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Every placenta should be examined, as it reflects disease in the mother and the fetus. Frequently these processes are unsuspected previously. The information the placenta contains is often unavailable from any other source. The necessary examination will vary with the clinical situation and ranges from simple visual inspection to detailed molecular studies.

Some appreciation of placental development and structure is necessary to understand its examination and certain pathology. While the placenta shows extensive growth and histologic change in the second and third trimesters, the basic gross morphology is established early in pregnancy, before the end of the first trimester.

The umbilical cord is the lifeline of the fetus. Complete cord occlusion often leads to fetal demise while intermittent obstruction has been associated with intrauterine brain damage. Cord compression and vasospasm are important factors in fetal distress. Careful umbilical cord examination often reveals significant lesions which may be associated with these processes.

The peripheral membranes and fetal placental surface are continuous, and most processes are seen in both. The layer of membrane closest to the fetus is amnion. External is the chorion, which is minimal on the peripheral membranes and more extensive on the disk. The remnant of the yolk sac lies between the amnion and chorion (Figure 4.1). The chorion is continuous with all the villous tissue. There is close proximity of the surface membranes to the maternal blood of the intervillous space, while the peripheral membranes abut the decidua and its blood vessels. This relationship permits maternal cells access to the membranes.

The general gross morphology of the placenta is established before the end of the first trimester, and further change is largely limited to growth and histologic maturation of villi. During placental examination the villous tissue is examined from the maternal side before and after transverse cuts have been made. While visual inspection is important, palpation of the placenta may be even more revealing of pathologic processes. Most villous lesions show diagnostic gross morphology. The common abnormalities are predominantly related to placental circulation (Figure 5.1). Alterations in the fetal and maternal components can be recognized and distinguished.

Today in the United States, at least one in 100 births is a multiple gestation and the examination of these placentas is one of the most important aspects of gross placental pathology. Twins account for a disproportionate percentage of perinatal morbidity and mortality and have significantly higher rates than singletons. Placentas of multiple gestations demonstrate all the abnormalities seen in singletons, as well as their own special pathology. While most of the following discussion relates to twins, the same principles are used when evaluating triplet and quadruplet placentas. Multiple births have become more common with assisted reproductive techniques, but refinements of procedure have fortunately decreased the number of higher order births currently conceived. Special twin placenta report forms are useful in examination (Appendix A-2).