When discussing the normal development of early pregnancy from an echographic point of view, it is useful to divide the first trimester into three periods: 4 to 6 weeks, 7 to 8 weeks, and 9 to 11 weeks. During the embryonic period, all major organs are formed. In the fetal period they grow and complete their functional development.
Such a distinction is somewhat arbitrary and is based on terminology accepted in embryology. These terms, particularly gestational age, differ from those used clinically.
Embryologists usually count gestational age from the moment of conception, while in clinical practice it is customary to use menstrual age, which is counted from the first day of the last menstrual period, since, as a rule, its date is known.
Although the time interval between the first day of the last menstrual period and the moment of conception is usually two weeks, it can vary up to 8 days. All data presented here is described based on the calculation of gestational age based on the date of the last menstrual period.