Updated: Mar 16, 2022
Were you told you likely are having a boy during an ultrasound at about 12-16 weeks? However, we could clearly see during the 20-week ultrasound that your daughter was most certainly not a boy? Your baby’s gender is determined at the moment of conception – when the sperm contributed a Y chromosome, which creates a boy, or an X chromosome, which creates a girl. Boys’ and girls’ genitals develop along the same path with no outward sign of gender until about nine weeks. It’s at that point that the genital tubercle begins to develop into a penis or clitoris. However, it’s not until 14 or 15 weeks that you can clearly begin to see the differentiated genitalia. So how can we predict gender before then? At 12 weeks, we may be able to use ultrasound to determine gender based on the angle of the genital tubercle. This is sometimes called “nub theory.” Using this method, we look at whether the tubercle is pointing up toward the baby’s head, which indicates a boy, or whether it remains flat or points down, indicating a girl. A 2016 study looked at the accuracy of such tests. The researchers found that out of 668 cases, sex determination was possible 89 percent of the time, and the prediction was correct 79 percent of the time. While those percentages may seem pretty high, it’s not as easy as it sounds.
Difficulties determining gender during the first trimester:
There are a number of variables that can affect whether we can determine gender as early as 12 weeks. First, your baby needs to be in a position for us to get a good view. That includes not having their legs closed! A mother’s weight also affects our ability to predict gender. The more body tissue the ultrasound waves must travel through, the fuzzier the images may be. The 2016 study found that a body mass index below 23.8 was the best cut-off value for gender prediction at 11 to 13 weeks. The odds of an accurate prediction fall for women above that number.
There’s also a chance that our prediction simply will be wrong. We tend to over predict boys more often than girls. This can happen, for example, if the baby is developing slowly and the tubercle hasn’t begun to point up or the umbilical cord is mistaken for a penis.
While gender prediction is much more accurate during the 20-week ultrasound, there’s still a chance it can be wrong.