Pregnancy is a long road to a new life; it begins at the moment when the spermatozoon fertilizes the ovum, and ends with delivery. This period, which is of paramount importance both for the mother and child, can be divided into the following main stages of antenatal (pre-delivery) development.
• Embryonic stage starting at the moment of fertilization (at two weeks of pregnancy)1 to full 10 weeks of pregnancy (full 8 weeks after fertilization) sees the following events:
- 3-8 weeks: formation of rudiment organs in the fetus;
- 2-4 weeks: formation of the heart and vessels;
- 4-5 weeks: beginning of lung formation, early development of the nervous system;
- 7-8 weeks: kidney formation.
• Fetal (syn.: antenatal) stage lasting from 11th-week of pregnancy until birth (8-38 full weeks from fertilization or 10-40 weeks from the first day of the last menstrual period):
- 8-12 weeks: sex differentiation;
- 15-20 weeks: intensive growth and maturation of the cerebral cortex;
- 20-24 weeks: formation of major functional systems of the fetus. Embryology (from Greek ἔμβρυον - fetus, embryo, logos - study) developed as a study of embryogenesis, intrauterine development from the moment of conception until birth.
The first notions of the child's intrauterine development emerged in ancient times; they were propounded in the works of philosophers and doctors of Ancient India, Egypt and Greece (Hippocratic Corpus). Some of them (like Anaxagoras in the 5th cent. BC) thought that the paternal or maternal semen contains in miniature all the parts of the future fetus (Fig. 1.1).
Thus there should exist a small human being not discernible by the eye; its development means that it merely grows in size (the idea of premorphism from Latin praeformare, to form in advance).