Nub Theory: a Way to Predict Baby’s Sex Early in Pregnancy?

What is the Nub Theory, and how accurate is it in determining whether you’re having a boy or girl? Here’s how to use “the angle of the dangle” in predicting baby’s sex.
ByChristian Pope, DO, FACOG
OB-GYN
Updated
Oct 2020
expecting couple taking selfie on the beach together
Photo: Inti St. Clair / Getty Images

One of the most common questions I get asked during the first prenatal visit is, “ when can we find out the sex of the baby?” My answer is dependent on what tool the expectant parents would like to use to help determine this. Your two main options are a blood test called cell-free DNA that can be done as early as 10 weeks of pregnancy, and an anatomical ultrasound, which can be used to determine fetal sex at 18 to 20 weeks. Recently, however, an alternative ultrasound method has emerged that is said to help determine fetal sex, known as the Nub Theory. So what is the Nub Theory, exactly, and is it actually reliable? Here’s the deal.

What Is the Nub Theory?

Before 14 weeks of gestation, both male and female fetuses have an anatomical body part between the legs called a genital tubercle, otherwise known as “the nub.” This genital tubercle, or nub, is what develops into the penis in male babies or the clitoris in female babies. According to the Nub Theory, specific ultrasound measurements taken between 12 and 14 weeks of pregnancy can be used to calculate the angle of the genital tubercle in relation to the fetal spine, which can determine whether the fetus is a boy or a girl. In other words, the “angle of the dangle” (yes, it’s a term circulating on the internet) can tell you if that nub will go on to develop into a penis or clitoris.

Nub Theory Results for a Boy or Girl

To put the Nub Theory into practice, an ultrasound technician would need to capture a sonogram of baby in clear profile, so the full length of their spine is visible. Next, the technician would locate the genital tubercle protruding from between baby’s legs and determine the angle between the nub and the fetal spine. If the nub angle is more than 30 degrees, baby is likely a boy. If the nub is parallel to the spine and less than 30 degrees, baby is likely a girl.

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Nub Theory Accuracy

You won’t find Nub Theory published in mainstream radiological textbooks, but it is founded in science and evidenced-based. According to a 2012 study of 1,222 patients, it was possible to reliably determine baby’s sex using the Nub Theory with an accuracy of 96.6 to 100 percent, starting at 12 weeks gestation. The bigger baby was at the time of the ultrasound (aka the farther along in pregnancy), the greater Nub Theory accuracy proved to be. When the length of the baby is too small, the sex cannot be reliably predicted. Between 12 weeks and 12 weeks 2 days, sex determination was possible 90.5 percent of the time, and the method had an accuracy rate of 96.6 percent (99.1 percent in males and 93.5 percent in females). When the angle measurements were taken past 12 weeks 2 days, feasibility increased to 97.4 percent and Nub Theory accuracy jumped to 100 percent in both male and female babies.

An earlier 2006 study showed similar results. Tested with 656 patients, Nub Theory was able to determine baby’s sex 96 percent of the time when used between 12 weeks 4 days and 12 weeks 6 days (and 97 percent of the time when used between 13 weeks and 13 weeks 6 days). It accurately identified males 99 to 100 percent of the time. For females, the method proved to be 91.5 percent accurate at 12 weeks to 12 weeks 3 days, 99 percent accurate at 12 weeks 4 days to 12 weeks 6 days, and 100 percent at 12 weeks to 12 weeks 6 days.
The research indicates that the Nub Theory has a high accuracy rate at 12 to 14 weeks of pregnancy. But of course, other aspects come into play, including the mom-to-be’s body build, the position of the baby, the ultrasound diagnostics and the experience of the ultrasonographer.

Still, Nub Theory remains just that—a theory—and is not widely used among doctors or ultrasound technicians. The most common means of fetal sex diagnosis is either by blood work as early as 10 weeks or an anatomical ultrasound mid-pregnancy.

About the expert:

Christian Pope, DO, FACOG, is a board-certified obstetrician and gynecologist and fellow of the American College of Obstetricians and Gynecologists in private practice at Hawthorn Medical Associates in Massachusetts. He is also a medical staff member at St. Luke’s Hospital of Southcoast Hospitals and at Women and Infants’ Hospital in Providence, Rhode Island. Additionally, he serves as a clinical instructor of obstetrics and gynecology at the Warren Alpert Brown University School of Medicine. A graduate of the Philadelphia College of Osteopathic Medicine, Pope received his obstetrics and gynecology training at the Tufts University School of Medicine, Baystate Medical Center.

Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.

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