What's the most weight in a week in first trimester?

Quite variable. Most women gain about 2-4lbs.During the first trimester and 1 lb./wk from there. Some none, other my lose due to n/v. An average weight woman should gain a total of 25-35 lbs., underweight women 28-40 lbs., and overweight women 15-25 lbs. You don't have to "eat for two", but be sure to eat a variety of nutritious foods. Moderate exercise is also recommended, but talk to your doctor for more info.