Body Mass Index (BMI) is a measurement used to determine whether a person’s weight is within a healthy range based on their height. While BMI is useful before pregnancy, its role during pregnancy changes as weight gain is expected for the baby’s development.
Knowing your pre-pregnancy BMI is important because it helps healthcare providers:
✔ Assess the risk of pregnancy complications.
✔ Provide personalised weight gain recommendations.
✔ Monitor maternal and baby health throughout pregnancy.
This guide explains how BMI is used during pregnancy, its effects on weight gain, labour, and post-pregnancy recovery, and how to maintain a healthy weight for a safe pregnancy.
BMI is calculated using the following formula:
Metric Formula:
BMI = Weight (kg) ÷ Height (m²)
Imperial Formula:
BMI = (Weight (lbs) ÷ Height (inches²)) × 703
However, BMI is only relevant before pregnancy since weight gain is normal during pregnancy. Instead of BMI, doctors track:
✔ Total weight gain throughout pregnancy.
✔ Baby’s growth via ultrasound and check-ups.
✔ Blood pressure and glucose levels to monitor health risks.
| Pre-Pregnancy BMI Range | Category | Potential Risks During Pregnancy |
|---|---|---|
| Below 18.5 | Underweight | Risk of premature birth, low birth weight baby |
| 18.5 – 24.9 | Healthy Weight | Lowest health risks for mother and baby |
| 25 – 29.9 | Overweight | Higher risk of gestational diabetes, high blood pressure |
| 30 and above | Obese | Increased complications during pregnancy and delivery |
Find Your Pre-Pregnancy BMI Category
A healthy pre-pregnancy BMI reduces the risk of complications, while a very high or low BMI may require additional monitoring.
| Pre-Pregnancy BMI | Recommended Weight Gain |
|---|---|
| Underweight (BMI < 18.5) | 12.5 – 18 kg (28 – 40 lbs) |
| Healthy Weight (BMI 18.5 – 24.9) | 11.5 – 16 kg (25 – 35 lbs) |
| Overweight (BMI 25 – 29.9) | 7 – 11.5 kg (15 – 25 lbs) |
| Obese (BMI 30 and above) | 5 – 9 kg (11 – 20 lbs) |
NHS Pregnancy Weight Gain Guidelines
The NHS advises gradual weight gain:
✔ First trimester: 1–2 kg (2–5 lbs).
✔ Second & third trimesters: ~0.5 kg (1 lb) per week.
✔ Higher risk of preterm labour.
✔ Low birth weight babies may need neonatal care.
✔ Inadequate nutrient intake can affect baby’s growth.
✔ Gestational diabetes – Increases risk of large baby and C-section.
✔ High blood pressure and preeclampsia – Can lead to preterm birth.
✔ More difficult labour and delivery complications.
✔ Eat whole grains, lean proteins, fruits, and vegetables.
✔ Avoid excess sugar, processed foods, and trans fats.
✔ Follow the NHS Eatwell Guide for pregnancy nutrition.
✔ Walking, swimming, and prenatal yoga are ideal.
✔ Strength training (with light weights) can maintain muscle tone.
✔ Avoid high-impact or contact sports.
✔ Increased risk of long labour and C-section.
✔ Higher chance of needing labour induction.
✔ Epidural or anaesthesia difficulties due to excess weight.
✔ Increased risk of preterm birth.
✔ Lower birth weight babies may require neonatal care.
✔ Most women lose half their pregnancy weight within six weeks.
✔ Gradual weight loss is healthier than crash dieting.
✔ Breastfeeding can help burn extra calories.
✔ Regular walking and light exercise improve recovery.
✔ Eat nutrient-rich meals rather than skipping meals.
✔ Yes, being underweight or overweight can lead to irregular menstrual cycles.
✔ No, BMI calculators do not apply during pregnancy due to natural weight gain.
✔ Plan for a healthy pregnancy with NHS-approved weight guidelines.
✔ Find out your weight status before conception.
