pl. placentae pl. placentas [L. placenta, flat cake]
The oval or discoid spongy structure in the uterus of eutherian mammals from which the fetus derives its nourishment and oxygen.
DELIVERY OF THE PLACENTA
placental (′ăl), adj.
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
SEE: Abruptio placentae.
A placenta separate from the main placenta.
A placenta in which the trophoblastic cells invade and grow into the uterine musculature, resulting in difficult or impossible separation of the placenta. Complications of manually separating the placenta include hemorrhage (which is sometimes severe enough to threaten the life of the mother), damage to the uterus, and hysterectomy.
SYN: placenta creta
A placenta that remains adherent to the uterine wall after the normal period following childbirth.
SEE: placenta accreta
A placenta that extends like a belt around the interior of the uterus.
SYN: zonary 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.
A placenta with two separate portions attached to separate sites on the wall of the uterus, occasionally found in humans.
A placenta consisting of two lobes.
SYN: dimidiate placenta
A placenta divided into two separate parts.
A placenta in which the allantoic mesoderm and vessels fuse with the inner face of the serosa to form the chorion.
A cup-shaped placenta.
A placenta whose membranes wrap over the edge of the fetal surface of the organ.
A placenta with appearance of varicose veins.
A placenta having a marginal indentation, giving it a heart shape.
SEE: Placenta accreta.
A placenta whose maternal part is shed with delivery.
SEE: Bilobate placenta.
A placenta with a flat, circular shape.
A placental mass of the two placentae of a twin gestation.
A placenta in which the syncytial trophoblasts of the chorion penetrate to the blood vessels of the uterus.
A placenta in which the chorion is next to the lining of the uterus but does not invade or erode the lining.
A placenta in which a portion of the placental tissue is thinning or absent.
That part of the placenta formed by aggregation of chorionic villi in which the umbilical vein and arteries ramify.
A placenta attached to the uterine wall within the fundal zone.
A placenta in which the maternal blood is in direct contact with the chorion. The human placenta is of this type.
A placenta in which the maternal blood is in contact with the endothelium of the chorionic vessels.
A formation in which the two placentae of a twin gestation are united.
A placenta retained in the uterus due to incomplete separation from the uterine wall or by irregular uterine contractions after delivery.
A form of placenta accreta in which the chorionic villi invade the myometrium.
A placenta attached to the lateral wall of the uterus.
A portion of the placenta that develops from the decidua basalis of the uterus.
Thinning of the placenta from atrophy.
A placenta with more than three lobes.
A placenta that does not shed the maternal portion.
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.
ABBR: 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).
SEE: Nursing Diagnoses Appendix
SYMPTOMS AND SIGNS
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.
Painless bleeding during the last 3 months and a placenta found in the lower portion of the uterus are diagnostic.
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.
IMPACT ON HEALTH
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.
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, and therefore 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 (A) Low implantation. (B) Partial placenta previa. (C) Central (total) placenta previa. (Adapted from Beare, PG: Davis's NCLEX-RN Review, ed 3. FA Davis, Philadelphia, 2001, p 61, with permission.)
placenta previa partialis
A placenta that only partially covers the internal os of the uterus.
An abnormal placenta in which the margin is thickened and appears to turn back on itself.
A kidney-shaped placenta.
A placenta not expelled within 30 min after completion of the second stage of labor.
An outlying portion of the placenta that has not maintained its vascular connection with the decidua vera.
An accessory placenta that has a vascular connection to the main part of the placenta.
A placenta with three lobes.
A three-lobed placenta attached to a single fetus.
A placental mass of three lobes in a triple gestation.
The maternal part of the 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.
A placenta in which the chorion forms villi.
SEE: Annular placenta.