The chorionic villi, or sponge-like protrusions, which surround the newly embedded embryo rapidly increase in both number and size At about the 6th week of pregnancy blood vessels are formed within the foetus, and by the 8th week within the chorionic villi, thus enabling them to transport materials mote readily to the developing foetus. As the embryo enlarges it gradually fills and then commences to distend the cavity of the uterus and by the 12th week of pregnancy the active growth of the pregnancy itself is beginning to exert pressure on the walls of the uterus and cause its further growth and distension. It is at this stage that the chorionic villi at the site of the original implantation develop with immense speed to form the placenta (or afterbirth), while the chorionic villi over the remainder of the embryo gradually shrivel and die. After the 14th week a proper placenta is formed and takes over the functions of basic nourishment and hormone production.
The newly formed placenta gradually enters larger maternal blood vessels, but the chorionic villi penetrate only as far as the middle layers of the cells lining the inner surface of the uterus. They do not penetrate the muscle of the uterus, nor do they penetrate the major blood vessels which are supplying the uterus.
The action of the chorionic villi upon the maternal tissues is the creation of a lake or pool which fills with maternal blood and surrounded on its outer side by the cells lining the inner side of the uterus, and on the foetal side by the placenta itself, and into which are suspended the individual villi or finger-like processes of the placenta. Blood arrives at the placenta from the foetus by means of the umbilical arteries; it circulates through the placenta and through the chorionic villi and returns to the foetus by way of the umbilical vein.
The placenta is responsible for the selective transfer to the foetus of oxygen and other necessary substances as well as removal from the foetus of its waste products. Everything that is required for the growth and maturation of the foetus must pass the placenta, so the placenta is responsible for the passage of not only oxygen, but also carbohydrate, fatty acids, protein, complicated amino acids, vitamins and essential elements such as calcium. Excretion products from the foetus are absorbed into the maternal circulation and some of these, such as carbon dioxide, are exhaled by the mother from her lungs, whilst others such as urea are excreted by her kidneys.
The placenta reaches maturity at approximately the 34th week of pregnancy. After this it slowly undergoes very mild ageing or degenerative changes.
At term the placenta, or afterbirth, weighs approximately 570 gm. or about one-sixth of the baby’s weight, and is a flattened disc-like mass measuring approximately 18 cm. in diameter and 3-2 cm. in thickness.
The umbilical cord is usually attached to the centre of the foetal surface of the placenta, which has a shiny bluish colour on which can be seen the blood vessels which supply the placenta. The maternal, or deeper, surface of the placenta is usually divided into twelve to fifteen lobules or segments and is a dull red or brownish colour.
The maternal and foetal circulations are entirely independent. The foetal blood circulates from the foetus into the placenta and returns again to the baby. The maternal circulation arrives at the uterus by means of the uterine artery and leaves it via the uterine veins. This blood forms the pool, or lake, of blood into which the lobes of the placenta are immersed. At no stage, with the exception of a few cells as described below, do the circulations or cells from the cir-culations of the mother and foetus mix. Materials are transmitted rapidly across the barrier formed by the placenta either from the foetus to the mother or from the mother to the foetus. This barrier is sometimes only one cell thick.
Bleeding occurring during pregnancy, either early or later, comes from the pool of maternal blood lying immediately beneath the placenta. The blood is always maternal blood and never blood from the foetal circulation, since it is impossible for blood to escape from the foetal circulation.
Small finger-like processes of the placenta may occasionally break off and be washed away with the maternal blood into the mother’s circulation. This may occasionally include a few of the circulating red cells from the foetus. The intrusion of these foetal cells into the maternal circulation occurs intermittently throughout the whole of pregnancy, and careful examination of the mother’s blood will occasionally demonstrate cells either from the placenta itself or from the foetal circulation. At the time of delivery quite a large number of both placental cells and circulating foetal blood cells escape into the maternal circulation and it is the marked leakage of cells at this particular time that is responsible for Rhesus sensitization.
Hormones Produced by the Placenta
The placenta produces a large number of hormones, one of the most important being progesterone. It begins to produce progesterone after about the 1oth week of pregnancy and, by the end of the 14th week, should be manufacturing sufficient to ensure the survival of the entire pregnancy and so replace the progesterone previously made by the corpus luteum.
The placenta makes human placental lactogen (H.P.L.) and oestrogens (excreted as oestriol). These hormones may be used later in pregnancy as an indicator of placental activity and function.
Abnormalities of the Placenta