In the maternity hospital, doctors use the first day of the last menstruation, to which we add 280 days, as well as the ultrasound diagnostic method to determine the expected date of birth. The most accurate time to determine the gestational age on an ultrasound scan is 11-13 weeks. During an ultrasound examination, the doctor will measure the coccyx-parietal size of the fetus, and using special tables, determine the gestational age to days and, accordingly, the expected date of birth. And, if the difference between the period calculated from the first day of the last menstruation and using ultrasound is less than 5 days, then we focus on the date according to the last menstruation, and if the difference is more than 5 days, then on the date that turned out during the ultrasound examination .