Many expecting parents are anxious to find out the sex of their unborn baby, and I frequently get asked, "How early can we find out what it is?" If you have done any research on this, chances are you've come across comments from people in mommy blogs or Facebook groups claiming that their ultrasound tech told them it was a boy or girl as early as 11- 12- 13 weeks. Now, even though I like to think that I'm fairly good at determining what it is, I like to tell my patients that finding the sex through ultrasound is never 100% accurate... you can't really know "for sure" (I'm sure you've all heard stories about someone who thought they were having a male or female, and were shocked when the baby came out the opposite). But when it comes to finding the sex in the first trimester, it is even less accurate. In fact, it is mostly a guess based upon a theory that doesn't ALWAYS hold true for every fetus.
A baby's sex organs develop between week 7 and week 12, but a penis and clitoris remain approximately the same size until approximately week 14, which is why differentiating between the two at that stage can be tricky. But if you examine the genital tubercle in a sagital plane (at an angle similar to the one you'd need to view the profile of the face- only you would be looking at the lower half of the body instead of the top half), you can use what is known as the Nub Theory to guess male or female. This theory uses the "angle of the dangle" to guess the sex in fetuses between 11 and 14 weeks.
This theory states that, in a male fetus, the genital tubercle will be angled upwards at 30 degrees or more when compared with the spine. In a female, the genital tubercle will be parallel to the spine.
How accurate is the Nub Theory, exactly? Based upon studies, the accuracy increases with gestational age. At 11 wks, the accuracy is only approx. 68%, while at 14 weeks it is 98% accurate. That sounds pretty dang good, however, you have to keep the size of your baby and the development into consideration. While it is fun to guess the sex based upon the Nub Theory, I personally like to wait until the anatomical survey (at approximately 20 weeks) to commit to a specific sex. Having a bigger baby means having a more developed baby, which means we are more likely to view the genitals better and get a more accurate prediction.