The Fetal “Gender Nub”: How To Learn Your Baby’s Gender at the First Trimester Screening

Kudos to people who can wait until the birth to find out if they are having a boy or girl. The feminist in me feels guilty about this, but I never wanted to wait that long. Pretty much the second I learned I was pregnant, I started trying to figure out if it was a boy or or girl.

The good news for people like me: ultrasounds can now detect a baby’s gender as early as 12-13 weeks gestation.

Genetic testing through CVS remains the most accurate way to determine fetal sex in the first trimester. But since this test carries a slight risk of miscarriage, many of us choose not to have it performed. (New blood-based tests that rely on cell free DNA can detect your baby’s gender as early as 9 weeks, without increasing the risk of miscarriage, but these are only about 95% accurate in the first trimester).

Until recently, forging genetic testing meant we had to wait until the second trimester screening to learn our baby genders, sometime around 18 to 20 weeks gestation. By that time, the penis, testes, and labial folds are clearly visible on an ultrasound.

The latest research, however, suggests we can learn the gender months earlier, during the ultrasound for the first trimester screen, which is performed sometime between 11 weeks 0 days to 13 weeks 6 days of gestation. The key is scheduling your scan towards the end that window.

Male and female fetuses look pretty similar throughout most of the first trimester. The genitalia are just starting to develop from their root, the “genital tubercle,” which slowly develops into either a penis or clitoris. This genital tubercle is same size in boys and girls until around 14 weeks gestation, when the penis begins to elongate.

A sonographer therefore has to rely on more subtle clues to determine gender in the first trimester. The angle of the genital tubercle is one important clue. By 12-13 weeks gestation, the angle of the penis begins to point up, towards the baby’s head, while the clitoris remains flat or points slightly down.

The ultrasound images below illustrate this. A male fetus is shown on the left, a female fetus on the right. The angle of the genital tubercle is noted with faint white lines.

Screen Shot 2014-11-07 at 4.06.27 PM
From Efrat et al. 1999

In small study of 172 pregnancies, sonographers  were able to accurately detect fetal gender using this angle from about 12 weeks on. Before then, sonographers correctly identified only 70% of fetuses. They were most prone to misclassify the boys: roughly half of the male fetuses were misclassified as girls. So at 11 weeks, when the sonographers guessed that a fetus was a boy, they were usually correct. But when they guessed it was a girl, they were often wrong.

But by 12 weeks this method’s accuracy shot up to 98%. By 13 weeks, there were no more misidentifications; 100% of their classifications were correct. In a follow-up study of 656 pregnancies, sonographers were again perfectly accurate by 13 weeks.

At this point in pregnancy, using the angle of the genital tubercle is more accurate than trying to detect the labia or testes, the method commonly used to determine gender in the second trimester. Classification by the latter method was only about 75% accurate until 14 weeks.

(Incidentally, several websites, like this post at CafeMom and this one at About Health, claim that fetal gender can be detected with nearly 100% accuracy at the 6-8 week ultrasound using “Ramzi’s Method”. I tracked these claims down. All of them appear to derive a paper written by Dr. Saad Ramzi Ismail, which was posted to last year (and has since been removed). In it, Dr Ismail claims that the placenta is almost always on the left side for girls and on the right side for boys. While her paper is formatted and reads like a legitimate scientific article, it was never published in an actual scientific journal. Moreover, it seems extremely unlikely that, if we could determine gender by something as simple as the location of the placenta, we would already know this and use this method all the time. So, as far as I can tell, “Ramzi’s method” is pure junk science.)

The Bottom Line

If you want to learn your baby’s gender, schedule your first trimester scan for 13 weeks gestation. The methods used to detect gender at this fetal age are pretty new, so some sonographers may be hesitant to give your their best guess. But don’t let this deter you. Ask! They are very likely to get it right.

Additional Reading

Like this post? You may also like my take on fetal sex tests based on cell free DNA. With a simple blood draw, these tests can determine fetal sex as early as 9 weeks.


Efrat Z. Akinfenwa O.O, and Nicolaides K. H. (1999). First-trimester determination of fetal gender by     ultrasound. Ultrasound Obstet Gynecol 13:305–307.

Efrat Z., Perri T., Ramati E., Tugendreich D., and Meizner I. (2006). Fetal gender assignment by first-trimester ultrasound. Ultrasound Obstet Gynecol.27(6):619-21.

Emerson D.S., Felker R.E., and Brown DL. (1989). The sagittal sign. An early second trimester sonographic indicator of fetal gender. J Ultrasound Med. Jun;8(6):293-7.

Author: Amy Kiefer

As a former research scientist and proud mama of three little munchkins, I love digging into the research on all things baby-related and sharing it with my readers.

13 thoughts on “The Fetal “Gender Nub”: How To Learn Your Baby’s Gender at the First Trimester Screening”

  1. The claim about accurately guessing gender by placenta location cracked me up. Yet another wives’ tale about gender prediction, it seems like. We found out we were having a boy at 15 weeks, and I do remember reading about the angle method around that time (when “researching” the accuracy of early gender determination) and I find it fascinating! Good post, like always.

    1. Generally speaking, I’m enjoying your fact- and logic-filled articles about pregnancy (and labor). Those seem unfortunately rare, and the combination of thoughtfulness and data is refreshing (from the perspective of a mom who is also a researcher by training).

      Given that your writing is so refreshing in so many ways, I was a bit bummed to see misuse of the term “gender”. Gender is a social and psychological construct, the assessment of which through ultrasound would be quite the trick. (See The expectation is that a baby’s gender will match its biological sex, but what we are able to learn just by looking at baby or DNA is, indeed, the baby’s sex. Differentiating between the two terms is important so that we can have meaningful conversations about when sex and gender are the same or different for any given individual.

      1. Thanks for the comment, Amanda.

        I did think about this issue before posting. I went back and forth between using gender or, the technically correct term, sex. The Google Trends related search feature was what ultimately pushed me towards using gender instead of sex. Baby’s gender is pretty much used only when people perform searches related to determining their baby’s sex. Baby’s sex, on the other hand, generates a lot of, let’s say, noise.

        I felt okay about this decision since it is clear that we cannot assign a societal and psychological construct to fetuses. But I can see how someone could argue against conflating the two, ever.

  2. I asked my midwife to follow the procedure explained in your post during the 13th week ultrasound. And guess what, it worked. We just got the confirmation at the 5th month ultrasound: it’s a boy! This is the most wonderful anecdotal evidence I’ve ever seen 🙂 Thanks for sharing this piece of science with us.

  3. Hey!
    I am trying chinese gender prediction..but cant get my age right..
    I was born on 27 sept 1992..
    soo i m 23 n will b 24 on 27thsept..
    But wen i conceived i m 23 rite na..
    so shud i write 23 in age box or 24..plzz help..

  4. pls tell me i have boy or girl baby now iam 5months pregnant i send my 14weeks 3 days scan report pls reply me how i send my scan report

  5. I was in my 13 weeks when I had one done I need to know boy or girl they said boy but that was an elective ultrasound

  6. actually i had my ultrasound during my 12th week , and my doctor knew the baby’s gender right away and was very sure of it, he said it was a boy, and yes it is, and recognized it also in my second baby before i finished my first trimester and he was quite sure of it too that it was a girl, i was surprised that he can guess the baby’s gender this early, and both were correct

    1. I’m afraid I don’t do ultrasound readings but I hope you find out soon. Congrats on your pregnancy.

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