Nub theory calculator: measure the nub angle on your scan

Mark the lower spine and the nub on your scan. The calculator measures the angle between them and places it in one of three zones used by researchers: over 30° leans boy, under 10° leans girl, and anything in between is too close to call.

BPBabyPeek EditorialSources & editorial policyUpdated Sep 2026

Measure the nub angle

Stays on your device
Choose or drop your scan JPG, PNG or a screenshot. It is opened in your browser and never uploaded.

Choose a scan image to start.

S1–S2: spine lineB–T: nub, base to tip

The nub angle is the tilt of the genital tubercle, measured against a line through the skin over the baby's lower spine. In studies, specialists measuring live scans at 12–14 weeks were usually right; at 11 weeks they often were not. A reading from a photo of your printout has never been tested, so treat the result as a clue, not an answer.

How to use the calculator

Where to place the four points: S1 and S2 along the skin over the lower spine, B at the base of the nub, T at its tip angle S1 S2 B T Side profile, head to the right (schematic) skin line over the lower spine
A schematic, not a real scan. The dashed lines show how each line extends; the angle is where they meet.
  1. Pick a side-on image. Use the clearest view where the baby lies in profile and the lower spine runs as a line along the bottom. A screenshot from the clinic's app is better than a phone photo of paper.
  2. Place the spine line (S1, S2). Tap two points along the skin surface over the lower back, near the bottom. Spread them apart: a longer line is more stable.
  3. Place the nub line (B, T). Tap the base of the nub where it leaves the body, then its tip.
  4. Fine-tune. Zoom in with the buttons, a pinch or the mouse wheel, and drag each handle. A magnifier appears while you drag. Undo steps back; Clear points starts over.

Not sure which bump is the nub? Read how to find the nub on your scan first. The legs and umbilical cord are the usual mix-ups.

How to read the three zones

The popular rule has two outcomes: 30° or more is a boy, anything flatter is a girl. It comes from the first study, in 1999. The larger 2006 study by the same lead author used three zones, and this calculator follows it:

  • Over 30°, leans boy. The nub has tilted clearly up and away from the spine line.
  • Under 10°, leans girl. The nub runs parallel to the spine line or points towards it.
  • 10° to 30°, can't tell. In the 2006 study the specialists made no call in this range. They could not assign sex in 43 of 656 babies (7%).

A 2024 study of 312 fetuses at 11–13+6 weeks shows why the middle zone matters. The average angle was 35.9° in boys and 21.6° in girls, so the average girl sits in the grey zone. The best single cutoff there was about 28°. It caught 92% of boys but also called about 15% of girls boys. When two readers measured the same images, their agreement was only moderate. For more on the rule itself, see angle of the dangle.

Accuracy by week

These figures come from trained sonographers measuring on the ultrasound machine, not from photos.

WeekCorrect, Efrat 1999Pooled sensitivity, 2025 review
11 weeks70.3%69%
12 weeks98.7%89%
13 weeks100%96%

The 1999 study covered 172 pregnancies. At 11 weeks, 56% of boys were read as girls, because a boy's nub may not have tilted up yet. So an early "leans girl" result is the weakest one.

The 2006 study followed 656 pregnancies at 12–14 weeks. Boys were read correctly 99–100% of the time. Girls were read correctly 91.5% of the time at 12+0 to 12+3 weeks, 99% at 12+4 to 12+6 and 100% at 13 weeks. Across the nine first-trimester studies in the 2025 review that reported accuracy, the mean was 85%. For the full picture, read nub theory at 11, 12 and 13 weeks.

Why your number may differ from a specialist's

  • Tilt. The angle only means something on a true side profile. If the baby is turned a little, a flat nub can look raised, and the other way round.
  • The photo itself. A picture of a printout adds blur, glare and perspective. The printout is also a single frozen frame, while the sonographer could adjust the live image.
  • Short lines. The nub is only a few pixels long, so a small nudge of the tip moves the angle by several degrees. Measure twice. If the result jumps between zones, it is too close to call.
  • The week. Before 12 weeks many boys still look flat.

Compare what you see with the drawings in boy vs girl nub, or start from the nub theory guide. If you want a reliable answer this early, ask your provider about a prenatal blood test or wait for the anatomy scan.

Keep your read with the rest

In BabyPeek you measure the nub on your 12-week scan, then the app adds Ramzi, skull theory, the Chinese calendar and a playful Photo Pick, labels how much evidence each has, and turns the majority verdict into a shareable reveal.

Download on theApp Store

FAQ

It is free and needs no sign-up. The image is opened and drawn inside your browser, and the page sends nothing to a server. Closing the tab removes it.
In the 2006 study by Efrat and colleagues, more than 30° from the spine line was read as a boy and less than 10° as a girl. Between 10° and 30° no call was made. The popular two-way rule, 30° or more for a boy, comes from the original 1999 study.
No study has measured that. The published accuracy comes from specialists measuring the live ultrasound at 12–14 weeks. A photo of a printout can be tilted, blurry and cropped, so treat the number as a fun clue.
You can, but the answer is unreliable. In the 1999 study specialists were right 70.3% of the time at 11 weeks, and 56% of boys were read as girls because their nub had not tilted up yet.
A few pixels at the tip of a short nub can move the angle by several degrees. Even professionals disagree: in a 2024 study, agreement between two readers measuring the same images was only moderate.

Sources

  1. Efrat Z, Akinfenwa OO, Nicolaides KH. First-trimester determination of fetal gender by ultrasound. Ultrasound Obstet Gynecol. 1999;13(5):305–7. PubMed
  2. Efrat Z, Perri T, Ramati E, Tugendreich D, Meizner I. Fetal gender assignment by first-trimester ultrasound. Ultrasound Obstet Gynecol. 2006;27(6):619–21. PubMed
  3. Alfuraih AM, Almajem BM, Alsolai AA. The predictive accuracy of anogenital distance and genital tubercle angle for first-trimester fetal sex determination. Diagnostics (Basel). 2024;14(16):1811. PubMed
  4. Amankona E et al. Effectiveness of prenatal ultrasound in fetal sex identification: a systematic review and meta-analysis. J Ultrasound. 2025;28(3):563–74. PubMed

Written by the BabyPeek editorial team from the studies above. How we research and write. For entertainment, not medical advice.