Anatomically, development of prenatal life is divided into embryonic and foetal periods. Although the embryonic period is universally divided into 23 stages, there are fewer consensuses on the stages of the foetal period.
From a radiobiological viewpoint, prenatal life is divided into pre-implantation, organogenesis, and the foetal period after all major organ precursors have developed.
In humans, this period typically extends until 10 days, and corresponds to the period after fertilisation has occurred and before the developing embryo has implanted into the uterine wall.
This period extends until 6 – 7 weeks in humans, during which time the major organs become differentiated from each other. The organs are not fully differentiated, rendering them highly susceptible to radiation effects.
The foetal period extends from about 40 days until delivery. As most of the organs have formed, congenital abnormalities are far less common following radiation exposure. Microcephaly and mental retardation have been reported following radiation exposure, particularly in the late phases of the 1st trimester and early 2nd trimester.