doctor advised

Any women have a doctor advise weight loss around 23 weeks pregnant I was 155 at 5'6 to start and now am 174, due to complications in my last pregnancy they want me a lower weight to avoid any risks possible..plus they said I'm a little ahead of myself..I'm classified as overweight also according to the bmi chart they said if I was 154 at the start I wouldn't have to worry so much 1 pound difference blows my mind on how you can be considered overweight or not..but what I'm asking is any women have a diet they followed that helped them all they did was give me a paper saying to watch portions and sweets...oh.. they also expect me down a couple pounds by my 27 weeks appointment.. I'm nervous and just want some help