at a healthy BMI, 31–50 lb (14–23 kg) if overweight and 25–42 lb (11–19 kg) with obesity. It set no range for underweight twin pregnancies and no week-by-week rates, so this calculator shows the total only.

For triplets, teenage pregnancy, a history of eating disorders or bariatric surgery, your care team will set a target specifically for you.

## Pregnancy Weight Gain Chart (One Baby)

Recommended total gain so far, by week, for a healthy pre-pregnancy BMI (18.5–24.9), from the IOM first-trimester assumption plus 0.35–0.5 kg a week afterwards.

| Week | Total gain so far (kg) | Total gain so far (lb) |
|---|---|---|
| Week 8 | 0.3 – 1.2 kg | 0.7 – 2.7 lb |
| Week 13 | 0.5 – 2.0 kg | 1.1 – 4.4 lb |
| Week 20 | 3.0 – 5.5 kg | 6.5 – 12.1 lb |
| Week 24 | 4.4 – 7.5 kg | 9.6 – 16.5 lb |
| Week 28 | 5.8 – 9.5 kg | 12.7 – 20.9 lb |
| Week 32 | 7.2 – 11.5 kg | 15.8 – 25.4 lb |
| Week 36 | 8.6 – 13.5 kg | 18.8 – 29.8 lb |
| Week 40 | 10.0 – 15.5 kg | 21.9 – 34.2 lb |

A straight-line guide from the IOM rates, rounded to 0.1. Real gain is uneven week to week. Other BMI bands use the weekly rates shown above.

## How to Use the Pregnancy Weight Gain Calculator

You need four numbers, and the last one is optional.

1. **Enter your height** — In centimetres or feet and inches.
2. **Enter your weight before pregnancy** — Your weight just before you conceived, or at your first appointment if you are not sure. This sets your BMI band.
3. **Enter how many weeks pregnant you are** — Counted from the first day of your last period, as your provider counts it.
4. **Add your current weight (optional)** — The calculator shows whether your gain so far is below, within or above the range for this week.

## The Formulas Behind This Calculator

Computed in your browser. Nothing is uploaded and nothing is stored between visits.

- **Pre-pregnancy BMI:** `BMI = weight before pregnancy (kg) ÷ height (m)²` — Bands: under 18.5, 18.5–24.9, 25–29.9, 30 and above (WHO).
- **Range up to week 13:** `gain = (0.5 to 2 kg) × week ÷ 13` — The IOM assumes 0.5–2 kg in the first trimester for every BMI band.
- **Range from week 14:** `gain = (0.5 to 2 kg) + weekly rate × (week − 13)` — Weekly rates: 0.44–0.58, 0.35–0.50, 0.23–0.33 and 0.17–0.27 kg by BMI band (IOM 2009).
- **Extra calories:** `first trimester +0 · second +340 · third +452 kcal/day` — Singleton pregnancy, on top of pre-pregnancy needs (IOM 2005 energy requirements).

## Sources and Method

- **[Institute of Medicine and National Research Council (2009) — Weight Gain During Pregnancy: Reexamining the Guidelines](https://nap.nationalacademies.org/catalog/12584/weight-gain-during-pregnancy-reexamining-the-guidelines)** — Total and weekly gain ranges by pre-pregnancy BMI for singleton pregnancies, the first-trimester assumption of 0.5–2 kg, and provisional ranges for twins.
- **[ACOG Committee Opinion 548 — Weight Gain During Pregnancy](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/01/weight-gain-during-pregnancy)** — Endorses the IOM ranges and recommends calculating BMI at the first prenatal visit.
- **[CDC — Weight Gain During Pregnancy](https://www.cdc.gov/maternal-infant-health/pregnancy-weight/index.html)** — Public summary of the IOM ranges for one baby and twins.
- **Institute of Medicine (2005) — Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids** — Estimated energy requirement in pregnancy: no addition in the first trimester, +340 kcal/day in the second and +452 kcal/day in the third.

## Frequently Asked Questions

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

For one baby, the Institute of Medicine recommends 28–40 lb (12.5–18 kg) if your pre-pregnancy BMI was under 18.5, 25–35 lb (11.5–16 kg) at 18.5–24.9, 15–25 lb (7–11.5 kg) at 25–29.9 and 11–20 lb (5–9 kg) at 30 or above. Your provider may adjust this for you.

### How much weight should I gain each week of pregnancy?

Very little in the first trimester — about 0.5–2 kg (1–4.5 lb) in total by week 13. After that, about 0.35–0.5 kg (0.8–1 lb) a week at a healthy pre-pregnancy BMI, 0.44–0.58 kg if underweight, 0.23–0.33 kg if overweight and 0.17–0.27 kg with obesity.

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

The IOM provisional ranges for twins are 37–54 lb (17–25 kg) at a healthy BMI, 31–50 lb (14–23 kg) if overweight and 25–42 lb (11–19 kg) with obesity. There is no IOM range for underweight twin pregnancies, so your provider will set one.

### How many extra calories do I need when pregnant?

For one baby, usually none in the first trimester, about 340 extra calories a day in the second and about 450 extra a day in the third, on top of what you needed before pregnancy. Twins need more; ask your provider for a target.

### Is it normal to lose weight in the first trimester?

A small loss from nausea and food aversions is common in the first trimester. Tell your provider if you keep losing weight, cannot keep food or fluids down for more than a day, or feel dizzy and weak.

### What if I am gaining more than the range?

Mention it at your next appointment rather than dieting on your own. A calorie deficit is not recommended in pregnancy unless your obstetric provider sets one. Often small changes — fewer sugary drinks, regular meals, daily walks — are enough.

### Which weight do I use if I do not know my pre-pregnancy weight?

Use the weight recorded at your first prenatal visit. In the first few weeks it is usually within a kilo or two of your pre-pregnancy weight, which is how ACOG suggests BMI is calculated.

### Can I use SlimAI while pregnant?

SlimAI is a general calorie and macro tracker, not a pregnancy app, and it does not create pregnancy-specific targets. If you want to log meals, choose a Maintain weight goal and, with SlimAI Pro, enter the calorie and protein targets your provider gives you.

Source: https://www.slimai.ai/calculators/pregnancy-weight-gain-calculator

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