No weight gain by the 20th week

Anyone else tiny and haven’t gained much weight by week 20? I started out 93lbs, I gained a few pounds and lost weight due to puking, I got down to 90lbs. I’m now 97lbs but just the other day I was 95 so it fluctuates. II’ve been trying to gain weight but it seems impossible. I’m not worried since my baby is measuring how he’s suppose to but I cannot gain weight for the life of me and they said I should gain 40lbs, good luck with that 😂