A. The healthy weight gain during pregnancy rate should be: 4 pounds during the first trimester. If underweight, add 1 pound. 1 pound per week thereafter. Add ¼ pound if underweight. During the last month, it’s normal to gain less even though baby is still gaining. Average weight gain is 1-2 pounds, although that can vary. Most women gain weight during the second trimester, which coincides with the period of most rapid weight gain of baby (from 1 ounce to 2 pounds). It’s not abnormal to bounce up 5 to 10 pounds quickly between 15 and 20 weeks of pregnancy.