At the start of pregnancy, your doctor may tell you your body mass index, or BMI.
This is an estimate of your body fat based on your weight and height. By calculating your BMI, your doctor can establish whether you are a healthy weight for your height or whether your weight could cause problems during pregnancy.
Your BMI is calculated by multiplying your weight in pounds by 703, dividing the answer by your height in inches, then dividing the answer by your height in inches again. A BMI of 18.5-24.9 is considered normal; 25-29.9 is considered overweight; 30-39.9 is classified as obese; and over 39 is very obese. If you have A BMI of below 18.5, you are thought to be underweight.
Being underweight in pregnancy can make you more likely to give birth prematurely, or have a "small-for-dates" baby. Being overweight means that you are at a higher risk of problems such as high blood pressure, preeclampsia, and gestational diabetes, and are more likely to have a larger baby. These factors increase the risk of complications at the time of delivery.