Sorry, this is a major pet peeve of mine and I had to say it.
Gender is a psychological/social construct that can take years to develop.
Sex is the biological makeup that makes you male or female.
Oh and bonus PSA: the 20 week scan is not a "gender" scan or even a sex scan. It is an anatomy scan.
::gets down off soap box::
You can expect one of these posts a few times a month.