I completely understand why all my friends chose to find out their baby's sex via sonogram, before l3b.co middle-aged sonographer thought I was joking when I told her that I didn't want to know. “Really? You're the first person in months to not want to know,” she said. But it wasn't that I didn't. It involves inserting a needle into the uterus to remove a small amount of amniotic fluid. We usually do this test at around 16 weeks, so in this instance, the mother can find out the baby's sex a little sooner. This test isn't without risk, however, and isn't generally performed simply to determine the baby's sex. Boy or Girl?. 17 Aug From early on in pregnancy, one of the first questions you might have is when can you tell the sex of the baby. There are multiple ways to potentially determine whether you're having a boy or girl, but not all of them may be accurate at all times. Take a look at what doctors use when.