Our Findings After Nearly 20 Years of Nub Theory

After nearly two decades of assessing nub theory and analysing many thousands of ultrasound scans, we’ve seen one thing time and time again- accuracy increases with gestation.

The model below is based on our own observations and experience. It is not a clinical study or medical evidence, but a reflection of the trends we have consistently seen over the years.

Our accuracy estimates assume that each scan is assessed by an experienced nub theory practitioner. Correctly identifying the nub, assessing the surrounding anatomy, interpreting the nub’s position, angle and development, and deciding whether a scan is suitable for assessment all require specialist knowledge and experience. Gestational age, image quality, fetal position and clear visualisation of the genital area can all influence the confidence of a prediction. Even when viewing the same scan, someone without specialist nub theory experience is unlikely to achieve the same level of accuracy.

Typical Estimated Confidence By Gestation

Gestation

Our Observed Accuracy*

12+0 to 12+2: 80–85%

12+3 to 12+6: 85–90%

13+0 to 13+3: 90–95%

13+4 to 13+6: 95–98%

14+0 weeks onwards: 98–99%

*These approximate confidence ranges are informed by almost 20 years of practical experience assessing ultrasound scans and our own assessment algorithm. They represent our assessors’ professional estimates, not measured success rates or clinically validated accuracy figures.

The ranges assume suitable, good-quality images with clear visibility of the genital area. Each scan is assessed individually, and confidence may vary with gestation, image quality, foetal position and surrounding anatomy.

Nub theory is an informed interpretation of ultrasound anatomy, not a diagnostic medical test. No assessment or confidence rating guarantees your baby’s sex. Nothing but birth is 100%.