BMI Calculator

BMI Calculator for Pregnancy

Pre-pregnancy BMI is an important health indicator that helps midwives and GPs assess risk and provide appropriate care. BMI should be calculated using pre-pregnancy weight, as weight gain during pregnancy is normal and expected.

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Use our bmi calculator with these considerations in mind.

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Key Considerations

  • Use pre-pregnancy weight for BMI calculation — not current pregnant weight
  • Pre-pregnancy BMI is recorded at your booking appointment (around 8-12 weeks)
  • BMI over 30 at booking may mean additional monitoring during pregnancy
  • BMI under 18.5 at booking may also require extra nutritional support
  • Weight gain during pregnancy varies: 11.5-16kg for normal BMI, less for higher BMI
  • The NHS does not recommend dieting during pregnancy regardless of BMI

Specific Adjustments

Underweight (BMI < 18.5)

Recommended gain: 12.5-18kg. You may receive additional nutritional support and monitoring from your midwife.

Overweight/Obese (BMI 25+)

Recommended gain: 7-11.5kg. Additional monitoring may be offered. Weight loss is not recommended during pregnancy.

Tips for Best Results

1Record your pre-pregnancy weight — this is what your midwife will use for BMI
2Do not attempt weight loss during pregnancy — focus on eating nutritiously
3If your BMI is above 30, your midwife will discuss any additional care needed
4Gestational diabetes screening is offered to all women with BMI over 30
5After pregnancy, allow at least 6 months before focusing on weight management

Frequently Asked Questions

How does BMI affect pregnancy?
Higher pre-pregnancy BMI is associated with increased risk of gestational diabetes, pre-eclampsia, and delivery complications. However, many women with higher BMI have perfectly healthy pregnancies. Additional monitoring helps manage any risks.
Should I lose weight before getting pregnant?
If your BMI is above 30, achieving a healthier weight before pregnancy can reduce risks. The NHS recommends a gradual, healthy approach to weight management before conception rather than crash dieting.
How much weight should I gain during pregnancy?
NHS guidance: BMI 18.5-24.9 → 11.5-16kg gain. BMI 25-29.9 → 7-11.5kg. BMI 30+ → 5-9kg. These are guidelines, not rigid targets. Your midwife will monitor your health individually.

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