And some hospitals have a policy of not telling the sex, whatever the mum's stage of pregnancy.
If you want to pay to find out what your baby's gender is before your anomaly scan, some private clinics offer "gender" ultrasound scans from 16 weeks.
These images were taken at 13 weeks of pregnancy, and show the genital nubs of two different babies.
The first baby has a genital nub at an angle that's greater than 30 degrees, so he's probably a boy.
An obvious marker is the first day of the last menstrual period (LMP). Well, another variable is the length of menses but everyone has a first day so to be consistent, that is the marker used.
We generally use gestational age when discussing pregnancy.
Find out more about how your baby’s genitals develop.
Testosterone prompts the bud to grow and develop into a penis and scrotum.In a girl, the genital nub will become the clitoris and labia.Nub theory is based on the idea that it's possible to tell which type of genital nub your baby has from the dating scan image.In terms of nub theory, these scans provide an unusually good view of the babies' emerging genitals on an early scan. Getting the gender right at this stage is far more likely.Even then, most NHS hospitals will only disclose the gender at the anomaly scan if the parents ask.