Placenta

Fetal-Placental-Uterine Circulation

Oxygenated maternal blood is brought to the placenta through 80 to 100 end branches of the uterine arteries, the spiral arteries. Maternal blood enters the intervillous space near the central part of each placental lobule, where it flows around and over the surface of the villi. Maternal blood returns to the maternal circulation through the endometrial and then uterine veins. A very thin layer normally separates the fetal blood from the maternal blood. This layer is composed of the capillary wall, the trophoblastic basement membrane, and a thin rim of cytoplasm of the syncytiotrophoblast. The fetal placenta is anchored to the maternal placenta by the cytotrophoblastic shell and anchoring villi. It provides a large area where materials may be exchanged across the placental membrane and interposed between fetal and maternal circulation.

Oxygen- and nutrient-rich blood is carried back to the fetal systemic circulation through the umbilical vein. In the fetal abdomen, some of the blood is distributed to the liver, whereas the rest passes through the ductus venosus into the inferior vena cava and continues to the heart through the right atrium across the foramen ovale and into the left atrium. This oxygen-rich blood then passes into the left ventricle and the ascending aorta and supplies the brain and upper part of the body through the brachiocephalic circulation. Unoxygenated blood from the superior vena cava passes into the right atrium, through the right ventricle, and across the main pulmonary artery. A minor portion of blood from the right ventricle supplies the lungs, whereas the majority of the blood passes through a fetal shunt, the ductus arteriosus, into the aorta arch. This shunt protects the lungs against circulatory overload. The fetal blood continues inferiorly through the descending aorta, internal iliac arteries, and umbilical arteries and into the umbilical cord to return to the placenta for oxygen and nutrient exchange. By term, approximately 40% of the fetal cardiac output is directed through the umbilical circulation.

The placenta is dedicated to the survival of the fetus. Even when exposed to a poor maternal environment (e.g., when the mother is malnourished, ill, smokes, or abuses drugs), the placenta can compensate by becoming more efficient. Unfortunately, there are limits to the placenta’s ability to cope with external stressors. Eventually, if multiple or severe enough, these stressors may lead to placental insufficiency, fetal growth restriction, intrauterine death, and pregnancy loss.

During pregnancy, maternal blood volume increases 40% to 50% by term compared with the nonpregnant state. Maternal placental circulation may be affected by maternal conditions that decrease uterine blood flow, such as severe hypertension and renal disease. Intrauterine growth restriction (IUGR) is a fetal change that may be associated with decreased placental flow.

Cordal Attachments

The location of the umbilical cord attachment to the placenta is important. The majority of the time, the umbilical cord inserts on the fetal surface of the placenta more than 3 cm from the margin. Abnormal cord insertions include marginal insertion and velamentous insertion. With a marginal insertion , also referred to as a battledore placenta , the insertion of the umbilical cord is at or close to the placental margin within less than 2 cm of the edge (less than 10%) ( Fig. 56.3 ). A velamentous insertion refers to an umbilical cord that inserts into the fetal membranes and then travels to the placenta within the membranes (1%) ( Fig. 56.4 ). These submembranous vessels are fragile and, in a small number of cases (less than 2%), may be associated with significant fetal hemorrhage, especially if the membrane carrying the vessels is positioned across the internal cervical os (vasa previa).

Placental Implantation

Placental location should be documented during the second-trimester sonographic examination. Normally the placenta will be located on the anterior, fundal, posterior, or lateral wall of the uterus. The relationship of the placental edge to the internal cervical os should be evaluated during each sonogram. Occasionally, placental implantation will occur within the lower uterine segment (LUS), resulting in a placenta previa or a low-lying placenta .

Membranes

The fetal membranes consist of the chorion, amnion, allantois, and yolk sac. The chorion originates from the trophoblastic cells and remains in contact with the trophoblasts throughout pregnancy. The amnion develops at the 28th menstrual day and is attached to the margins of the embryonic disk. As the embryo grows and folds ventrally, the junction of the amnion is reduced to a small area on the ventral surface of the embryo to form the umbilicus.

Expansion of the amniotic cavity occurs with the production of amniotic fluid. Usually, by 16 weeks of gestation, fusion of the amnion with the chorion occurs and will no longer be visualized sonographically as two separate membranes. If the amnion/chorion separation extends beyond 16 weeks of gestation, it may be associated with polyhydramnios, aneuploidy, or prior amniocentesis. Placental hemorrhage may mimic the appearance of amnion/chorion separation on sonogram.

The secondary yolk sac forms after the regression of the primary yolk sac on the ventral surface of the embryonic disk at 28 menstrual days. The yolk sac has a role in transferring nutrients to the embryo during the second and third weeks of gestation while the uteroplacental circulation is developing. It is connected to the midgut by a narrow yolk stalk. By 5 menstrual weeks, the amniotic sac and secondary yolk sac are pressed together with the embryonic disk between them. This structure is suspended within the chorionic cavity. The yolk sac becomes displaced from the embryo and lies between the amnion and the chorion (see Fig. 56.1 ). By 9 weeks, the yolk sac has diminished to less than 5 mm in diameter.

Sonographic Evaluation of the Umbilical Cord

The vessels of the cord may be followed with real-time ultrasound from the placenta to the fetal abdomen, and the sonographer should document the umbilical cord insertion into the placenta ( Fig. 56.5 ) and the fetal abdomen. The intraabdominal portion of the umbilical vein courses superiorly. The ductus venosus shunts a significant amount of the oxygenated blood from the umbilical vein directly into the inferior vena cava. A portion of the umbilical venous blood also supplies the liver ( Fig. 56.6 ).

Sonographically, the ductus venous appears as a thin intrahepatic channel with echogenic walls branching from the umbilical vein ( Fig. 56.7 ). It lies in the groove between the left lobe and the caudate lobe of the liver. The ductus venosus is patent during fetal life until shortly after birth, when closure occurs. After closure, the remnant of the ductus venosus is known as the ligamentum venosum .

The two umbilical arteries carry deoxygenated blood from the fetus to the placenta. They ascend into the umbilical cord and are a branch of the internal iliac arteries. Their normal position extends around the fetal bladder. There are two acceptable ways to document the umbilical arteries. The first approach is to image the fetal pelvis in an axial scanning plane at the level of the fetal bladder. The umbilical arteries will be visible along the lateral aspect of the fetal bladder and then ascend toward the umbilical cord insertion. Color and power Doppler can be used to visualize the umbilical arteries. The second approach is to evaluate a free loop of cord in a short axis plane to visualize the umbilical vein and two smaller arteries. After birth, the umbilical arteries become the superior vesical arteries.