Nub theory: Is It Accurate?


During pregnancy, the female is excited and impatiently waiting for the baby to deliver. Around 18 to 22 weeks of pregnancy ultrasound scan will provide her with all different kinds of important information about her growing baby, even help in the determination of sex of the baby. There are ample theories available on the internet for sex prediction.

One such theory is “nub theory “. This got attention all over the world. With the internet, many people claimed of knowing the analysis of the early stages of the baby’s genital development to help in figuring out whether the little appendage will transform into the boy part or girl part.

nub theory test

What Is The Nub Theory?

Nub theory is based on the genital tubercle, which usually forms in early pregnancy over the lower abdomen of the baby. As time proceeds, the tubercle or “nub” transforms into a penis in a male baby or a clitoris in a female baby. This nub is often referred to as ‘angle of the dangle’. The theory simply states, during early pregnancy on 12-week ultrasound, if the angle of the nub is beyond 30° from the spine, it is considered to be a boy whereas if the angle is below 10° then it is considered to be a girl.

Is It Accurate?

The accuracy of the nub theory is dependent on the timing of the ultrasound. Sex determination accuracy improves with an increase in gestational age starting from week 11 to 13+ 6.

  • At 11 weeks the accuracy rate of sex determination is 71.9 %
  • At 12 weeks the accuracy rate is about 92%
  • At 13 weeks the accuracy rate is around 98.3%

Thus, waiting longer will give more accurate results in the determination of sex.

How Does This Theory Work?

The ultrasound scan is essential in determining the sex of the baby using nub theory during the first trimester. The angle of the dangle is seen in the profile view of the baby. This angle is then compared to the baby’s lower spine. The profile view or the side view on an ultrasound image is called a sagittal plane. Therefore, the angle is often referred to as a sagittal sign and this helps in determining the sex of the baby. Following method helps in determining the sex of the baby :

Draw A Baseline

On the saggital image of the scan, determine the baby’s base spine. The lower line in the image signifies the lumbosacral skin surface. It is the lowest part of the back, just above the tailbone. This forms the reference line in the nub theory.

Draw A Nub Line

As soon as the baseline is marked, find the nub. It is difficult to identify the exact location of the nub as the ultrasound images are often unclear. Thus, it requires the practice of reading the ultrasound image of some pregnancy week. Once located, the line is drawn across it.

Calculation of The Angle

Now the angle between the two lines in the scanned image should be determined. If the angle is above 30° then it may indicate to be a boy but if the angle is less than 10° or parallel to the nub line, then it may be considered as a girl. If the angle in between 30° and 10°, then it is hard to predict the sex. In such a case, the female should undergo another scan after two days.


How Do Experts Predict The Sex Of The Baby?

The gender experts conclusion depends upon the visibility of the genital tubercle which is affected by the exact location of the baby and their movements. The amount of amniotic fluid, the thickness of the abdominal wall as well as the image quality of the scan helps in determination. Following are the criteria used by experts to predict the sex of the baby :

  • Visibility
  • The Nub’s angle with the baby’s spine
  • The shadows on and nearby the nub captured in the scan.
  • The length as well as the shape of the baby according to the gestational age.

Initially, around 10 weeks of pregnancy, the genital tubercle appears the same for both boy and girl babies. At 11 weeks, the view is better and can help in predicting the sex of the baby under perfect conditions. The accuracy of the nub theory is considered highest in or after the 12th week of gestation and sex prediction before 12 weeks is usually not encouraged.

Why Is It Medically Important?

In case, if the baby suffers from any sex-linked disorder which mostly affects the male offspring, then sex determination at early pregnancy can help parents to refer for counselling. If the nub theory predicts a girl baby, then no further invasive testing like amniocentesis or CVS is required whereas, if the nub theory determines a boy, additional tests are necessary to determine the extent of the disorder and evaluate the health of the unborn male offspring.


Sex determination through nub theory is completely dependent on the ultrasound image. It should be clear enough to determine the exact location of the nub. Through the following tips, the chances of a good image improve :


Drinking plenty of water before the ultrasound

It is said, the more the amniotic fluid, the clearer is the ultrasound image. Therefore, drinking plenty of water a week or before till the due day for ultrasound can help in increasing the volume of amniotic fluid inside the womb.

Consumption of apple or orange juice before the scan

For a better image, the ultrasound requires the baby to be active during the process and show movements so that the sonographer can capture the right nub shot. It is therefore believed, a glass of apple or orange juice about 30 minutes before the scan can make the baby more active inside the womb.

Difference Between The Nub Theory and Skull Theory

Often people are confused between the Nub theory and skull theory of sex prediction. But skull theory is different from the Nub theory. In skull theory, certain male and female markers in the shape are considered to determine the shape of the skull to determine the baby’s sex. The ultrasound image of 12 weeks pregnancy along with the magnifying lens and the knowledge of the shapes of the female and male skulls help in predicting the sex of the baby.


While in the nub theory, the baby must be in the right position for the sonographer to capture the pictures. Here it is not easy to determine the nub until the genitals are completely developed. Therefore, it is advised to wait until the baby grows a little more and go for another ultrasound scan.


Such theories cannot be trusted 100 percent as they aren’t most accurate. The most accurate time to determine the sex of the baby is when the little warm bundle has safely entered the real world and reached the arms of the mother.