Many expectant parents are impatient to learn if they are having a boy or a girl. So they may be attracted to new companies, like SneakPeek, which claim to be able to determine your baby’s sex with 99% accuracy early in the first trimester, with only a simple blood draw.
These new tests rely on what is known as cell free fetal DNA (cffDNA)–small fragments of DNA shed from the placenta into your bloodstream.
Unfortunately, according to a 2011 meta-analysis published in JAMA, some of these companies overstate the accuracy of their tests. The meta-analysis found that none of these cfDNA-based tests are accurate before 7 weeks. (The meta-analysis also examined urine-based tests. None of these were accurate.)
Between 7 and 20 weeks, accuracy for these blood-based tests ranged from 95-98%, depending on the specific techniques employed and the baby’s actual sex. (The tests were more accurate when they use a DNA amplification technique known as real-time quantitative polymerase chain reaction.) Only after 20 weeks were the blood-based tests 97-99% accurate.
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.
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 obgyn.net last year (2017 update: the paper has since been taken down. A summary of her claims is described here.)
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 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! Odds are good they’ll get it right.
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.