Placental Causes of Fetal Loss
Vol.1 No.7
INTRODUCTION
The term placenta was
introduced in 1559 and is derived from the Latin word for a "circular cake". The
placenta or "afterbirth" is the organ of metabolic exchange between the fetus
and mother. It has a portion derived from the developing embryo and a maternal portion
formed by modification of the uterine lining. There is no direct mixing of fetal and
maternal blood. The intervening tissue is sufficiently thin to permit the exchange of
nutrients and oxygen into the fetal blood and the release of carbon dioxide and waste
materials from it. The placenta in the third trimester of pregnancy is a disk-shaped organ
measuring approximately 20 centimeters (cm) in diameter and 2 to 3 cm in thickness. It has
a maternal surface, attached to the uterus, or womb, and a fetal surface. The umbilical
cord extends from the fetus to the fetal surface of the placenta. There are many potential
abnormalities of the placenta that can result in fetal death: 1. Placental abruption 2.
Trauma 3. Circulatory disturbances 4. Abnormalities of placentation 5. Tumors of the
placenta 6. Abnormalities of the umbilical cord
PLACENTAL ABRUPTION
Placental abruption is
defined as separation of the maternal surface of the placenta from the uterus before
delivery of the fetus. It occurs in approximately 0.9% of pregnancies and accounts for 15%
to 25% of all perinatal mortality (stillbirths and neonatal deaths). Unfortunately,
placental abruption often occurs without advance notice. The most common symptom of
abruption is painful vaginal bleeding, but the clinical presentation is variable. Some of
the bleeding of placental abruption usually escapes through the cervix, resulting in
recognizable external hemorrhage. Less commonly, the blood does not escape externally but
is retained between the detached maternal surface of the placenta and the uterus,
resulting in a concealed hemorrhage. Although abruptions may occur any time during a
pregnancy, approximately 42% occur after 37 weeks (term). The primary cause of placental
abruption is unknown, but there are several associated conditions including: maternal
hypertension (both pregnancy-induced and chronic hypertension), cigarette smoking, cocaine
use, advanced maternal age, increasing parity (number of births), abdominal trauma
(especially motor vehicle accidents), and preterm premature rupture of the membranes.
Placental abruption may be total or partial. Treatment for placental abruption varies
depending upon the condition of the mother and fetus. If there is significant bleeding,
blood transfusions and prompt delivery may be lifesaving for the mother and fetus. If the
mother is stable and the fetus is immature (preterm) and not compromised, then expectant
management with very close observation and continuous electronic fetal heart rate
monitoring in hospital may be beneficial. However, facilities and personnel for immediate
intervention must be available. The risk of recurrent abruption in a subsequent pregnancy
is high, approximately 1 in 8 pregnancies. The frequency of placental abruption fatal to
the fetus has declined to about 1 in 800 deliveries.
TRAUMA
Trauma and accidents are the
leading cause of death in young reproductive age women. It is estimated that 1 in 12
pregnancies will be complicated by trauma. Motor vehicle accidents are the most common
cause of blunt trauma to the pregnant woman. The use of seat-belts with shoulder straps is
recommended at all times, including while pregnant. Other causes include falls and,
unfortunately, assaults, which appear to be increasing in frequency. Traumatic placental
abruption reportedly complicates 1% to 6% of "minor" injuries and up to 50% of
major injuries. Placental abruption is discussed above and usually develops early
following trauma. In the absence of placental abruption fetal injury and death are
uncommon. If the placenta is lacerated, fetal blood may hemorrhage into the maternal
circulation, a condition termed fetomaternal hemorrhage.
CIRCULATORY DISTURBANCES
Infarction or infarct refers
to an area of cell death and tissue necrosis resulting from insufficient blood supply.
Microscopic thrombi (blood clots) may form within blood vessels, impeding blood flow, and
are a common cause of infarction. This is usually what occurs during a heart attack
("myocardial infarction") secondary to occlusion of a coronary artery.
Constriction or closure of blood vessels (vasoconstriction) can occur for a variety of
reasons, most commonly as a result of hypertension. Additionally, certain substances, for
example cocaine, are "vasoactive" and are known to cause closure of blood
vessels and subsequent infarction. The placenta is a highly vascular organ. Any process
that adversely affects blood vessels can damage placental blood vessels as well as the
uterine blood vessels (spiral arteries) that "feed" the placenta. Placental
infarcts are common features of a normal "aging" placenta. They are found in
approximately 25% of uncomplicated term pregnancies are appear to be of no clinical
significance. However, certain maternal diseases, such as severe hypertension and
connective-tissue disorders (e.g., lupus, antiphospholipid antibody syndrome, scleroderma,
and rheumatoid arthritis) may lead to extensive placental infarction. If the placenta is
partially compromised (uteroplacental insufficiency) the fetus may not be able to grow
appropriately (intrauterine growth retardation--IUGR). However, in severe cases, blood
flow to and from the placenta may not be enough to keep the fetus alive.
ABNORMALITIES OF PLACENTATION
When the placenta is located
over or very near the internal opening (os) of the cervix, it is termed placenta previa.
Placenta previa is classified as marginal, partial, or total, depending on the
relationship of the placenta to the internal opening of the cervix (i.e., a total placenta
previa completely covers the cervix). Placenta previa occurs when the zygote implants very
low in the uterus, in close proximity to the internal cervical opening. These placentas
usually "migrate" away from the cervix as the pregnancy progresses and the
uterus increases in size to accommodate the growing fetus. Placenta previa complicates
approximately 1 in 200 deliveries. The most common presentation is painless vaginal
bleeding in the third trimester of pregnancy. The major complications of placenta previa
are maternal hemorrhage and shock, and significant perinatal mortality (stillbirths and
neonatal deaths). Although approximately half of patients are near term when bleeding
first develops, preterm delivery remains a major cause of perinatal death. The primary
cause of placenta previa is unknown, but there are several risk factors including:
advanced maternal age, high parity (number of births), prior cesarean section, prior
elective abortion, multiple fetuses, and cigarette smoking. Placenta previa may be
associated with abnormal attachment of the placenta to the uterus (placenta accreta,
increta and percreta), especially if the placenta previa is located over a previous
cesarean section scar. As with placental abruption, the treatment of placenta previa
varies depending upon the condition of the mother and fetus.
TUMORS OF THE PLACENTA
Tumors may develop in the
placenta as in other tissues. Chorioangiomas, the most common placental tumor, are benign
hemangiomas of the fetal blood vessels. They have been reported in approximately 1% of
placentas. Small tumors are usually asymptomatic and of no clinical significance. However,
large tumors (greater than 5 cm in diameter) may be associated with polyhydramnios (too
much amniotic fluid) and premature labor, or antepartum hemorrhage. Fetal death and
malformations are uncommon complications. Metastases of malignant tumors to the placenta
are exceedingly rare. Malignant melanoma is reportedly the most common malignancy
metastatic to the placenta (others include leukemia and lymphomas). Gestational
trophoblastic disease is a complicated topic referring to a spectrum of pregnancy-related
placental trophoblast growth abnormalities. Briefly, gestational trophoblastic disease can
be divided into hydatidiform mole (complete and partial molar pregnancy) and gestational
trophoblastic tumor (invasive mole, choriocarcinoma, and placental-site tumor). Complete
moles do not contain a fetus. The fetus of a partial mole is not viable. Hydatidiform
moles (complete and partial) tend to present as incomplete or threatened abortions
(miscarriage). Of note, rarely there may coexist 2 placentas with a hydatiform mole
developing alongside a normal appearing placenta and its fetus. Gestational trophoblastic
tumor (invasive mole, choriocarcinoma and placental-site tumor) almost always develop with
or follow some form of pregnancy (normal, molar, and ectopic pregnancy, miscarriage, or
elective abortion). Malignancy is rarely identified in the placenta of a normal appearing
pregnancy, but may follow an otherwise normal pregnancy. With prompt treatment by
experienced physicians specializing in these tumors, the prognosis and cure rates for
patients are excellent.
ABNORMALITIES OF THE
UMBILICAL CORD
Abnormalities in
cord length. Umbilical cord length varies considerably. The average length is
approximately 55 cm. Abnormal extremes of cord length range from apparently no cord
(achordia) to lengths of up to 300 cm. Vascular occlusion by thrombi (blood clots) and
true knots are more common in excessively long cords. Long cords are also more likely to
prolapse through the uterine cervix prior to delivery of the fetus. Cord prolapse is more
common when the fetus is small (e.g., preterm deliveries) and in certain types of breech
presentations (e.g., footling breech). Cord prolapse impairs blood flow to the fetus and
is an obstetric emergency requiring immediate delivery by cesarean section. Fortunately,
the incidence of cord prolapse is relatively low, complicating approximately 0.5% of all
births. Footling breech presentations are typically delivered by elective cesarean section
to prevent this and other potential complications of vaginal delivery. Rarely, abnormally
short umbilical cords may rupture or cause placental separation (placental abruption).
Abnormalities of cord insertion. The umbilical cord usually inserts near the center of the
fetal surface of the placenta. The blood vessels in the umbilical cord are protected by a
jelly-like substance (Wharton's jelly). In certain instances, the umbilical cord inserts
at a distance from the placenta, and its blood vessels must travel relatively unprotected
in the fetal membranes to reach the placenta. This condition is termed velamentous
insertion of the umbilical cord and occurs in approximately 1% of pregnancies, but is more
frequent with twins and triplets. Rarely, these unprotected vessels may rupture and result
in fetal death from hemorrhage. Additionally, with velamentous insertion of the umbilical
cord, some of the blood vessels traveling unprotected in the fetal membranes may cross the
cervix, a condition termed vasa previa. With vasa previa, rupture of the fetal membranes
("breaking the bag of water"), either spontaneously or by the
obstetrician/nurse-midwife (amniotomy), may be accompanied by rupture of a fetal blood
vessel, which can result in fetal death from hemorrhage. Unfortunately, the amount of
fetal blood loss enough to kill the fetus is relatively small. In contrast, hemorrhage
from placental abruption is lost from the mother, and a much larger hemorrhage may be
associated with a good outcome for the mother and fetus. Absence of one umbilical artery.
The umbilical cord normally contains 3 blood vessels (1 vein and 2 arteries). Two vessel
cords, with only 1 artery, are found in less than 1% of pregnancies (more common in twins,
and fetuses of mothers with diabetes). Approximately 30% of all fetuses with 2 vessel
cords have associated congenital anomalies. Additionally, fetuses with 2 vessel cords have
a higher incidence of intrauterine growth retardation (IUGR), preterm delivery, and
miscarriage (spontaneous abortion). Cord abnormalities ("accidents") capable of
interfering with blood flow. Several abnormalities of the umbilical cord are capable of
impairing blood flow between the placenta and fetus. True knots are thought to result from
active fetal movements and are found in approximately 1% of pregnancies. Fetal death may
result in approximately 6% of pregnancies complicated by true knots. The incidence of true
knots may be increased with abnormally long umbilical cords and is especially high in
monoamniotic twins. Loops of umbilical cord frequently become coiled around the fetus,
most commonly the neck (nuchal cord). Fortunately, nuchal cords are an uncommon cause of
fetal death. The umbilical cord normally becomes twisted as a result of fetal movements, a
condition termed torsion of the cord. Rarely, twisting of the cord on itself is so severe
that blood flow is compromised, resulting in fetal death. In monoamniotic twins, with no
fetal membrane separating the fetuses, the 2 umbilical cords may become twisted around
each other. Rarely, hematomas of the umbilical cord result from rupture of 1 of the
umbilical blood vessels, usually the umbilical vein. Cysts of the umbilical cord may form
but are rarely clinically significant. Of note, all of the so-called "cord
accidents" are rare causes of fetal death and it is probably unwise to attribute
fetal death to a cord accident until other causes have been ruled out.
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