- Harvey J. Kliman, M.D., Ph.D.
- New Haven, Connecticut
- Department Obstetrics and
Gynecology,
- Yale University School of Medicine
The Placenta is the single
most important factor in producing a healthy baby. The placenta, which is in fact part of
the fetus, is critical for all aspects of pregnancy from implantation to delivery. As
early as three days after fertilization, the trophoblaststhe major cell type of the
placentabegin to make human chorionic gonadotropin, a hormone which insures that the
endometrium will be receptive to the implanting embryo. Over the next few days, these same
trophoblasts attach to and invade into the uterine lining, beginning the process of
pregnancy. Over the next few weeks the placenta begins to make hormones which control the
basic physiology of the mother in such a way that the fetus is supplied with the necessary
nutrients and oxygen needed for successful growth. The placenta also protects the fetus
from immune attack by the mother, removes waste products from the fetus, induces the
mother to bring more blood to the placenta, and near the time of delivery, produces
hormones that matures the fetal organs in preparation for life outside of the uterus. In
many ways the placenta is the SCUBA system for the fetus while at the same time being the
Houston Control Center guiding the mother through pregnancy.
The placenta is dedicated to
the survival of the fetus. Even when exposed to a poor maternal environmentfor example
when the mother is malnourished, diseased, smokes or takes cocainethe placenta can often
compensate by becoming more efficient. Unfortunately, there are limits to the placenta's
ability to cope with external stresses. Eventually, if multiple or severe enough, these
stresses can lead to placental damage, fetal damage and even intrauterine demise and
pregnancy loss.
Just as the rings of a cut
tree can tell the story of the tree's life, so too the placenta can disclose the history
of the pregnancy. In cases of poor pregnancy outcome, microscopic examination of the
placenta often reveals the stresses that caused the fetal damage observed in an affected
newborn.
The major pathologic
processes observable in the placenta that can adversely affect pregnancy outcome include
intrauterine bacterial infections, decreased blood flow to the placenta from the mother
and immunologic attack of the placenta by the mother's immune system. Intrauterine
infections, most commonly the result of migration of vaginal bacteria through the cervix
into the uterine cavity, can lead to severe fetal hypoxia as a result of villous edema
(fluid build up within the placenta itself). Both chronic and acute decreases in blood
flow to the placenta can cause severe fetal damage and even death. As well as supplying
the fetus with nutrition, the placenta is also a barrier between the mother and fetus,
protecting the fetus from immune rejection by the mother, a pathologic process that can
lead to intrauterine growth retardation or even demise. In addition to these major
pathologic categories, many other insultssuch as placental separation, cord accidents,
trauma, viral and parasitic infectionscan adversely affect pregnancy outcome by affecting
the function of the placenta.
A trained
placental pathologist can examine a placenta and assist in the elucidation of the causes
of poor pregnancy outcome. A complete placental examination is most useful shortly after
the time of delivery when the affected family is most in need of understanding what
happened to their baby. If a full placental examination is not possible at the time of
delivery because no placental pathologist is available, then the placenta can be
transferred to a center that is prepared to make such an examination. As long as tissue
blocks are saved from the placenta, a microscopic examination of the placenta is always
possible at a later time if the need arises.
- Today, only a few specialized
centers for placental examination exist in the US. As the cost of processing and examining
placentas decreases, more of the 4 million placentas delivered every year will be able to
be examined by appropriately trained physicians. This trend will lead to a better
understanding of causes of poor pregnancy outcomes, which in turn will lead to better
diagnostic and therapeutic approaches to complicated pregnancies. The ultimate goal of
placental examination and research is to insure that wanted babies are healthy babies.