Yeah they can
in the uk at the 20 week scan you can ask them to tell you the sex of the baby, they may not do so unless you ask. You can also book a privet scan that will tell you the sex of the baby from 16 weeks but it may cost around £100.00. (sorry cant remember what the companys called)
they wont tell you the sex of the baby before then due to the fact that some people will have late terminations if the baby is not the sex they want as horrible as that sounds. your baby's external genitalia are normaly recognizably but still not fully formed from around 12 - 14 weeks.
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