What Is Nub Theory And How Does It Predict Your Baby's Sex?

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Nub theory is a method to predict fetal gender. It is observed that between 11 and 13 weeks, the genital tubercle is present between the legs of the babies. According to the theory, based on the angle of the tubercle, the gender may be determined. Tubercle pointed towards the baby’s head means a boy and it pointing downwards means a girl (1). The nub’s angle can be determined by carefully examining the ultrasound scans. Read on to know more about the nub theory and the criteria for predicting the gender of the baby with the help of this theory.

How Accurate Is The Nub Theory Of Sex Prediction?

The accuracy of the method depends on the timing of your ultrasound.

According to a research study published in the journal Acta Obstetricia et Gynecologica Scandinavica, the ultrasound scans (weeks 11 to 13+6) of 496 pregnant women in their first trimester were primarily screened, and the difference in angle of genital tubercle was measured to determine the sex of the baby.

It was then combined with screening tests for assessing abnormalities. It was reported that sex determination accuracy improved with increasing gestational age, starting from week 11 to 13+6.

  • At 11 weeks, the accuracy rate was 71.9%.
  • At 12 weeks, the accuracy rate was 92%
  • At 13 weeks, the accuracy rate was 98.3% (2)
Did you know?
A prospective study concluded “Around 200 ultrasound scans will need to be performed by a sonographer to achieve an accuracy greater than 70% in determining fetal sex at the first trimester (5).”

How Does Nub Theory Work?

Use an ultrasound scan to determine your baby's sex using the nub theory


An ultrasound scan is necessary to determine the sex of your baby using the nub theory in the first trimester. The nub angle is visible in the profile view of the baby. The angle of the nub is compared to the baby’s lower spine.

The side view or profile of the baby in an ultrasound image is known as the sagittal plane. Hence, the angle of the nub is referred to as the sagittal sign, which is used to determine the sex of the baby. Here’s how (3):

1. Draw the baseline

Consider the sagittal image of the scan and determine the baby’s base spine. This is the lower line you can see in the image and is technically called the lumbosacral skin surface. It is the lowest part of the back just above the tailbone. This becomes the reference line in the nub theory.

2. Draw the nub line

Once you mark the baseline, find the nub. Ultrasound images are not always clear, and a little bit of practice reading the ultrasound images of the same pregnancy week can help in identifying and locating the nub. Once you find it, draw a line across it.

3. Calculate the angle

Now that there are two lines in the scanned image, you should determine the angle between them. If the angle is greater than 30°, you may say it is a boy. And if the angle is less than 10°, or the nub line is parallel to the baseline, you may say it is a girl. If the angle is between 10 and 30°, the result is inconclusive. In that case, you might try to know the sex again by getting another scan after a couple of days.

How Do Experts Predict The Sex Of The Baby Using Nub Theory?

Gender experts depend on the visibility of the genital tubercle to predict the sex of your baby using the nub theory. This visibility can be affected by the position of your baby and their movements.

The amount of amniotic fluid, the thickness of the abdominal wall, and the scan quality may also determine the results.

The following are the criteria for predicting the sex of a baby using the Nub Theory:

  • Visibility
  • The nub’s angle to the baby’s spine
  • The shadows on and around the nub captured in the scan
  • The length and shape of the baby as per the gestational age

Before the tenth week of pregnancy, the genital tubercle looks alike in boy and girl fetuses. At 11 weeks, the view is slightly clearer and may allow you to predict the sex of your baby under perfect conditions. The accuracy of nub theory is the highest in or after the 12th week of gestation and sex prediction before 12 weeks is not encouraged (4).

Quick fact
Wrong gender prediction can happen when the baby is developing slowly, the tubercle hasn’t yet begun to point up or the umbilical cord is mistaken for a penis (1).

Why Is The Nub Theory Sex Determination Medically Important?

Some of the sex-linked disorders affect only male offspring. Therefore, determining the sex in the early stages of pregnancy can allow parents to go for counseling if the fetus is suspected of having any genetic disorders.

If the nub theory predicts a baby girl, it requires no further invasive testing such as amniocentesis or CVS to find if the baby is healthy. On the other hand, if the nub theory determines a boy, additional tests may be required to evaluate the health of the unborn.

Tips To Get A Better Nub Shot

Sex prediction through nub theory largely depends on the picture of the ultrasound. The nub shot must be perfect for a chance to determine the sex accurately. Here is how you may improve the chances of a good image:

1. Drink more water before the ultrasound.

The more amniotic fluid you have, the clearer your ultrasound image will be. So, drink ample water a week or two before you are due for an ultrasound. This might increase the volume of the fluid in your womb.

2. Consume apple/orange juices before the scan.

For a good image, you may want your baby to be a little active during the ultrasound so that they move a little, and the sonographer can capture the right nub shot. It is believed that having a glass of apple or orange juice about 30 minutes before the scan might make babies more active in utero.

Often, people confuse the Nub theory with the skull theory of sex prediction.

Is There Any Difference Between Nub Theory and Skull Theory?

Skull theory looks for certain male and female markers in the skull shape

Image: Shutterstock

Skull Theory is different from Nub Theory. This method looks for certain male and female markers in the shape of the skull to determine the baby’s sex. The baby’s 12th-week scan picture, magnifying lens, and knowledge about the shapes of female and male skulls are needed to predict the sex of the baby.

For Nub Theory to work, the baby must be in the right position for the sonographer to capture the pictures. If your baby is turning away or kicking wildly, you may not get to see the actual angle.

The nub theory works effectively in certain circumstances. Since it is not easy to identify the nub until the genitals are fully developed, it may pose certain challenges. Therefore, it is best to be patient and wait until the baby grows a little more and then get an ultrasound. The other option is to wait until you deliver, as it will allow you to keep the sex of the baby a surprise. However, if you want to find out the gender early, you could try this method by looking at the scan reports—so good luck with that and the foreseeable parenthood.

Infographic: Why A Gender Detection Before A Baby’s Birth?

Gender detections before birth may help identify the risk of certain disorders and conditions. Ultrasound and blood tests can predict the gender of the baby. Gender detection for non-medical reasons is not permitted or covered by insurance in many countries. Check the infographic to know some of the reasons for gender detection before birth.

fetal gender detection [infographic]
Illustration: MomJunction Design Team


MomJunction's articles are written after analyzing the research works of expert authors and institutions. Our references consist of resources established by authorities in their respective fields. You can learn more about the authenticity of the information we present in our editorial policy.
1. Patricia Santiago-Munoz; Boy or girl? The difficulties of early gender prediction; The University of Texas Southwestern Medical Center (2016)
2. C.H. HSIAO et al.; Fetal gender screening by ultrasound at 11 to 13+6 weeks; Acta Obstetricia et Gynecologica Scandinavica (2010)
3. Marek Lubusky et al.; Ultrasound evaluation of fetal gender at 12–14 weeks; Biomedical papers of the Medical Faculty of the University Palacky, Olomouc, Czech Republic (2012)
4. Manette Kearin et al.; Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans; Australas J Ultrasound Med. (2014)
5. Choi WahKong et al.; Learning Curve in Determining Fetal Sex by First Trimester Ultrasound Scan; Science Direct (2016)
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