Placental Causes of Fetal Loss
Vol.1 No.7 
 

by David M. Lima, M.D. 
Department of Obstetrics and Gynecology 
The Yale University School of Medicine  

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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