Gusmom,
I assume that you will be 24 weeks pregnant and will have had no history of premature labour or pregnancy complications, in either pregnancy.
The risks of flying during pregnancy have been well evaluated in the literature. Consquently airlines, in line with the research, do not restrict pregnant flyers until the pregnancy is approximately 28 weeks gestation and often much later. You will need to carry an obstetric letter confirming your gestation and that you are fit to fly.
http://www.nhs.uk/Livewell/travelhealth/Pages/Travelandpregnancy.aspx
http://pregnancy.about.com/cs/travelduringpreg/a/aa052299.htm
If you fly to the US, you should consider the risk or likelihood of developing a pregnancy related disorder whilst you are visiting the US, which would prevent or delay your return to Hong Kong.
If you were forbidden/ advised, not to make the return trip, you have to ensure that your medical insurance covers you for worldwide healthcare including the US. If you purchase holiday insurance, determining whether the insurance covers pregnancy travel to the US, is significant. After a certain gestation, pregnancy is often an exclusion.
If you are not a US citizen, the potential costs for delivery of a baby in the US could be up to $10,000 USD. If your baby needed special care baby unit admission, that could be $1,000 USD per day. If you were significantly ill, the costs could be seriously elevated.
I have lived all over the world, amongst expat Mom's travelling by air during pregnancy, it is a usual part of their lives. Sometimes, there are women who fly on holiday and go into premature labour. Considering all the possibilitie, like you are doing, allows you to make an appropriate decision suitable for your needs.
Best wishes.