✔ 最佳答案
Usually doctor will arrange an ultrasound exam to check the size and structure of the bb at 20 weeks. If the bb is co-operate, you can see the sex.
However, it can also be seen at early gestation, but the accuracy may not be as good as 16 to 20 weeks.
As I know, some public hospitals (e.g. KWH / PWH) is free for this (provided you are their booked case and they have the quota). In private, they normal charge a few hundreds $500-800. Private doctor usually refer other specialist to do the exam.