My husband, two children and I moved to Japan (for 3 years) when my daughter was 20 months. Its a long trip! From the time we left our home in Illinois, drove to the airport, flew, layover, flew and drove to our new home was over 24 hours. Needless to say, we were exhausted. That being said, we chose to make the trip back and forth 4 times in 3 years but only because we learned a few things to help.
My advice is, if your OB says its safe and you and hubby would like to see Hong Kong, then definitely go. Don't miss out on a trip because of what could go wrong.
If you choose to make the trip to Hong Kong here's a few suggestions:
1. Definitely get your little one a seat. My daughter sat on my lap a lot but it was nice to have the extra room. She can kneel on the floor and play on the seat even.
2. My pediatrician actually recommended this so dont freak out about it. You can mash up half a Dramamine pill, mix with a little juice/milk and give to your daughter. It'll help in case of turbulence and make her a little drowsy too.
3. I found during our travels that the more you sleep on the plane, the better you adjust once you get there. Encourage your daughter (you and hubby too) to sleep as much as possible. Having an overnight flight, if possible, would be best.
4. Take as much food as allowed. I have found a lot of flights serve food from the country your heading towards. My little one didn't like Japanese food much so we always packed food we knew she'd like. Some flights have special kid's meals but you'll have to check and just because they say a kid's meal will be there for her, doesn't mean it actually will be. I've had it happen more than once.
5. Pack more diapers in your carry-on then you think you'll need. Also pack a change of clothes for everyone. You never know when turbulence can cause a spill or a nasty diaper that makes a mess on her and you. Also pack large baggies to put the potential wet/soiled clothing in.
6. When checking into your flight, request seating in the bulk head. Its the first row and always has more leg room/play room than the rest of the seating. (Unless you decided to upgrade to business)
7. Rest when you need to rest. Don't push you and your daughter too hard to see everything you want to see. If both of you are tired, its best to go back to the hotel and take the afternoon off from sightseeing. Even if everyone goes on without you.
Hope this helps. Sorry its so long. :-)