A healthy BMI for pregnancy is between 18.5 and 24.9, which is considered the normal weight range. Women in this category are recommended to gain between 11.5 kg and 16 kg (25 to 35 lbs) during their entire pregnancy. However, healthy pregnancies occur across all BMI categories — underweight, overweight, and obese women can all have healthy pregnancies with appropriate weight gain guidance from their midwife or doctor.

Recommended weight gain during pregnancy depends on your pre-pregnancy BMI. Underweight (BMI under 18.5): 12.5–18 kg (28–40 lbs). Normal 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+): 5–9 kg (11–20 lbs). For twins, weight gain recommendations are higher. These guidelines are based on the Institute of Medicine (IOM) and are widely used by NHS midwives in the UK.

BMI (Body Mass Index) is calculated using your pre-pregnancy weight, not your current pregnant weight. The formula is: BMI = weight in kilograms divided by height in metres squared (kg/m²). For example, if you weighed 65 kg and were 1.65 m tall, your BMI would be 65 ÷ (1.65 × 1.65) = 23.9. It’s important to use your pre-pregnancy weight because pregnancy weight gain is not considered body fat and would give an inaccurate BMI reading.

In the first trimester (weeks 1–12), weight gain is typically minimal — around 0.5 to 2 kg (1–4 lbs) total. Most women gain very little or even lose a small amount due to morning sickness. In the second and third trimesters, weight gain should be steady. For a normal BMI woman, this is approximately 0.4 to 0.5 kg (about 1 lb) per week. Underweight women may need to gain slightly more per week, while overweight and obese women should aim for slightly less, around 0.2 to 0.3 kg per week.

No, it is not safe to diet or restrict calories during pregnancy. Your baby needs a steady supply of nutrients for healthy development, and restrictive dieting can harm fetal growth. Even if you are in the obese BMI category, pregnancy is not the time to lose weight. Instead, focus on eating a balanced, nutrient-rich diet and engaging in safe, moderate exercise as recommended by your midwife. Any concerns about weight should be discussed with your healthcare provider.

If your pre-pregnancy BMI is very high (35+) or very low (under 17), your midwife or doctor will provide personalised guidance and may refer you to a specialist dietitian. Women with a high BMI may have additional monitoring during pregnancy and may be offered a glucose tolerance test. Women with a very low BMI may need extra nutritional support. In both cases, the goal is a healthy pregnancy, and your healthcare team will work with you to ensure both you and your baby are well cared for.

Pre-pregnancy BMI can influence certain aspects of delivery. Women with a higher BMI may have a slightly increased risk of complications such as gestational diabetes, pre-eclampsia, or needing a caesarean section. Women with a very low BMI may be at higher risk of having a small-for-gestational-age baby. However, the vast majority of women across all BMI categories have healthy pregnancies and deliveries. Regular antenatal care and following your midwife’s advice are the best ways to ensure a positive outcome.

It’s a good idea to start tracking your weight from your first antenatal appointment, usually around 8 to 12 weeks. Your midwife will record your weight at this visit and use it as a baseline. After that, weighing yourself once a week or once a fortnight at home is sufficient — there’s no need to weigh yourself daily as natural fluctuations in fluid retention can cause misleading changes. Bring your weight records to your antenatal appointments so your midwife can monitor your progress.