Baby Nub Theory: Your Best Bet for Early Gender Prediction

| Reviewed By Sarah Schulze, MSN, APRN, CPNP

Determining the sex of your baby is an important moment in your pregnancy journey. Whether you choose to find out early or later on, it will be a memory you’ll cherish forever. But how exactly do doctors determine the gender of your baby? The nub is commonly used as a determining factor.

What is the nub theory? The nub theory indicates that a baby’s gender may be identified by the angle of the genital tubercle. If tit is angled 30° or higher from the spine, it’s a boy. If it’s angled less than 30°, it’s a girl. Most evidence supporting this is anecdotal, but limited studies indicate accuracy of 50-70%.

Keep reading to learn more about the nub theory, its accuracy, and alternatives.

The Genital Tubercle

The genital tubercle is the part of a fetus that eventually becomes the penis or clitoris. It can be found in the ventral caudal, or lower frontal, part of the body. 

The genital tubercle develops around 4 weeks gestation at what’s called the “indifferent stage” and becomes distinguishable around 9 weeks.

Though it can be seen this early, doctors will not attempt to look at it until at least 12 weeks during the first ultrasound scan. 

Development in Females

In females, the genital tubercle develops into the clitoris. Without the presence of androgens, there is no influence for the genital tubercle to form into a penis.

So, the vestibular plate forms a mound called the vestibular groove. This remains unfused with other parts of the developing tubercle and instead forms the labia minora. 

Development in Males

In males, the genital tubercle develops into the penis. Hormones, called androgens, are responsible for the development of male sex characteristics.

These hormones cause the canalization and infusion of various parts of the genital tubercle to form the penis.

Nub Theory Explained

The nub theory is all about the angle of the dangle. It states that the gender of a baby may be determined by the angle of the caudal notch.

Nub Theory: 11 Weeks

The nub is only visible at 11 weeks gestation given the perfect position and intrauterine conditions.

In general, you will not be able to check for the angle of the caudal notch until 12 weeks at your first ultrasound scan. Additionally, the nub theory is more accurate at later gestational ages. 

Nub Theory: 12 Weeks 

Around 12 weeks gestation is when the first ultrasound scan typically occurs to check the baby’s gestational age and heartbeat. During this scan, there will be a small nub visible at the base of the spine that may or may not be forked in shape.

Several factors determine an ultrasound technician’s ability to detect the angle of the caudal notch, including the quantity of amniotic fluid, the position of the baby, the thickness of the abdominal wall, and the resolution of the ultrasound.

At this point, using the nub theory is only slightly better than taking a guess at around 50-60% accuracy.

It’s at the ultrasound technician’s discretion if they want to make a gender prediction, and many will not so as not to get parents’ hopes up given the low accuracy rate.

Alternatively, you may ask the ultrasound technician to point out the nub to make a guess yourself. 

Nub Theory: 12 Weeks Boy

A male nub will be 30° or higher from the base of the spine at 12 weeks gestation. The ultrasound technician can only view the baby’s caudal notch if the baby gives a perfect side profile under optimal conditions. 

Nub Theory: 12 Weeks Girl

A female nub will be less than 30° from the base of the spine at 12 weeks gestation. The ultrasound technician can only view the baby’s caudal notch if the baby gives a perfect side profile under optimal conditions.

Nub Theory: 13 Weeks

At 13 weeks gestation, the genital tubercle will be slightly more developed. The larger, more protruding, and differentiated nub aids in determining gender based on its angle.

Nub Theory: 14 Weeks

At 14 weeks gestation, the genital tubercle has mostly formed into the external genitals. Given the right conditions, good equipment, and experience, an ultrasound technician should be able to determine gender.

At this point, the accuracy of the nub method has risen to about 70%. 

Nub Theory: Boy vs. Girl

If the caudal notch is angled 30° or more in relation to the tail of the spine, then it’s a boy. If it is angled less than 30°, then it’s a girl. This is assumed because the penis forms outward, whereas the clitoris would form downward.

How Accurate Is the Nub Theory? 

The nub theory is 50-70% accurate depending on the baby’s gestational age. The first opportunity to look at the nub occurs around 12 weeks.

At that point, using the nub theory is slightly better than guessing at around 50% accuracy. By 14 weeks, many ultrasound technicians can identify gender accurately when given the right conditions.

Sagittal Sign Ultrasound

Each gender has corresponding sagittal signs. These signs can be observed by looking at a baby’s profile, otherwise known as the midline sagittal plane.

Then, by evaluating the angle of the caudal notch, ultrasound technicians can make an educated prediction regarding the baby’s gender. 

In essence, an ultrasound with a view of the sagittal plane is the best tool to predict gender using the nub theory. 

Nub and Skull Theory

The skull theory examines the shape and size of the skull as a method of gender prediction. Though it examines a different part of the body, both the nub and skull theory claim accuracy as early as 12 weeks gestation. 

The skull theory states that there are contrasts between male and female skulls that can be used to cite gender. However, many studies show that these changes don’t actually occur until puberty.

Most of the evidence in support of the skull theory is anecdotal, meaning it’s based on testimonials. 

The differences observed between male and female skulls are as follows:

  • The male cranial mass is more blocky, wide, and deeper compared to the female’s, which is rounder and tapered. 
  • The temporal ridge is more prominent in males than females.
  • A male’s ridge above the eyes is round and dull.
  • The zygomatic bone is more pronounced on the male skull.
  • The mandible or lower jaw bone of a male is squared, whereas a female’s mandible is rounded.
  • The male forehead is lower and more sloping.
  • The superciliary arch is large and pronounced in men.
  • The low, backward point of a man’s jaw bone, called the gonion, tends to be more sharply angled and outward.
  • Teeth tend to be bigger in males than in females.

Nub Theory App

While there are no applications currently on the market, there are several websites that you can visit to purchase an ultrasound evaluation.

These evaluations examine pictures from the 12-week ultrasound scan for indications that the caudal notch is revealing gender. Some of the most popular websites to purchase nub theory evaluations are listed below. 

Earliest They Can Tell Gender

The earliest that a baby’s gender can be predicted is at 11 weeks via noninvasive prenatal testing. This type of testing isn’t typically covered under insurance, so the out-of-pocket cost is high.

Following that, a definitive gender can be given at 18 weeks gestation via ultrasound.

When Is a Baby’s Sex Determined? 

Gender is determined at the time of conception when the sperm provides either an X or Y chromosome. Genital growth following this point goes down the same path until around nine weeks when external genitals start forming. 

What Gender Do All Babies Start at in the Womb? 

All babies appear as ambisexual in the womb in early pregnancy, meaning that they present neither girl nor boy genitals. However, this stage of development is far closer to the female genitalia than the male. 

Can Gender Change During Pregnancy? 

No, the gender of the baby will not change over the course of your pregnancy. However, sometimes mistakes are made when visual methods such as ultrasounds are used to predict gender.


The nub theory is a moderately accurate method of early gender determination. It can be used from 12 weeks of gestation until the external genitals are fully formed.

That said, intrauterine conditions are not always conducive for using this method.