I went to a public hospital too. If they say everything is normal, it means everything is normal. Don't worry. Weight is just an estimation, and is not exact. So they measure the size of the tummy to do the estimate.
I gained around 18 lbs only and my baby was over 6 pounds.
--> I was having check up at Tsan Yuk. as far as I know, unless you or the BB has any abnormality, the clinic only offers one ultrasound scan at around 25th week.
you can check at C+ of you wish. it costs about 1000. I went to the 胡寶星 clinic which is on the same building of Tsan Yuk to have 3D scan and found that my BB was undersize and they refered me to have follow up scan at Tsan Yuk. My BB continued to be that way after a few scans though I gained over 30 lbs during my pregnancy. she was 5lb11oz when born.
size doesn't mean everything. so less worry, both you and your BB will be more healthy. add oil ar!