It is usually possible to tell a baby's sex after about 18 weeks. However, at this stage it is still quite easy for the ultrasound to make a mistake, for instance if the baby is facing a different way or has its legs crossed in front. So hospitals often prefer not to tell the parents about the baby's sex at this point, as the information isn't completely reliable. It can be possible to tell even earlier - at around 12 weeks - but this is even less reliable and the technique would only be used if there was a special reason, such as wanting to check for a certain gender-related illness.

From 20 weeks it becomes much easier to determine the baby's gender, and this is around the time that most mothers have their scans. You can learn a little more about the process here, but your doctor will be able to tell you a lot more.