Everyone's different, some hold water and bloat, some don't! The recommendation is to put on roughly 1lb a week from second trimester onwards, you don't need to eat any additional cals until then and baby / uterus are too small to cause a significant amount of weight gain, so if this were the case it is likely to be from overeating or holding water. But it is down to each woman and he lifestyle choices you have including diet etc.