Free Pregnancy Tool
Work out pre-pregnancy BMI, the range it falls in, and the weight gain usually recommended. Private - nothing leaves this page.
Height
Weight before pregnancy, or at the first antenatal appointment.
The NHS uses lower BMI ranges for these backgrounds.
The NHS advises not using a BMI calculator if you have, or think you may have, an eating disorder.
Please check the height and weight.
| Range | BMI | BMI (lower thresholds) | Total gain (one baby) |
|---|---|---|---|
| Underweight | Below 18.5 | Below 18.5 | 12.5–18 kg (28–40 lb) |
| Healthy weight | 18.5–24.9 | 18.5–22.9 | 11.5–16 kg (25–35 lb) |
| Overweight | 25–29.9 | 23–27.4 | 7–11.5 kg (15–25 lb) |
| Obese | 30 and above | 27.5 and above | 5–9 kg (11–20 lb) |
Lower thresholds apply to Asian, Middle Eastern, Black African and African-Caribbean family backgrounds (NHS). Weight gain ranges are US Institute of Medicine guidance; the UK has no formal target.
Body mass index (BMI) uses height and weight to show whether weight is in a healthy range for someone's height. In pregnancy, BMI is always worked out from the weight before pregnancy - not the current weight - because the bump, baby, and extra fluid would skew it.
BMI is weight in kilograms divided by height in metres squared. The result is placed in a range - underweight, healthy, overweight, or obese. People with Asian, Middle Eastern, Black African or African-Caribbean family backgrounds have a higher risk of conditions like type 2 diabetes at a lower BMI, so the NHS uses lower thresholds for them. The calculator also shows the total pregnancy weight gain usually recommended for each range (US Institute of Medicine guidance).
BMI is usually calculated at the booking appointment, and it can shape the care that's offered - extra scans, a glucose test, a higher dose of folic acid. Knowing what the number means helps you understand the plan, ask better questions, and back her up without making weight the topic.
FAQ
How BMI is used in pregnancy, and what the ranges mean for care.
Use the weight from before pregnancy, or the weight at the first antenatal appointment if that's all you have. Midwives calculate BMI in pregnancy from pre-pregnancy weight.
The healthy range is 18.5 to 24.9 for most people, or 18.5 to 22.9 for people with Asian, Middle Eastern, Black African or African-Caribbean family backgrounds. Most people outside these ranges still have a straightforward pregnancy - it just means extra care may be offered.
A higher BMI slightly raises the chance of complications such as gestational diabetes, high blood pressure and pre-eclampsia. With a BMI of 30 or above, the midwife may offer consultant-led care, a glucose tolerance test, and a higher dose of folic acid. Dieting in pregnancy isn't recommended.
With a BMI under 18.5, extra growth scans and support from a dietitian may be offered, as there's a higher chance of a smaller baby or early birth. Most people with a low BMI have a healthy pregnancy and baby.
BMI is a rough guide. It can't tell muscle from fat, so very muscular people may score high. The NHS advises not using a BMI calculator if you have, or think you may have, an eating disorder - speak to a GP or midwife instead.
Compare weight gained so far with the typical range for this week of pregnancy, based on pre-pregnancy BMI. Singles and twins.
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