Gain only 15 lbs for the whole pregnancy?

My doctor suggested I gain 15-20 lbs for my whole pregnancy, I thought it was doable but I'm at 26 weeks and have just reached 15 lbs. I don't think I can only gain 5 more lbs in 14 weeks.  I eat healthy for the most part, I'll occasionally go out for dinner on special occasions. I could walk more but it hurts my pubic symphysis when I do. Have any of you ladies kept on track and stayed within the 15-20 range? If so how did you do it?