Measure the angle of the nub on your own ultrasound. Tap three points and see what nub theory would guess. It's free, instant, and your scan never leaves your device.
Your scan never leaves your device. The image is opened and measured right here in your browser. Nothing is uploaded, saved or sent to us.
For fun only, not medical advice. Nub theory is not scientifically proven and works best from about 12 weeks. The 20-week anatomy scan or an NIPT blood test is the real answer.
A side-on (profile) view from around 12–14 weeks works best. JPG, PNG or WebP.
2. Mark three points on the scan
Choose an ultrasound image to begin.
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Pick an ultrasound photo or screenshot from your phone or computer. It opens only in your browser.
Tap a point along the baby's spine, the bright line running along the back.
Tap where the nub meets the spine line. This sets the spine line and the corner of the angle.
Tap the tip of the nub. The angle between the two lines appears instantly.
Nub theory looks at the genital tubercle (the "nub") on a side-on scan and compares its angle with the line of the spine. A nub lying roughly parallel to the spine, under 30 degrees, is read as a girl guess. A nub pointing upward at more than 30 degrees is read as a boy guess. Angles within a few degrees of 30 are too close to call.
It's a popular game in pregnancy forums, but it is not scientifically proven. The baby's position, the angle of the probe, blur and simply where you tap can all change the number. Learn more in our nub theory guide, and remember that an NIPT blood test or the 20-week anatomy scan is the real answer.
You open your ultrasound image in the page and tap three points: one along the spine, one at the base of the nub where it meets the spine line, and one at the tip of the nub. The checker draws the spine line and the nub line and measures the angle between them. Nub theory treats an angle under 30 degrees as a girl sign and over 30 degrees as a boy sign.
No. The image is opened with your browser's own file reader and drawn on your screen. It is never uploaded, stored or sent to us, and nothing is analysed by AI. Closing the page discards it.
Nub theory is not scientifically proven. Small studies and anecdotal reports vary, and a measurement taken from a photo of a scan is easily thrown off by the baby's position, the image angle and where the points are placed. Treat the result as a bit of fun.
Nub theory is usually tried from about 12 weeks, ideally on a clear side-on (profile) view of the baby. Earlier than that the nub is very hard to read. For a reliable answer, wait for an NIPT blood test or the 20-week anatomy scan.
If your measured angle lands within 3 degrees of 30, nub theory can't read it either way. Small changes in where you tap can move the angle by a few degrees, so a borderline result really is a coin flip.
The Nub Angle Checker is provided for informational and entertainment purposes only. Results are not a substitute for professional medical advice, diagnosis, or treatment. Always consult your doctor or qualified healthcare provider with any questions regarding a medical condition.
Do not make health or medical decisions based on these results. Read our full medical disclaimer.
