Before becoming pregnant, you are encouraged to try to attain a healthy body mass index (BMI). BMI is calculated by taking your weight (in kilograms) and dividing that by the square of your height (in metres). It is used to determine whether your weight may have an impact on your health, and is used in addition to waist circumference and/or skin fold thickness to check for a healthy weight.   

A BMI between 18.5 to 24.9 is considered within the healthy range, a BMI between 25 to 30 is considered overweight, and a BMI over 30 is considered obese. A BMI between 25 to 30 can increase your risk for certain conditions and pregnancy and birth complications, however, the increase in risk is slight. While a BMI over 30 can greatly increase your risk for certain conditions and pregnancy and birth complications.

*Note: While BMI can be a useful tool, it is not necessarily a comprehensive indicator of health. It doesn't account for differences in body fat and muscle mass or fitness levels, and there are also racial differences in BMI that can be differentially associated with health outcomes.

BMI 18.5 to 24.9. Women whose BMI is between 18.5 to 24.9 may ensure they maintain a healthy diet and exercise habits ahead of conception. A healthy diet and physical activity are important habits to ensure your health and the health of your future baby. See Lifestyle and Pregnancy for information about physical activity and diet and nutrition.

BMI 25 to 30. For women whose BMI is between 25 to 30 (what is considered the overweight range), the risks are slightly higher in comparison to women who have a lower BMI and are considered to be at a healthy weight. If your BMI is considered overweight, you may choose to safely achieve a healthy weight or a slight weight reduction to reduce any risks to your health and that of your baby. If you are unable to achieve a healthy weight or slight weight reduction before becoming pregnant, remember that the increase in risks are slight. Focusing on maintaining a healthy diet and physical activity habits regardless of your weight are important ways you can maintain your health and wellbeing and your baby’s health and wellbeing.

BMI 30+. For women whose BMI is above 30 (what is considered the obese range), these risks can be much higher and affect the health and wellbeing of the mother and baby. Women who are considered obese tend to have a higher risk for many serious conditions such as heart disease, type 2 diabetes and certain cancers (endometrial, breast, and colon). They are at especially high risk of complications such as C-section, pre-eclampsia, gestational diabetes, preterm delivery, labor induction, and postpartum hemorrhage (bleeding after birth). They also have higher rates of complications for their baby, such as shoulder dystocia (baby’s shoulder is stuck after head comes out); needing help breathing immediately after birth; poor heart rate, breathing, muscle tone, reflexes, and skin colour upon birth; and being abnormally large for gestational age. Women who are considered obese can also tend to have a harder time conceiving and are more likely to miscarry early in pregnancy and face challenges with production of breast milk after giving birth. 

If your BMI is considered obese, talk with your doctor about ways to reach and maintain a healthy weight before you get pregnant. Achieving a healthy weight or even a small reduction in weight can reduce the risk of pregnancy complications that may impact you or your baby. However, it is important to make sure that achieving a healthy weight does not compromise your mental health or wellbeing. Any dietary or physical activity changes should add to your wellbeing and mental health.

The key to achieving and maintaining a healthy weight is not about short-term dietary changes.  It’s about a lifestyle that includes healthy eating and regular physical activity to maintain your wellbeing and prevent illnesses. These things are also great to know and practice before you conceive so that you can teach your child healthy eating habits