Uteroplacental physiology - more detailed information.

Certain structural changes occur in the uterine blood vessels during pregnancy, which are essential to the satisfactory formation of the placenta and development of an adequate fetoplacental circulation. The pivotal change occurs within the uterine arterial circulation especially the resistance vessels, the spiral arterioles. This is a continuous process throughout pregnancy. Invasion by non-villous trophoblast occurs in the first trimester, usually before the 12th week. Trophoblastic invasion of luminal smooth muscle has usually taken place by the 20th week. The process of placentation results in the conversion of the uterine circulation from a high resistance, low flow circuit, to a low resistance, high flow system, ensuring good communication between (dilated, tortuous spiral arterioles) uteroplacental arteries and the intervillous space. The pelvic venous vasculature also undergoes a process of maeked dilation, serving as a reservoir for the circuit in later pregnancy when large amounts of blood are circulated.

Fetal physiology - more detailed information.

While the fetus eventually develops some degree of autonomy during pregnancy, structurally, it is the placenta that remains in direct contact with the external environment. In practise therefore, both fetus and placenta serve as a single functional unit for the purpose of vascular, respiratory, metabolic and endocrine activity. (Continued on next page).