 [ Symptoms ] when should i start gaining weight during pregnancy [ Disease ] when it comes to pregnancy pounds timing matters chances are weight gain during the first three months will be minimal    and should be according to the iom institute of medicine guidelines you do not need any extra calories during the first trimester although the baby is growth is rapid he is so small that his development does not warrant additional energy however you may add some pounds to your frame during the first three months because you are hungrier or because of fluid retention some women even lose weight because they do not feel well enough to eat their usual diet once the second trimester starts and the baby is growth begins in earnest gaining weight on a steady basis is a must plan on putting on about a pound each week with a prepregnancy bmi of 24 9 and below and about a half pound a week if you had a prepregnancy bmi of 24 9 and above