Pregnancy Weight Gain
Pregnancy is divided into three trimesters, with each trimester lasting three months, or approximately thirteen weeks (a normal pregnancy lasts 40 weeks). Recommendations for weight gain during pregnancy are based on the Institute of Medicine (IOM) definitions of prepregnancy BMI range. The BMI is defined as weight in pounds, divided by height in inches, divided by height in inches, multiplied by 703 (or weight in
kilograms, divided by height in centimeters, divided by height in centimeters, multiplied by ten-thousand). The majority of weight gain should occur in the second and third trimesters. Weight gain can vary greatly in normal pregnancies with normal birth outcomes. Few studies have included women in their first trimester, so the importance of first-trimester weight gain on pregnancy outcome is unclear. However, a slow and steady rate of weight gain is considered ideal. The current recommended weight gain for the BMI ranges are outlined in the accompanying figure.
Poor weight gain during pregnancy is associated with prematurity, low birth weight, and small for gestational age. Among normal-weight women, weight gain above the recommended level corresponds to maternal fat stores and is not of benefit to fetal growth. In other words, fat gain during pregnancy parallels gestational weight gain, and women with greater weight gain also gain more fat. In addition, an inverse relationship exists between pre-pregnancy BMI and weight gain during pregnancy: women with a low pre-pregnancy BMI tend to gain more weight than women with a high prepregnancy BMI. On average, overweight women gain less weight than their thinner counterparts, though it is not unusual for obese women to achieve normal birth outcomes with less than the recommended weight gain.
