Nub Theory Boy Examples: A Photo Guide to Recognising a Boy Nub

If you've had your 12-week scan and you're trying to work out whether that little bump between your baby's legs points to a boy, you're not alone. Nub theory boy examples are some of the most searched-for pregnancy content in the UK, and for good reason — the 12-week scan is the earliest point where a trained eye can reasonably guess at sex using the nub angle method.

This guide walks you through what a boy nub looks like in practice. We describe a series of example ultrasound images, explaining the angle, the shape, and the visual cues that experienced nub theory readers look for. Because we cannot reproduce copyrighted ultrasound photos, each example is described in detail so you can compare the description against your own scan.

What Makes a Boy Nub a Boy Nub?
Example 1: The Classic 30-Degree Angle
Example 2: The Stacked Boy Nub
Example 3: The Forked Boy Nub
Example 4: Boy Nub at 11 Weeks
Example 5: Boy Nub at 13 Weeks
Example 6: The Caudal Notch
Common Mistakes When Reading Boy Nubs
Frequently Asked Questions

What Makes a Boy Nub a Boy Nub?

A boy nub is identified primarily by **angle**. The genital tubercle — the small protuberance between the baby's legs that will become the penis in a male or the clitoris in a female — is visible on a side-profile (sagittal) ultrasound at 11 to 13 weeks. In a male fetus, the tubercle begins to elongate and angle upward as it develops toward the penis. This creates the characteristic **30-degree or greater upward angle** relative to the horizontal axis of the baby's spine.

The secondary visual cues that nub theory readers look for in a boy nub include:

Stacked appearance: The tip of the nub may appear as layered white lines, sometimes described as a "stacked" look.
Forked tip: The nub may show a Y-shaped or forked appearance at the tip, which some associate with male development.
Caudal notch: A small dip or notch visible between the nub and the base of the spine.
Length: Boy nubs often appear slightly longer and more prominent than girl nubs at the same gestational age.

It is important to remember that angle is the primary indicator. The stacked, forked, and caudal notch features are secondary and less reliable on their own. A nub that points up at 30 degrees or more is the strongest visual signal of a boy.

Example 1: The Classic 30-Degree Angle

**Description**: In this example, the baby is lying in a clear side-profile (sagittal) view with the spine visible as a bright white line running horizontally along the bottom of the image. Between the baby's bottom and the legs, a small nub is visible pointing upward at approximately 30 degrees relative to the line of the spine. The nub appears as a short, bright line with a slightly raised tip.

What to look for: Draw an imaginary line along the spine. The nub should angle upward from that line by roughly 30 degrees — imagine the hand of a clock pointing to the one o'clock position. This is the textbook boy nub angle. The tip of the nub may show a faint forked or stacked appearance, but the angle alone is sufficient for a boy prediction at 12+ weeks.

Gestational age: 12 weeks 3 days. At this gestation, a 30-degree angle is a strong boy indicator.

Example 2: The Stacked Boy Nub

**Description**: This image shows the baby in profile with the spine along the lower portion of the scan. The nub angles upward at approximately 35 degrees. At the tip of the nub, you can see what appears to be two or three bright white lines stacked on top of each other — this is the "stacked" appearance that many nub theory readers associate with boys.

What to look for: The stack is not always obvious. It may look like a small bright patch at the tip of the nub rather than clearly separated lines. Do not rely on the stack alone — confirm the angle is 30 degrees or more before making a boy call. In this example, both the angle and the stack point to boy.

Gestational age: 12 weeks 6 days.

Example 3: The Forked Boy Nub

**Description**: The baby is in a good sagittal position with the spine visible along the bottom. The nub angles upward at roughly 30 to 35 degrees. At the tip, the nub splits into a Y-shape — two small prongs extending upward. This forked appearance is another secondary boy indicator.

What to look for: The fork should be at the tip of the nub, not at the base. It can be subtle and may require zooming in on the image. Some sonographers and experienced readers say the fork is caused by the early development of the glans of the penis. The angle in this example is clearly above 30 degrees, supporting the boy prediction.

Gestational age: 13 weeks 1 day.

Example 4: Boy Nub at 11 Weeks

**Description**: This scan was taken at 11 weeks 2 days. The baby is in profile, but the nub is less distinct than in the 12-week examples. The nub angles upward at roughly 20 to 25 degrees — in the ambiguous range. However, there is a faint stacked appearance at the tip.

What to look for: At 11 weeks, the nub is still developing. An angle of 20-25 degrees is not a confident boy call. This is why nub theory accuracy at 11 weeks is only around 70%. If you have an 11-week scan, treat any prediction as tentative and wait for a 12+ week scan if possible.

Gestational age: 11 weeks 2 days.

Example 5: Boy Nub at 13 Weeks

**Description**: At 13 weeks 2 days, the nub is more developed. In this example, the nub angles upward at approximately 40 degrees — a very pronounced boy angle. The nub is clearly elongated and the tip shows a stacked appearance.

What to look for: By 13 weeks, boy nubs tend to be more obvious because the genital tubercle has had more time to develop. An angle of 40 degrees is one of the clearest boy indicators. At this gestational age, nub theory accuracy is around 92-95%.

Gestational age: 13 weeks 2 days.

Example 6: The Caudal Notch

**Description**: In this image, the baby is in profile with the spine along the bottom. The nub angles upward at about 30 degrees. Below the nub, between it and the base of the spine, there is a small dip or notch — the caudal notch. Some nub theory readers consider the presence of a caudal notch a supporting boy indicator.

What to look for: The caudal notch is not always visible and not all boy nubs have one. It is a secondary feature. The primary indicator remains the angle. If the nub is at 30+ degrees and there is a caudal notch, this strengthens the boy prediction.

Gestational age: 12 weeks 4 days.

Common Mistakes When Reading Boy Nubs

Even with good examples to compare against, it is easy to misread a nub. Here are the most common mistakes:

Confusing the umbilical cord with the nub: The umbilical cord can sometimes appear as a bright line between the legs and may be mistaken for the nub. The cord is usually thicker and curves differently. If the line you are looking at connects to the baby's abdomen, it is likely the cord, not the nub.
Misjudging the angle: Estimating 30 degrees by eye is harder than it sounds. A nub at 20 degrees can look like 30 degrees if the image is slightly rotated. Always try to align the spine horizontally before judging the angle.
Reading a non-profile image: Nub theory only works on a sagittal (side-profile) view. If the baby is face-on or bottom-on, the nub cannot be assessed using the angle method.
Trusting the stack or fork over the angle: The stacked and forked appearances are secondary indicators. A girl nub can sometimes show a stacked appearance. Always prioritise the angle.
Ignoring gestational age: A 30-degree angle at 11 weeks is less reliable than at 12+ weeks. Always factor in the gestational age when making a prediction.
Overlooking image quality: A blurry or zoomed-out image makes angle assessment unreliable. If you cannot clearly see the nub and the spine, the prediction is not reliable.

How to Practise on Your Own Scan

If you want to compare your own scan against these examples:

Find your clearest profile (sagittal) image from your 12-week scan.
Orient the image so the spine runs horizontally.
Locate the nub between the baby's bottom and legs.
Estimate the angle of the nub relative to the spine.
Look for secondary features: stack, fork, caudal notch.
Compare your findings to the examples above.

Remember that even experienced readers can be wrong. Nub theory is a prediction, not a diagnosis. For confirmation, you will need NIPT or the 20-week anatomy scan.

Frequently Asked Questions

Can a boy nub ever point downward?

It is very unusual for a boy nub to point downward at 12+ weeks. If the nub clearly points downward or is parallel to the spine, it is more likely a girl. However, image quality and baby position can sometimes distort the appearance. If your scan shows a downward nub but you suspect it may be a boy, wait for NIPT or the 20-week scan for confirmation.

What angle is definitely a boy?

There is no angle that is "definitely" a boy, but an angle of 30 degrees or more upward relative to the spine is the classic boy indicator. At 12+ weeks, this angle has roughly an 85-95% chance of being correct. Angles above 30 degrees at 13 weeks are even more reliable.

Do all boy nubs have a fork or stack?

No. The forked and stacked appearances are secondary features and are not present in every boy nub. Some boy nubs are simply an upward-angled line with no visible fork or stack. The angle is the primary indicator.

Can the umbilical cord look like a boy nub?

Yes. The umbilical cord can sometimes appear as a bright line between the legs and may be mistaken for a nub. If the line seems to connect to the baby's abdomen rather than the genital area, it is likely the cord. Experienced readers distinguish the nub from the cord by location and appearance.

How accurate is nub theory for boys specifically?

Studies suggest nub theory is slightly more accurate for boys than for girls at 12+ weeks, with some studies reporting boy accuracy at 90-95% and girl accuracy at 85-90%. This is because the upward angle of a boy nub tends to be more visually distinct than the flat angle of a girl nub.

This article is for informational purposes only and does not constitute medical advice. Nub theory is a prediction method, not a diagnostic tool. For definitive sex determination, consult your healthcare provider about NIPT or your 20-week anatomy scan.

Last reviewed: June 2026 | Next review due: December 2026

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