Underweight

I'm 12w1d pre pregnancy I was around 95lb (I've been tiny my whole life born 5lb9oz) my first appointment was at 9w2d and I was 87lbs. My doctor told me to gain about 40lbs thoughtout the pregnancy. I have only thrown up 1 time. I eat both healthy food and occasionally some goodies but I eat regularly I don't skip meals and I eat inbetween meals also. I still haven't gained any weight even though I'm starting to show and they couldn't hear the heart beat but I have symptoms and no sign of miscarriage. I'm just scared the poor baby isn't growing