The Best Week for Nub Theory: When to Get a Nub Reading

12 to 13 weeks is the sweet spot. Before about 11 weeks, male and female fetuses look the same, so there is nothing reliable to measure. The genital tubercle becomes distinguishable from around week 12 and is largely complete by about week 14 — which is exactly why a 12–13 week scan gives nub theory the clearest, most readable image (StatPearls embryology).

This page is about timing — choosing the right week to get a reading. If you want the full breakdown of how accurate nub theory is and the studies behind it, see our guide to nub theory accuracy. For how the method works in the first place, start with our complete guide to nub theory.

Accuracy climbs steeply across these few weeks. In the original expert study, reported accuracy went from 70.3% at 11 weeks to 98.7% at 12 weeks and 100% at 13 weeks (Efrat et al., 1999). Those are best-case figures from skilled operators; real-world accuracy is lower — roughly 85–90% at 12–13 weeks (Manzanares et al., 2016). Either way, the pattern is the same: a little later is a lot more reliable.

Effectively, yes — 11 weeks is too early to rely on. The nub can usually be seen at 11 weeks, but it has not finished angling, so a flat-looking nub might still tip upward a week later. That is why reported accuracy at 11 weeks is only around 70% (Efrat et al., 1999) — close enough to a coin-flip on the harder calls that it is not worth banking on.

If your only scan so far is at 11 weeks, treat any guess as just that. Waiting even seven days to reach 12 weeks makes a real difference to how confident a reading can be.

Slightly, yes. Both 12 and 13 weeks fall inside the recommended window, but 13 weeks is the peak. In Efrat’s study, accuracy rose from 98.7% at 12 weeks to 100% at 13 weeks, and real-world reliability shows the same upward trend as the baby grows (Efrat et al., 1999).

The practical takeaway: 12 weeks is a good time to get a reading, and if you have a choice, a scan dated at 13 weeks gives the clearest, most differentiated nub. Both are far better than 11 weeks.

“Weeks” is really a proxy for how big the baby is, and the most reliable size measure is crown-rump length (CRL) — the distance from the top of the head to the bottom of the body. Because dating can be a few days out either way, CRL is often the better guide to whether a nub is ready to read.

A large first-trimester review of 2,314 pregnancies found that sex could be assigned successfully in about 90% of cases overall, and reliability climbed above 95% once CRL passed roughly 65 mm (first-trimester review, n=2,314). In other words, a well-grown baby at 12 weeks can give a clearer reading than a smaller baby at the same dates — so a longer CRL is good news for a nub reading.

Plenty of dating scans happen before 12 weeks, so this is common. Here is what to do:

• If your scan is before 11 weeks: it is too early — there is nothing reliable to read yet. Plan to use a later image.
• If your scan is at 11 weeks: you can look, but treat any result as a low-confidence guess. Waiting for a 12–13 week image is the safer plan.
• If you can book or wait for a 12–13 week scan: that is the ideal window — a clear side-on (mid-sagittal) image dated 12–13 weeks gives the best chance of a confident reading.

NubHub readings work best from 12 weeks, so we do not promise reliable results from a pre-12-week image. Once you have a clear 12-week-plus scan, our AI checks the nub’s angle, shape, and image quality, and then a real person double-checks the result before it reaches you — typically within about 2 hours, backed by a money-back guarantee. Read more about how our AI works.