The uterus is a thick-walled, pear-shaped organ measuring seven centimetres (about 2.75 inches) in length and weighing 30 grams (about one ounce) in an unpregnant woman in her later teens. It has a buttonlike lower end, the cervix, that merges with the bulbous larger portion, called the corpus. The corpus comprises approximately three-fourths of the uterus. There is a flat, triangular-shaped cavity within the uterus. At term, the uterus is a large, thin-walled, hollow, elastic, fluid-filled cylinder measuring approximately 30 centimetres (about 12 inches) in length, weighing approximately 1,200 grams (2.6 pounds), and having a capacity of 4,000 to 5,000 millilitres (4.2 to 5.3 quarts).

The greater size of the uterus as a result of pregnancy is due to a marked increase in the number of muscle fibres, blood vessels, nerves, and lymphatic vessels in the uterine wall. There is also a five- to tenfold increase in the size of the individual muscle fibre and marked enlargement in the diameters of the blood and lymph vessels.

During the first few weeks of pregnancy, the shape of the uterus is unchanged, but the organ becomes gradually softer. By the 14th week it forms a flattened or oblate spheroid. The fibrous cervix becomes remarkably softer and acquires a protective mucus plug within its cavity, but otherwise it changes little before labour. The lower part of the corpus, the isthmus, first becomes elongated and then, as the uterine contents demand more space, stretches and unfolds to form a bowl-shaped formation called the lower uterine segment. The fibrous nature of the cervix causes it to resist this unfolding action.

The uterine wall is stretched and thinned during pregnancy by the growing conceptus, as the whole product of conception is called, and by the fluid that surrounds it. By term, this process converts the uterus into an elastic, fluid-filled cylinder. It is only late in pregnancy that the cervix gradually thins out and softens; during labour it dilates for passage of the infant.

As pregnancy progresses, the uterus rises out of the pelvis and fills the abdominal cavity. It is top-heavy near term so that it falls forward and, because of the large bowel on the left side, rotates to the right. It presses on the diaphragm and pushes the other organs aside. The uterus may sink downward in the pelvis several weeks before term in a process that is known as lightening or dropping. This occurs as the fetal head descends into the pelvis. In some women, particularly those who have borne children, lightening does not occur until the onset of labour. Lightening may be impossible in women who have an abnormally small pelvis, an oversized fetus, or a fetus lying in an abnormal position.

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For a short time after fertilization, the conceptus, a minute bubblelike structure called a blastocyst, lies unattached in the uterine cavity. The cells that will become the embryo (the embryonic disk) form a thickened layer on one side of the bubble. Elsewhere, the walls of the bubble consist of a single layer of cells; these cells are the trophoblast, which has a special ability to attach to and invade the uterine wall. The trophoblast plays an important role later in the development of the placenta or afterbirth. The conceptus makes contact with the uterine lining about the fifth or sixth day after conception. After contact the blastocyst collapses to form a rounded disk with the embryonic mass on the surface and the trophoblast against the endometrium (uterine lining). The part of the trophoblast that is in contact with the endometrium grows into and invades the maternal tissue. Concomitant disintegration of the endometrium allows the conceptus to sink into the uterine lining.

Soon the entire blastocyst is buried in the endometrium. Proliferation of the trophoblast over the part of the collapsed bubble that is opposite the embryo is part of the implantation procedure that helps to cover the blastocyst. After a few days, a cavity forms that bears the same relation to the embryonic disk that the blastocyst cavity did before; this cavity will become the fluid-filled chorionic cavity containing the embryo. Ultimately it will contain the amniotic fluid that surrounds the fetus, the fetus itself, and the umbilical cord.

The body stalk, which will become the umbilical cord, then begins to separate the embryo from the syncytiotrophoblast, the outer layer of the trophoblast lying against the endometrium; the inner lining of the trophoblast is called cytotrophoblast. As the syncytiotrophoblast advances into the endometrium, it surrounds minute branches of the uterine arteries that contain maternal blood. Erosion of the endometrium about these blood sinuses allows them to open into the small cavities in the trophoblast. The cytotrophoblast, which lines the cavity, forms fingers of proliferating cells extending into the syncytiotrophoblast. After the placenta is developed, these fingers will be the cores of the rootlike placental villi, structures that will draw nutrients and oxygen from the maternal blood that bathes them. This is the first step in uteroplacental circulation, which supplies the fetus with all of the sustenance necessary for life and growth and removes waste products from it. During the third week of pregnancy, the syncytiotrophoblast forms a single layer of cells covering the growing villi and lining the syncytial lacunae or small cavities between the villi. The conceptus is buried in the endometrium, and its whole surface is covered at this time by developing villi. The greater part of the chorionic wall is now cytotrophoblast. Fingers of cytotrophoblast in the form of cell masses extend into the syncytial layer. Soon thereafter, a layer of connective tissue, or mesoderm, grows into the villi, which now form branches as they spread out into the blood-filled spaces in the endometrium adjacent to the conceptus.

By the end of the third week, the chorionic villi that form the outer surface of the chorionic sac are covered by a thick layer of cytotrophoblast and have a connective tissue core within which embryonic blood vessels are beginning to develop. The vessels, which arise from the yolk sac, connect with the primitive vascular system in the embryo. As growth progresses the layer of cytotrophoblast begins to regress. It disappears by the fifth month of pregnancy.

The layer of endometrium closest to the encroaching conceptus forms, with remnants of the invading syncytiotrophoblast, a thin plate of cells known as the decidua basalis, the maternal component of the mature placenta; it is cast off when the placenta is expelled. The fetal part of the placenta—the villi and their contained blood vessels—is separated from the decidua basalis by a lakelike body of fluid blood. This pool was created by coalescence of the intervillous spaces. The intervillous spaces in turn were formed from the syncytial lacunae in the young conceptus. Maternal blood enters this blood mass from the branches of the uterine arteries. The pool is drained by the uterine veins. It is so choked by intermingling villi and their branches that its continuity is lost on gross inspection.

The chorionic cavity contains the fluid in which the embryo floats. As its shell or outer surface becomes larger, the decidua capsularis, which is that part of the endometrium that has grown over the side of the conceptus away from the embryo (i.e., the abembryonic side) after implantation, becomes thinner. After 12 weeks or so, the villi on this side, which is the side directed toward the uterine cavity, disappear, leaving the smooth chorion, now called the chorion laeve. The chorion frondosum is that part of the conceptus that forms as the villi grow larger on the side of the chorionic shell next to the uterine wall. The discus-shaped placenta develops from the chorion frondosum and the decidua basalis.

At term, the normal placenta is a disk-shaped structure approximately 16 to 20 centimetres (about six to seven inches) in diameter, three or four centimetres (about 1.2–1.6 inches) in thickness at its thickest part, and weighing between 500 and 1,000 grams (1.1 and 2.2 pounds). It is thinner at its margins, where it is joined to the membrane-like chorion which spreads out over the whole inner surface of the uterus and contains the fetus and the amniotic fluid. The amnion, a thinner membrane, is adherent to and covers the inner surface of the chorion. The inner or fetal surface of the placenta is shiny, smooth, and traversed by a number of branching fetal blood vessels that come together at the point—usually the centre of the placenta—where the umbilical cord attaches. The maternal or uterine side of the placenta, covered by the thin, flaky decidua basalis, a cast-off part of the uterine lining, is rough and purplish-red, and has a raw appearance. When the placenta is cut across, its interior is seen to be made of a soft, crepelike or spongy matrix from which semisolid or clotted blood, caught when it is separated from the uterine wall to which it was attached, can be squeezed. Detailed examination shows that the villi and their branches form an arborescent (treelike) mass within the huge blood lake of the intervillous space. Anchoring villi extend outward from the fetal side and fuse with the decidua basalis to hold the organ’s shape. Others, algaelike, float freely in the blood lake. Dividing partitions, formed from the trophoblast shell, project into the intervillous space from the decidual side. They divide the placenta into 15 or 20 compartments, which are called cotyledons.

Maternal blood flows from the uterine vessels into the trophoblast-lined intervillous blood lake. Within each villus is a blood vessel network that is part of the fetal circulatory system. Blood within the villous vessel is circulated by the fetal heart. The blood vessel wall, the connective tissue of the villous core, and the syncytiotrophoblast covering the villus lie between the fetal and the maternal bloodstreams. This is known as the placental barrier. As pregnancy progresses, the fetal blood vessels become larger, the connective tissue stretches over them, and the syncytiotrophoblastic layer becomes fragmentary. As a result, the placental barrier becomes much thinner. Normally, blood cells and bacteria do not pass through it, but nutrients, water, salt, viruses, hormones, and many other substances, including many drugs, can filter across it.