Gain weight in pregnancy.

I’m in week 29 and I’ve gained 2kg and my baby is 1.1kg. Doctor is concerned that I’ve not gained enough weight though my baby is growing well. I don’t really restrict my diet and have been drinking milk, eating loads of desserts and durians. I also exercise moderately with yoga and swimming at least 3 times a week How can I gain weight? I’ve always had high metabolism so gaining weight with food isn’t that easy for me..