Pregnancy BMI Calculator
Calculate your BMI and get personalised weight gain recommendations based on NHS guidelines
Calculate Your Pregnancy BMI
Your Results
Weight Gain Recommendations
BMI Categories in Pregnancy
Underweight (BMI < 18.5)
Women with a BMI below 18.5 may need additional nutritional support during pregnancy. Recommended weight gain is typically 13-18kg (28-40 lbs).
Healthy Weight (BMI 18.5-24.9)
This is the ideal BMI range for pregnancy. Recommended weight gain is 11-16kg (25-35 lbs) for a singleton pregnancy.
Overweight (BMI 25-29.9)
Women in this category should aim for more modest weight gain of 7-11kg (15-25 lbs) to reduce pregnancy complications.
Obese (BMI ≥ 30)
Limited weight gain of 5-9kg (11-20 lbs) is recommended. Additional monitoring and care may be required during pregnancy.
Weight Gain During Pregnancy
Most pregnant women gain between 10kg and 12.5kg (22-28 lbs) during pregnancy, with most weight gain occurring after week 20. The weight you gain supports your baby’s growth and prepares your body for breastfeeding.
Where Does Pregnancy Weight Come From?
- Baby: 3-4kg (6.5-8.8 lbs)
- Placenta: 0.7kg (1.5 lbs)
- Amniotic fluid: 0.8kg (1.8 lbs)
- Breast tissue: 1kg (2.2 lbs)
- Blood volume: 1.5kg (3.3 lbs)
- Fat stores: 2-4kg (4.4-8.8 lbs)
- Fluid retention: 1-2kg (2.2-4.4 lbs)
Important Note
Your BMI calculation during pregnancy should be based on your pre-pregnancy weight, not your current pregnant weight. This calculator provides general guidance based on NHS recommendations, but individual circumstances may vary.
Healthy Pregnancy Weight Management
Healthy Eating During Pregnancy
Focus on eating a balanced diet rich in nutrients rather than counting calories. You don’t need to “eat for two” – only an extra 200 calories per day are needed in the last three months of pregnancy.
Key Nutrients for Pregnancy
- Folic Acid: 400mcg daily (5mg if BMI ≥30)
- Vitamin D: 10mcg daily
- Iron: Include iron-rich foods like lean meat, leafy greens
- Calcium: Essential for baby’s bone development
Safe Exercise During Pregnancy
Aim for at least 30 minutes of moderate exercise daily, such as walking or swimming. If you’re new to exercise, start with 15 minutes three times per week and gradually increase.
Risks Associated with BMI in Pregnancy
Higher BMI Risks
Women with a BMI of 30 or above have increased risks including:
- Gestational diabetes (3x higher risk)
- Pre-eclampsia (2-4x higher risk)
- Blood clots
- Longer labour and increased caesarean risk
- Baby complications including higher birth weight
Lower BMI Risks
Women with a BMI below 18.5 may experience:
- Increased risk of premature birth
- Low birth weight baby
- Nutritional deficiencies
Frequently Asked Questions
How often should I weigh myself during pregnancy?
Your midwife will weigh you at your booking appointment and may weigh you again at 28 and 36 weeks. Avoid frequent weighing at home as normal pregnancy weight fluctuations can cause unnecessary worry.
What if I’m gaining too much weight?
Focus on healthy eating rather than dieting. Pregnancy is not the time to lose weight. Speak to your midwife about nutrition advice if you’re concerned about excessive weight gain.
What if I’m not gaining enough weight?
Some naturally slim women remain slim during pregnancy and have healthy babies. However, insufficient weight gain can indicate nutritional deficiencies. Discuss your concerns with your healthcare provider.
Does my BMI affect where I can give birth?
Women with a BMI over 40 are recommended to give birth in hospital. Those with BMI 35-40 may have restricted birth place options depending on individual circumstances and previous birth history.
References
- NHS. Weight gain in pregnancy. NHS website. Available at: https://www.nhs.uk/pregnancy/related-conditions/common-symptoms/weight-gain/ (Accessed September 2025)
- Oxford University Hospitals NHS Foundation Trust. Pregnancy and body mass index (BMI) Information leaflet. June 2022.
- Royal College of Obstetricians and Gynaecologists. Being overweight in pregnancy and after birth. Patient information leaflet. July 2025.
- NHS Tayside. Weight in Pregnancy – optiMUM Programme. NHS Scotland.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 548: Weight gain during pregnancy. Obstet Gynecol. 2013;121:210-2.
- NICE. Maternal and child nutrition. NICE guideline [NG247]. Published 14 January 2025.
- Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington, DC: The National Academies Press; 2009.