Basic Embryology
The study of embryology can be considered from aspect of the embryonic and fetal periods. The embryonic period is considered to be the period from fertilization to the end of the eighth week of development. Meanwhile, the fetal period is considered to be the period from the beginning of the ninth week of development to the birth of the fetus.
Embryonic Period
During the embryonic period of development, the major structures and organ systems (while not fully functioning yet) are formed. By the end of the eighth week of development, the features of an embryo that are discernable include: facial features, the digits, and the limbs.
Meanwhile, from the ninth week of development through to birth, the fetus matures in what is known as the fetal period. During this time, the fetus will continuously increase in size and complexity. During this stage of development, the previously formed organ systems continue their functional development.
Clinical Notes:
Embryologists consider development as beginning on day 1, or the point of fertilization while clinicians use the date of the last menstrual period to determine the period of gestation. Due to this, there is a 2 week discrepancy between an embryologist’s timeline of development vs a clinician’s timeline.