Placenta

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placenta

 [plah-sen´tah] (pl. placentas, placen´tae) (L.)
an organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble bloodborne substances through apposition of uterine and trophoblastic vascularized parts. See also afterbirth. adj., adj placen´tal.

In anatomic nomenclature the placenta consists of a uterine and a fetal portion. The chorion, the superficial or fetal portion, is surfaced by a smooth, shining membrane continuous with the sheath of the umbilical cord (amnion). The deep, or uterine, portion is divided by deep sulci into lobes of irregular outline and extent (the cotyledons). Over the maternal surface of the placenta is stretched a delicate, transparent membrane of fetal origin. Around the periphery of the placenta is a large vein (the marginal sinus), which returns a part of the maternal blood from the organ.

The major function of the placenta is to allow diffusion of nutrients from the mother's blood into the fetus's blood and diffusion of waste products from the fetus back to the mother. This two-way exchange takes place across the placental membrane, which is semipermeable; that is, it acts as a selective filter, allowing some materials to pass through and holding back others.

In the early months of pregnancy the placenta acts as a nutrient storehouse and helps to process some of the food substances that nourish the fetus. Later, as the fetus grows and develops, these metabolic functions of the placenta are gradually taken on by the fetal liver.

The placenta secretes both estrogens and progesterone. After birth of the infant the placenta is cast off from the uterus and expelled via the birth canal.
placenta accre´ta one abnormally adherent to the myometrium, with partial or complete absence of the decidua basalis.
battledore placenta one with the umbilical cord inserted at the edge.
placenta circumvalla´ta one encircled with a dense, raised, white nodular ring, the attached membranes being doubled back over the edge of the placenta.
placenta fenestra´ta one that has spots where placental tissue is lacking.
placenta incre´ta placenta accreta with penetration of the myometrium.
placenta membrana´cea one that is abnormally thin and spread over an unusually large area of the myometrium.
placenta percre´ta placenta accreta with invasion of the myometrium to the peritoneal covering, sometimes causing rupture of the uterus.
placenta pre´via low implantation of the placenta so that it partially or completely covers the cervical os. Percentages are used to designate the amount of obstruction; e.g., 100 per cent is total placenta previa, and 50 per cent indicates that about half the opening is obstructed. The condition occurs with greater frequency in women who have had multiple pregnancies or are over 35. The exact cause is not known.

With the onset of any contractions and cervical dilation, or when the cervix begins to dilate at the onset of labor and the upper and lower uterine segments differentiate, the placenta is stretched and pulled from the uterine wall, producing bleeding. The bleeding usually is abrupt and painless and may stop on its own. However, if it continues it can be life-threatening for the mother since it is maternal blood that is being lost. The life of the fetus is in jeopardy because of anoxia resulting from separation of the placenta from its blood supply.

Diagnosis can be established by ultrasonography or radiologic placentography. Once diagnosis is made, treatment will depend on the gestational age of the fetus and the percentage of placenta covering the cervical os. Cesarean delivery is recommended if 30 per cent or more of the opening is obstructed by the placenta. If there is minimal bleeding that stops on its own, the fetus is not in distress, and if the gestational age is such that continuing the pregnancy is necessary for delivery of a viable fetus, the pregnancy may be continued under careful monitoring in the hospital, or at home if the mother is able to stay in bed. However, if the life of the mother or fetus is threatened by continued and excessive bleeding, delivery is indicated.

Vaginal examinations are carried out in an operating room so that if hemorrhage does occur as a result of manipulation of the uterus, a cesarean section can be done immediately to remove the placenta, stop the bleeding, and deliver the child safely.
Patient Care. Premature separation of the placenta is an emergency. The maternal signs are monitored every 15 minutes and blood loss is evaluated. Fetal heart tones also are monitored to detect fetal distress. The amount of bleeding is estimated and documented. Oxygen equipment should be at hand in the event signs of fetal distress indicate anoxia.

Postpartal hemorrhage and infection are more likely in women who have had placenta previa. Placement of the placenta in the lower segment predisposes to more bleeding because that portion of the uterus does not contract as forcefully as the upper segment. Additionally, the misplaced placenta has enlarged its bed to compensate for its poor location, so that there is a larger denuded area after delivery of the placenta. The same denuded area is also more susceptible to infection because it is located near the cervical opening where infectious organisms may enter.

Vaginal bleeding during pregnancy or labor is frightening for the mother. She will need reassurance and frequent explanations of what is happening to her throughout the period of monitoring and delivery. Some emotional stress can be alleviated by encouraging the mother to be aware of fetal movements and allowing her to listen to normal fetal heart sounds.
placenta reflex´a one in which the margin is thickened, appearing to turn back on itself.
placenta spu´ria an accessory portion without blood vessels connecting it with the main placenta.
placenta succenturia´ta an accessory portion with an artery and a vein connecting it with the main placenta.

pla·cen·ta

(plă-sen'tă),
Fetomaternal organ of metabolic interchange between the embryo or fetus and mother. It has a portion of embryonic origin, derived from a highly developed area of the outermost embryonic membrane (chorion frondosum), and a maternal portion formed by a modification of the part of the uterine mucosa (decidua basalis) in which the chorionic vesicle is implanted. Within the placenta, the chorionic villi, with their contained capillaries carrying blood of the embryonic circulation, are exposed to maternal blood in the intervillous spaces in which the villi lie; no direct mixing of fetal and maternal blood occurs, but the intervening tissue (the placental membrane) is sufficiently thin to permit the absorption of nutritive materials, oxygen, and some harmful substances, like viruses, into the fetal blood and the release of carbon dioxide and nitrogenous waste from it. At term, the human placenta is disc shaped, about 4 cm in thickness and 18 cm in diameter, and averages about 1/6-1/7 the weight of the fetus; its fetal surface is smooth, being formed by the adherent amnion, with the umbilical cord normally attached near its center; the maternal surface of a detached placenta is rough because of the torn decidual tissue adhering to the chorion and shows lobular elevations called cotyledons or lobes.
[L. a flat cake]

placenta

(plə-sĕn′tə)
n. pl. placen·tas or placen·tae (-tē)
1.
a. A membranous vascular organ that develops in female eutherian mammals during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord. Following birth, the placenta is expelled.
b. A similar organ in marsupial mammals, consisting of a yolk sac attached to the uterine wall.
c. An organ with similar functions in some nonmammalian animals, such as certain sharks and reptiles.
2. Botany The part within the ovary of a flowering plant to which the ovules are attached.

pla·cen′tal adj.

pla·cen·ta

(plă-sen'tă)
Fetomaternal organ of metabolic interchange between embryo or fetus and mother. It has a portion of embryonic origin, derived from the outermost embryonic membrane (villous chorion), and a maternal portion formed by a modification of the part of the uterine mucosa (decidua basalis) in which the chorionic vesicle is implanted. Within the placenta, the chorionic villi, with their contained capillaries carrying blood of the embryonic circulation, are exposed to maternal blood in the intervillous spaces in which the villi lie; no direct mixing of fetal and maternal blood occurs, but the intervening tissue (the placental membrane) is sufficiently permeable to permit the absorption of nutritive materials, oxygen, and some harmful substances, such as viruses, into the fetal blood, and the release of carbon dioxide and nitrogenous waste from it. At term, the human placenta is disc shaped, about 4 cm thick and 18 cm in diameter, and averages about one sixth to one seventh the weight of the fetus; its fetal surface is smooth, being formed by the adherent amnion, with the umbilical cord usually attached near its center; the maternal surface of a detached placenta is rough because of the torn decidual tissue adhering to the chorion and shows lobular elevations called cotyledons.
[L. a flat cake]

placenta

(pla-sent'a ) (-sent'e?) plural.placentaeplural.placentas [L. placenta, flat cake]
Enlarge picture
DELIVERY OF THE PLACENTA
The oval or discoid spongy structure in the uterus of eutherian mammals from which the fetus derives its nourishment and oxygen. See: illustrationplacental ('al), adjective

Anatomy

The placenta consists of a fetal portion, the chorion frondosum, bearing many chorionic villi that interlock with the decidua basalis of the uterus, which constitutes the maternal portion. The chorionic villi lie in spaces in the uterine endometrium, where they are bathed in maternal blood and lymph. Groups of villi are separated by placental septa forming about 20 distinct lobules called cotyledons.

Attached to the margin of the placenta is a membrane that encloses the embryo. It is a composite of several structures (decidua parietalis, decidua capsularis, chorion laeve, and amnion). At the center of the concave side is attached the umbilical cord through which the umbilical vessels (two arteries and one vein) pass to the fetus. The cord is approx. 50 cm (20 in.) long at full term.

The mature placenta is 15 to 18 cm (6 to 7 in.) in diameter and weighs about 450 gm (approx. 1 lb). When expelled following parturition, it is known as the afterbirth.

Maternal blood enters the intervillous spaces of the placenta through spiral arteries, branches of the uterine arteries. It bathes the chorionic villi and flows peripherally to the marginal sinus, which leads to uterine veins. Food molecules, oxygen, and antibodies pass into fetal blood of the villi; metabolic waste products pass from fetal blood into the mother's blood. Normally, there is no admixture of fetal and maternal blood. The placenta is also an endocrine organ. It produces chorionic gonadotropins, the presence of which in urine is the basis of one type of pregnancy test. Estrogen and progesterone are also secreted by the placenta.

abruption of placenta

Abruptio placentae.

accessory placenta

A placenta separate from the main placenta.

placenta accreta

A placenta in which the cotyledons have invaded the uterine musculature, resulting in difficult or impossible separation of the placenta. Complications of manually separating the placenta include hemorrhage, damage to the uterus, and, in rare cases, hysterectomy. Synonym: placenta creta

adherent placenta

A placenta that remains adherent to the uterine wall after the normal period following childbirth.
See: placenta accreta

annular placenta

A placenta that extends like a belt around the interior of the uterus. Synonym: zonary placenta

battledore placenta

A form of insertion of the umbilical cord into the margin of the placenta in which it spreads out to resemble a paddle or badminton racket.

bidiscoidal placenta

A placenta with two separate portions attached to separate sites on the wall of the uterus, occasionally found in humans.

bilobate placenta

A placenta consisting of two lobes. Synonym: dimidiate placenta

bipartite placenta

A placenta divided into two separate parts.

chorioallantoic placenta

A placenta in which the allantoic mesoderm and vessels fuse with the inner face of the serosa to form the chorion.

circinate placenta

A cup-shaped placenta.

placenta circumvallata

A placenta whose membranes wrap over the edge of the fetal surface of the organ.

circumvallate placenta

Placenta circumvallata.

cirsoid placenta

A placenta with appearance of varicose veins.

cordiform placenta

A placenta having a marginal indentation, giving it a heart shape.

placenta creta

Placenta accreta.

deciduate placenta

A placenta whose maternal part is shed with delivery.

dimidiate placenta

Bilobate placenta.

discoid placenta

A placenta with a flat, circular shape.

double placenta

A placental mass of the two placentae of a twin gestation.

endotheliochorial placenta

A placenta in which the syncytial trophoblasts of the chorion penetrate to the blood vessels of the uterus.

epitheliochorial placenta

A placenta in which the chorion is next to the lining of the uterus but does not invade or erode the lining.

placenta fenestrata

A placenta in which a portion of the placental tissue is thinning or absent.

fetal placenta

That part of the placenta formed by aggregation of chorionic villi in which the umbilical vein and arteries ramify.

fundal placenta

A placenta attached to the uterine wall within the fundal zone.

hemochorial placenta

A placenta in which the maternal blood is in direct contact with the chorion. The human placenta is of this type.

hemoendothelial placenta

A placenta in which the maternal blood is in contact with the endothelium of the chorionic vessels.

horseshoe placenta

A formation in which the two placentae of a twin gestation are united.

incarcerated placenta

A placenta retained in the uterus due to incomplete separation from the uterine wall or by irregular uterine contractions after delivery.

placenta increta

A form of placenta accreta in which the chorionic villi invade the myometrium.

lateral placenta

A placenta attached to the lateral wall of the uterus.

maternal placenta

A portion of the placenta that develops from the decidua basalis of the uterus.

membranous placenta

Thinning of the placenta from atrophy.

multilobate placenta

A placenta with more than three lobes.

nondeciduate placenta

A placenta that does not shed the maternal portion.

placenta percreta

A type of placenta accreta in which the myometrium is invaded to the serosa of the peritoneum covering the uterus. This may cause rupture of the uterus.

placenta previa

Abbreviation: PP
A placenta that is implanted in the lower uterine segment. There are three types: centralis, lateralis, and marginalis. Placenta previa centralis (total or complete PP) is the condition in which the placenta has been implanted in the lower uterine segment and has grown to completely cover the internal cervical os. Placenta previa lateralis (low marginal implantation) is the condition in which the placenta lies just within the lower uterine segment. Placenta previa marginalis is the condition in which the placenta partially covers the internal cervical os (partial or incomplete PP).

Symptoms

The condition is more common in multigravidas than primigravidas, and occurs in about 1 in every 200 pregnancies. Slight hemorrhage, recurrent with greater severity, appears in the seventh or eighth month of pregnancy. Gradual anemia, pallor, rapid weak pulse, air hunger, and low blood pressure occur.

Diagnosis

Painless bleeding during the last 3 months and a placenta found in the lower portion of the uterus are diagnostic.

Treatment

The blood supply before and during delivery should be conserved. Postpartum hemorrhage should be prevented or controlled. Anemia should be treated before and after labor. Prevention of sepsis is necessary.

Prognosis

The prognosis for the mother is good with control of hemorrhage and prevention of sepsis. Prognosis for the fetus depends on gestational age and the amount of blood lost, but continuous monitoring and rapid intervention help to prevent neonatal death.

Patient care

In a calm environment, the patient is told what is happening; then the procedure of vaginal ultrasound is explained. The patient is told that if the ultrasound examination reveals a placenta previa, sterile vaginal examination will be delayed if possible until after 34 weeks' (preferably 36 weeks') gestation (to enhance the chances for fetal survival) and then will be carried out only as a “double-setup” procedure, with all preparations needed for immediate vaginal or cesarean delivery. (If, however, the ultrasound examination reveals a normally implanted placenta, a sterile vaginal speculum examination is performed to rule out local bleeding causes, and a laboratory study is ordered to rule out coagulation problems.)

The patient is maintained on absolute bedrest and under close supervision (usually in the hospital) to extend the period of gestation until 36 weeks, when fetal lung maturity is likely (or can be stimulated to mature 48 hr before delivery). Intravenous access is established using a large-bore catheter, and continuous external electrode fetal monitoring is initiated. Maternal vital signs are closely monitored, and the amount of vaginal bleeding is assessed. The laboratory types and cross-matches blood for emergency use; the number of units is based on the assessment of the particular patient's possible requirements. The patient's hematocrit level is kept at 30% or greater. The patient is prepared physically and emotionally for cesarean delivery; vaginal delivery may be attempted, but only if the previa is marginal, bleeding is minimal, and labor is rapidly progressing.

After delivery, the patient is monitored closely for continued bleeding, which may occur from the large vascular channels in the lower uterine segment, even if the fundus is firmly contracted. Prophylactic antibiotic therapy may be prescribed because of the patient's propensity for infection. Oxytocic drugs are given to control bleeding; packed cells or whole blood also are given. The obstetrical surgery team remains available, in case further intervention is required. The patient's hemodynamic status is monitored continuously, to provide blood and fluid replacement needed to prevent and treat hypovolemia while avoiding hypervolemia.

Although maternal mortality remains a concern, the patient and her family should be assured that this is unlikely but not impossible in most large treatment centers because of the conservative regimen that is followed. A pediatric team is present at delivery to assess and treat neonatal hypoxia, anemia, blood loss, and shock. In the event of fetal distress or death, the family is informed that these are related to detachment of a significant portion of the placenta or to maternal hypovolemic shock, or both. All parents are provided opportunities to be with and touch their (usually premature) neonate in the critical care nursery. In cases of fetal demise, the infant is carefully wrapped and the parents encouraged to hold their baby, and to examine it as they desire. Infant photographs may be taken to provide memories for the family. The patient and family require the health care providers' empathetic concern and support. A social service consultation is set up if financial or home and family care concerns require agency referrals; spiritual counseling is supplied according to the patient's wishes. Reducing maternal anxiety helps reduce uterine irritability, so a mental health practitioner should be consulted if the patient does not respond to nursing interventions (e.g., relaxation techniques, guided imagery) or if the patient's previous coping skills are known to be ineffective.

placenta previa partialis

A placenta that only partially covers the internal os of the uterus.

placenta reflexa

An abnormal placenta in which the margin is thickened and appears to turn back on itself.

reniform placenta

A kidney-shaped placenta.

retained placenta

A placenta not expelled within 30 min after completion of the second stage of labor.

placenta spuria

An outlying portion of the placenta that has not maintained its vascular connection with the decidua vera.

succenturiate placenta

An accessory placenta that has a vascular connection to the main part of the placenta.

trilobate placenta

A placenta with three lobes.

tripartite placenta

A three-lobed placenta attached to a single fetus.

triple placenta

A placental mass of three lobes in a triple gestation.

placenta uterina

The maternal part of the placenta.

velamentous placenta

A placenta with the umbilical cord attached to the membrane a short distance from the placenta, the vessels entering the placenta at its margin.

villous placenta

A placenta in which the chorion forms villi.

zonary placenta

Annular placenta.

placenta

The part of the early developing EMBRYO, that differentiates to form an organ attached to the lining of the womb and provides a functional linkage between the blood supplies of the mother and the fetus. This allows for the passage of oxygen and nutrients from the mother to the fetus. The placenta is connected to the fetus by the UMBILICAL CORD and is discharged from the womb after the birth of the baby. Also known as afterbirth.
Placentaclick for a larger image
Fig. 253 Placenta . Section showing blood circulation.

placenta

  1. (in animals) the structure formed from the tissues of embryo and mother at the point of attachment of the embryo to the mother, through which the embryo is nourished. In placental mammals the embryonic blood supply may be from the ALLANTOIS (allantoic placenta) or the yolk sac (yolk sac placenta). Small molecules of oxygen and food materials pass to the embryo through the placenta, and urea and CO2 pass out from the embryo. The placenta itself produces the hormones OESTROGEN, PROGESTERONE and GONADOTROPHIC HORMONE. At birth the foetus is ejected from the uterus first, the umbilical cord broken and the placentalafterbirth is discharged last, except in abnormal circumstances. See Fig. 253 .
  2. (in plants) a part of the wall of the ovary on which OVULES are borne.

Placenta

The organ that allows interchange between the fetus and the mother. Blood from the fetus and the mother do not directly mix, but the thin placental membrane allows the fetus to absorb nutrients and oxygen from the mother. Waste products from the fetus can exit through the placenta.

pla·cen·ta

(plă-sen'tă)
Fetomaternal organ of metabolic interchange between embryo or fetus and mother.
[L. a flat cake]