In addition, an ultrasound diagnostic doctor can determine the gestational age.
If we are talking about short stages of pregnancy and the embryo has not yet been identified, but there is a fertilized egg in the cavity, then the calculation is carried out according to its average diameter, but this method is approximate. When the embryo is already clearly visible (and this is 7-8 weeks), the period is calculated based on the coccygeal-parietal size of the fetus, and this method is already more reliable.
At later stages, the doctor focuses on the size of the uterus, or more precisely on the height of the uterine fundus from the symphysis pubis. Everything is quite simple here: approximately every week the uterus grows by 1 cm (that is, if the period is 22 weeks, then the height of the fundus of the uterus from the womb will be approximately 22 cm).