How much weight to gain in pregnancy, by your starting BMI
This pregnancy weight gain calculator shows the healthy range of weight gain recommended for your whole pregnancy, and roughly where you might be by this week, based on your body mass index (BMI) before you became pregnant. It follows the 2009 guidelines of the US Institute of Medicine (now the National Academy of Medicine), which are used by doctors and midwives in many countries.
Enter your height, your weight before pregnancy, your current weight and the week you are in. It works in kg and cm or in lb and feet, covers one baby or twins, and draws your gain on a week-by-week chart.
It is information only and not a diagnosis. Weight is just one sign of a healthy pregnancy, and your care team knows your situation best.
How to use the pregnancy weight gain calculator
- Choose kg / cm or lb / ft, and One baby or Twins.
- Enter your height and your weight before pregnancy (or at your first antenatal visit if you are not sure).
- Enter the week of pregnancy and, if you like, your current weight.
- Read your recommended total gain, the suggested range for this week and where your gain sits on the chart.
Pregnancy Weight Gain Calculator features
IOM 2009 ranges
Totals and weekly rates for each BMI group, with the IOM’s own pound figures in lb mode.
Week-by-week chart
A shaded band from conception to 40 weeks with your current gain marked.
Twins
The provisional IOM twin ranges, with a clear note where the IOM gives none.
Metric or imperial
Switch units at any time; your numbers are converted for you.
Recommended weight gain (IOM 2009)
Total gain for the whole pregnancy by BMI before pregnancy, and the average rate in the second and third trimesters. Source: Institute of Medicine and National Research Council, Weight Gain During Pregnancy: Reexamining the Guidelines (National Academies Press, 2009); twin figures are the IOM’s provisional ranges. Checked against the CDC summary (updated 15 May 2024) on 24 October 2026.
| BMI before pregnancy | One baby | Rate, 2nd–3rd trimester | Twins |
|---|---|---|---|
| Underweight (under 18.5) | 12.5–18 kg (28–40 lb) | 0.51 kg (1 lb) a week | no IOM range (CDC: 50–62 lb) |
| Healthy weight (18.5–24.9) | 11.5–16 kg (25–35 lb) | 0.42 kg (1 lb) a week | 17–25 kg (37–54 lb) |
| Overweight (25.0–29.9) | 7–11.5 kg (15–25 lb) | 0.28 kg (0.6 lb) a week | 14–23 kg (31–50 lb) |
| Obese (30 or more) | 5–9 kg (11–20 lb) | 0.22 kg (0.5 lb) a week | 11–19 kg (25–42 lb) |
How the week-by-week range is worked out
The IOM assumes a gain of 0.5–2 kg (about 1–4 lb) in the first trimester. The calculator draws a straight line from zero to that range at week 13, then another straight line to your recommended total at week 40. For example, with a healthy-weight BMI at week 24 the suggested range is 0.5 + (11.5 − 0.5) × 11/27 = 5.0 kg to 2 + (16 − 2) × 11/27 = 7.7 kg.
This is an approximation: real gain is not a straight line, and the IOM publishes totals and average rates, not a weekly chart. For twins the IOM gives totals only, so the chart shows them as a box at full term.
Things to know
- BMI uses your weight before pregnancy, not your current weight. The BMI calculator explains the categories.
- The IOM uses WHO BMI cut-offs. Some Asian and Indian guidance uses lower cut-offs for overweight and obesity; ask your doctor which applies to you.
- Gaining less or more than the range is common and not a diagnosis. Sudden gain, especially with swelling of the face or hands, needs prompt medical advice.
- To date your pregnancy, use the due date calculator.
Pregnancy Weight Gain Calculator FAQ
How much weight should I gain during pregnancy?
It depends on your BMI before pregnancy: 12.5–18 kg if underweight, 11.5–16 kg if a healthy weight, 7–11.5 kg if overweight and 5–9 kg if obese, for one baby (IOM 2009).
How does a pregnancy weight gain calculator work in kg?
Choose kg / cm, enter your height and weights in metric, and the pregnancy weight gain calculator shows the IOM ranges in kilograms.
How much weight should I gain with twins?
For a healthy-weight BMI the IOM’s provisional range is 17–25 kg (37–54 lb); it is 14–23 kg if overweight and 11–19 kg if obese.
How much weight do you gain in the first trimester?
Usually little — the IOM assumes 0.5–2 kg (about 1–4 lb), and some people lose weight because of sickness.
Is it bad if I am above or below the range?
Not necessarily. The ranges are guides for populations. Your midwife or doctor will look at your health and your baby’s growth and advise you.
Do these guidelines apply in New Zealand, the UK and India?
They are US guidelines. New Zealand’s Ministry of Health aligned its Guidance for Healthy Weight Gain in Pregnancy (June 2014) with them. Other countries, including the UK and India, have their own antenatal advice and Indian practice may use lower BMI cut-offs, so follow your local care team.