18 weeks and up.
Anything before that could result in a mistaken gender (trust me, we know, lol.)
The fetus usually becomes one sex or the other at 13 weeks and the indifferent penis (penis with a vaginal opening) will either shrink down into a clitoris and vagina or the vaginal opening will close and the penis will grow. It is generally not visible until 16 weeks or more, but doctors generally will not do the ultrasound to determine gender until 18 weeks or more. Usually at 20 weeks.
at 8:52 PM on Apr. 23, 2009