我有BB about 5-6 weeks,但照超聲波又照唔到fetal heart, but HCG is上升正常　,

you would see some schedules for development of human fetus. the fetal heart can usually be seen at as early as 6-7 weeks. those are theoretical; clinically, there may be some problems, e.g. wrong dating, late ovulation etc, which makes you see the heart at a later date. This is common, and totally normal.
at near 8-9 weeks by dating, you should see the heart beat by transabdominal ultrasound
if you have the fetal sac, and your hCG doubles in 2 days, there is nothing to worry about.
some people on this web would tell you not to do ultrasound too early, or too frequent, as ultrasound is harmful to the fetus. this is ridiculous and not evidence-based. ultrasound is safer than x-ray as it does not impose a radiation on the fetus, so do it when needed. it is true that AT LEAST you need two ultrasounds at early pregnancy (about 7-13 weeks) for dating (check the size) +/- OSCAR (around 10-13 weeks); and then one more morphology scan at 20 weeks. this is the basic, but doesnt mean you cannot have more.

m~可能係太早照啦,所以唔係好清楚卦
我本人就唔係香港既,係外國既醫生都
唔見意咁早去照bb,都係叫你大約18weeks
先好去照,但我8weeks都死去照,個醫生
都好難照到d咩,因為係太細粒啦,而且我
又飲得唔夠多水,所以搞左好耐先照到d仔
睇到心跳(因為跳得好勁),因為你宜家先5-6weeks,
bb既體積都只係同d手指甲咁大,或者你等
到一,兩個月先啦or最好18weeks先好去照
記得要飲多d水先好去照會好d,照前一個鐘
要忍尿,放心啦,平常心就ok啦