Reading the nub at 12 weeks
At 12 weeks the nub is usually readable for the first time on a clean side-profile scan: a tilt of 30°+ suggests boy, near-parallel suggests girl. Accuracy is about 70% on a good image — and far lower on a poor one.
12 weeks is the earliest the nub reliably shows its angle. You need a true side-on view with the spine visible as a line; without it, the angle cannot be measured.
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Accuracy by week
The nub gets easier to read across the 12–13 week window:
| Week | What you see | Typical accuracy |
|---|---|---|
| 11 weeks | Too early — nub underdeveloped | Low |
| 12 weeks | Readable on a clear profile | ~70% |
| 13 weeks | Best window for a confident read | ~75% |
| 14 weeks | Still readable, profile harder to get | ~70% |
Tips for a clean 12-week read
Ask for a mid-sagittal (perfect side) profile where the spine runs as a clear line to the base of the body. Avoid tilted or front-on frames — they distort the angle and are the number-one cause of wrong reads.
If your 12-week image is borderline, a 13-week shot is often clearer. And remember a single nub read is one clue around 70%, not a verdict. Read the full method on our nub theory guide and compare it with ultrasound signs.
How BabyPeek does it
In the app, AI measures the nub angle on your 12-week scan and combines it with four other methods into one majority verdict — with an honest confidence, not a guess dressed up as a fact.