How to Read a Nub Theory Ultrasound: Step-by-Step Guide

So you've got your 12-week scan photo and you want to try nub theory. This step-by-step guide will walk you through exactly how to read your ultrasound image and make a gender prediction using the nub angle method.

What You Need
Step 1: Check the Image Type
Step 2: Identify the Spine
Step 3: Locate the Genital Area
Step 4: Find the Nub
Step 5: Measure the Angle
Step 6: Check for Stacking or Forking
Step 7: Make Your Prediction
Common Errors to Avoid
Frequently Asked Questions

What You Need

Before you start, make sure you have:

A clear ultrasound image from your 12-week scan (or a video you can pause)
The baby in a side-profile (sagittal) position — lying on its side with the full body visible
A gestational age of 12+ weeks (ideally 12+0 to 13+6)
A pen or digital tool to draw reference lines if needed
Good lighting to view the image clearly

Step 1: Check the Image Type

Not all scan photos work for nub theory. You need a **sagittal (side-profile) view** — the baby should be lying on its side, with the spine visible as a white line along the back.

Good images for nub theory:

Baby lying sideways with full body visible
Spine clearly visible along the bottom or top
Bottom and genital area visible
Image is clear and not overly zoomed

Bad images for nub theory:

Baby face-up or face-down
Only the face/head is visible
Image is zoomed in too closely
The genital area is obscured by legs or umbilical cord
Image is blurry or overexposed

If your scan photo doesn't show a good profile view, nub theory won't work — and that's not your fault. Sonographers capture images for medical purposes, not for gender prediction.

Step 2: Identify the Spine

The spine is your **reference line** for measuring the nub angle. On an ultrasound, the spine appears as a **bright white line** running along the baby's back.

Look for a line of bright white dots or a solid white line
The spine runs from the neck/base of the head down to the bottom
This line is your horizontal reference — the nub angle is measured relative to this line

If you can't see the spine clearly, you may need to try a different image from your scan or ask for a profile view at your next appointment.

Step 3: Locate the Genital Area

The genital area is located between the **baby's bottom** and the **legs**. On a profile view:

Find the baby's bottom (the curve at the base of the spine)
Look just below the bottom, between the legs
This is where the nub (genital tubercle) sits

The genital area is typically at the bottom of the baby's body in a profile view — it's the area between where the spine ends and where the legs begin.

Step 4: Find the Nub

The nub is a **small bump or line** protruding from the genital area. It can look like:

A tiny peg or bump pointing upward (boy)
A short flat line lying against the body (girl)
A small protuberance between the legs

Tips for Finding the Nub:

Look for a small white bump or line in the genital area
The nub is typically brighter than surrounding tissue
It may be very small — zoom in on your photo if needed
If you can't see anything, the nub may not be visible in this particular image

What's NOT the Nub:

The umbilical cord: Thicker, rounder, and goes toward the placenta
A leg or foot: Longer and thicker than the nub
The tailbone: This is part of the spine, not the nub
A shadow or artefact: Not a physical structure

Step 5: Measure the Angle

This is the **most critical step**. The nub angle relative to the spine determines the prediction:

Draw a line along the spine (mentally or with a digital tool)
Draw a line along the nub from its base to its tip
Measure the angle between these two lines

Angle Interpretation:

30 degrees or more upward → Boy
0–10 degrees (parallel/flat) → Girl
Pointing downward → Girl
10–30 degrees → Ambiguous (inconclusive)

The angle is the single most important factor in nub theory. If the angle is clearly 30+ degrees, it's very likely a boy. If it's clearly flat, it's very likely a girl.

Step 6: Check for Stacking or Forking

After checking the angle, look at the **tip of the nub** for additional clues:

Stacked Appearance

White layered lines at the tip
Looks like lines stacked on top of each other
Commonly indicates boy

Forked Appearance

Y-shaped or split tip
Looks like a small fork
Commonly indicates boy

Three Parallel Lines

Short, flat lines side by side
No stacking or forking
Commonly indicates girl

These visual cues are secondary to the angle. Always check the angle first, then use stacking/forking as confirmation.

Step 7: Make Your Prediction

Combine all your findings:

Strong Boy Prediction:

Angle 30+ degrees upward ✓
Stacked or forked tip ✓
Gestational age 12+ weeks ✓
Clear image quality ✓

Strong Girl Prediction:

Angle parallel or downward ✓
Three flat lines, no stacking ✓
Gestational age 12+ weeks ✓
Clear image quality ✓

Inconclusive:

Angle 10–30 degrees
Poor image quality
Baby in wrong position
Under 12 weeks gestation

Common Errors to Avoid

1. **Using a face-view image**: Nub theory ONLY works on profile views 2. **Confusing the cord with the nub**: The umbilical cord is thicker and goes in a different direction 3. **Trying to read at 10–11 weeks**: Too early — the nub hasn't developed enough 4. **Focusing on stacking without checking angle**: The angle is the primary indicator 5. **Over-analysing a poor-quality image**: If the image isn't clear, don't force a prediction 6. **Forgetting that baby position matters**: If the baby is curled up or in an awkward position, the angle may appear distorted

Frequently Asked Questions

### What if I can't see the nub on my scan?

If the nub isn't visible, it could be because the baby was in a poor position, the image wasn't captured at the right angle, or the photo you received was zoomed in on the face. You can try requesting a profile view at your next scan, or book a private scan specifically for gender prediction at 12+ weeks.

Can I use a video from my scan for nub theory?

Yes! Video can actually be better than a photo because you can pause it at the exact moment when the baby is in a perfect profile position. Look for a frame where the baby is lying sideways with the genital area clearly visible.

Do I need special software to measure the nub angle?

No. You can estimate the angle by eye — if the nub clearly points up at a noticeable angle (30+ degrees), it's likely a boy. If it's flat, it's likely a girl. You don't need to measure the exact degree. For a rough digital measurement, you can use any photo editing app to draw lines.

Should I ask the sonographer to check the nub?

NHS sonographers will not specifically check the nub for gender prediction — their role is to measure the baby and check development. However, you can ask for a profile view, which gives you the best image for nub theory. Some private scan clinics will specifically look at the nub if you ask.

Can someone else read my scan for me?

Yes! Many online communities (including our Nub Hub community) offer free nub theory readings. Posting your scan photo for experienced readers to analyse can give you a more confident prediction, especially if you're unsure what you're looking at.

This article is for informational purposes only. Nub theory is a prediction method, not a diagnostic tool.

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

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