Boy or girl?
Is the nub theory accurate at 12 or 13 weeks?
Partly true
It can be fairly accurate at 12–13 weeks in experienced hands, but it is an educated guess, not a diagnosis.
Early on, boys and girls both have a small bump between the legs called the genital tubercle, or “nub”. On a side view, a nub angled more than about 30° upwards from the spine suggests a boy, and a flatter nub suggests a girl.
Accuracy rises with gestational age. One study reported about 70% at 11 weeks and close to 100% by 13 weeks in expert hands, though a measurement isn’t always possible and everyday results may be lower. Image quality and the baby’s position matter a lot. Confirm with NIPT or the 18–22 week anatomy scan before buying anything pink or blue.
References
Related weeks