What's so important about BMI? Being too thin (a body mass index of less than 18.5) or overweight (a BMI of 25 or more) can affect your fertility and your baby's health.

Use our tool to calculate your pre-pregnancy BMI. This will give you a starting point for recommended weight gain. Women in the average BMI range should gain somewhere between 25-35 pounds during pregnancy, while women with a lower BMI should gain more, and women with a higher BMI should gain less, with their doctor's guidance.

UNDERWEIGHT (Less than 18.5) 28-40lbs recommended weight gain

NORMAL (18.5 to 24.9) 25-35lbs recommended weight gain

OVERWEIGHT (25.0 to 29.9) 15-25lbs recommended weight gain

OBESE (30 or greater) <15lbs recommended weight gain