They usually do a routine checkup/ultrasound in between 16 &20wks just to check the health of the baby, &they'll ask you if you want to know the sex.

Sometimes it isn't always possible to determine the sex by ultrasound. And many doctors do not think that determining gender is a good enough reason to order an ultrasound (they can be costly). Ultrasounds are used primarily for making sure that your baby is growing healthy. My 20 week ultrasound was not able to tell me boy or girl and I hear that happens alot. Luckily for me, I was able to have another one at 23 weeks b/c my little baby wouldn't sit still the first time, and they were not able to get accurate measurements ( the whole reason for getting an ultrasound). However, many women will only get the 20 week scan and then no others. So don't be too set upon finding out the gender b/c your baby may not cooperate :)