The placenta is the medium through which material passes from the maternal circulation to the fetal circulation by passive diffusion or active transport.
After fertilization of the sperm and ovum, four cell division leads to a morula (16 cells). Around the fourth day after fertilization, the morula enters the uterus as a blastocyst. The blastocyst divides into trophoblast and embryoblast. About 6 to 7 days later, the differentiating blastocyst implants into the uterine decidua. Then, the trophoblast further divides into the syncytiotrophoblast and cytotrophoblast; together, these compose the fetal component of the placenta. These two cell types lay on either side of villi. The cytotrophoblast resides on the inner layer of the chorionic villi. Syncytiotrophoblasts are multinucleated cells that are located on the outer layer of the chorionic villi. In between the syncytiotrophoblast layer, lacunae form to serve as a direct connection to the maternal blood. The remaining cells surrounding the lacunae, called trabeculae, penetrate deep into the uterine wall and, later, develop into the villous trees of the placenta. The maternal blood supply lies in the decidua basalis of the endometrium, which constitutes the maternal component of the placenta.
As the pregnancy progresses, the villous cytotrophoblast slowly disappears from the chorionic villi. Additionally, the villi structure develops to reduce the distance between the maternal blood and fetal vessels — this change benefits maternal-fetal-exchanges. By the end of a pregnancy, five types of villi comprise the placenta: mesenchymal villi, immature intermediate villi, stem villi, mature intermediate villi, and terminal villi. From the mesenchymal villi and immature intermediate villi, about three to five mature intermediate villi grow from the stem villous, and ten to twelve branches of the terminal villi grow from the mature intermediate villi. These villi help communication between the chorionic plate and decidua. Most of the villi float freely in the intervillous space while other villi attach to the decidua as structural stability for the placenta.
The placenta plays a vital role in maternal-fetal physiology. The placenta has numerous responsibilities:
Abnormalities in the placenta’s function can lead to poor pregnancy outcomes.
The chorionic villus sampling test (CVS) is a diagnostic, ambulatory procedure testing for chromosomal abnormalities. This test is recommended at the time of conception for women above the age of 35 if a parent is a carrier of a genetic disorder, women with abnormal ultrasound results, or women with previous children with chromosome abnormalities. Performance of the test may be transcervically or transabdominally; during the test, a small cell sample is taken from the fetal part of the placenta chorion between the 10th and 13 weeks of pregnancy. The fetus’ chromosomes are karyotyped to diagnose chromosomal abnormalities. There is a risk of miscarriage and, in some literature, limb defects when performing placenta chorion testing before the suggested testing period (before the tenth week). Other complications include vaginal bleeding, infection, rupture of membranes, and fetomaternal hemorrhage. In some cases, results can be inconclusive so that other tests may be necessary. Amniocentesis is an alternative to CVS as both are used to analyze genetic information.
The combined test is a screening tool used to primarily detect Down syndrome in the first trimester with a low false positive rate. This test is also known to detect trisomy 18 and 13 as well as monosomy X and triploidies at lower accuracy rates. It is comprised of ultrasound to determine nuchal translucency, pregnancy-associated plasma protein-A (PAPP-A) which correlates with aneuploidy, and free beta or total human chorionic gonadotropin (hCG). Ultrasound imaging is performed between 11 and 13 weeks while the collection of serum sample ranges from 9 to 13 weeks. If screening with the combined test suggests chromosomal abnormalities, CVS or amniocentesis should be offered for definitive prenatal diagnosis.
Placental growth factor (PIGF) is a potent angiogenic protein factor produced by the syncytiotrophoblast. The purpose of PIGF is to facilitate the development and maturation of the placental vasculature. Low PIGF suggests placental dysfunction associated with pre-eclampsia and intrauterine growth restriction.
Human chorionic somatomammotropin (HCS), also known as human placental lactogen (HPL) is produced by the syncytiotrophoblast starting in week 6. HCS functions to promote breast development and decrease maternal insulin sensitivity and increases glucose availability for the fetus. Literature indicates the measurement of HCS is clinically relevant during the third trimester as blood HCS results can predict the outcome by indicating functionality of the placenta and, indirectly, fetal growth.
Intrauterine growth restriction (IUGR) is a condition in which fetal growth is less than the tenth percentile of the predicted weight for gestational age. IUGR can lead to fetal morbidity and mortality. The restriction of growth can be the result of maternal, fetal, and placental causes. Most fetuses develop IUGR secondary to maternal causes like preeclampsia, poor nutrition, drug addiction, alcohol use, and tobacco use. Retardation occurs symmetrically in all organs with fetal causes like chromosomal abnormalities, congenital malformations, and congenital infections. Additionally, abruptio placentae, placental infarction, and a single umbilical artery (instead of two) cause asymmetric growth retardation, which is also known as “head sparing.” The common factor among these risk factors is that blood flow to the fetus is compromised either through vasoconstriction or loss of blood. Diagnoses of IUGR is difficult in utero. Therefore, the detection of highly associated conditions like unexplained oligohydramnios (about 85% of IUGR infants also have oligohydramnios) should warrant further evaluation. Assessment for IUGR includes doppler sonography as well as analysis of estimated fetal weight, the volume of amniotic fluid, and mother’s blood pressure status.
Choriocarcinoma is a germ cell malignant tumor of the cytotrophoblast and syncytiotrophoblast cells. The tumor cells mimic placental tissue; however, there are no villi to exchange nutrients. Choriocarcinoma can develop into a small hemorrhagic tumor and can easily invade maternal blood vessels leading to early hematogenous spread throughout the circulation. Patients usually present with a uterus larger than expected for gestational age. The evaluation includes ultrasound which shows a “snowstorm appearance.” Blood tests typically show elevated beta hCG, which is produced by syncytiotrophoblast. High levels of hCG can lead to thecal cysts in the ovary. Treatment is dilation and curettage with follow-up monitoring with beta-hCG levels. This germ cell tumor usually responds poorly to chemotherapy; however, there are some cases in which chemotherapy after resection may result in a successful outcome.
Preeclampsia, usually arising in the third trimester, is pregnancy-induced hypertension by increased total peripheral resistance but the pathogenesis is poorly understood. Literature provides evidence for an imbalance of angiogenic and anti-antigenic factors as well as defective spiral artery remodeling, which eventually leads to the former hypothesis. This disproportion favors anti-angiogenic factors leading to endothelial dysfunction in the maternal organs. In terms of diagnosing pre-eclampsia, hypertension couples with one of the following: elevated liver enzymes, reduced liver or kidney function, or pulmonary or cerebral edema. Preeclampsia resolves with delivery and removal of the maternal-fetal vascular interface. Methyldopa and magnesium sulfate are the recommendations for control of hypertension.
Placenta previa is the implantation of the placenta over the cervical os, which usually presents as painless vaginal bleeding in the third trimester. Additionally, the uterus is soft and non-tender on physical exam. The fetus is usually not in distress. Risk factors include a history of cesarean section or multiple vaginal births. A transabdominal ultrasound localizes the placenta, but placenta previa diagnosis is by transvaginal ultrasound. In these cases, mother and fetus must undergo careful monitoring and delivery is often by cesarean section.
Placental abruption (abruptio placentae), complete or partial, is the premature separation of the placenta from the decidua basalis, which is the number one cause of vaginal bleeding late in pregnancy. Unlike placenta previa, uterine bleeding is painful with uterine contractions and fetal distress is usually present. Commonly, bleeding is due to rupture of maternal vessels, but fetal-placental vessels can also cause the separation. Ultrasound is the preferred diagnostic tool. Conditions affecting circulation act as risk factors and include hypertension, tobacco, cocaine, trauma, and premature rupture of membranes. Treatment includes the delivery of the baby. However, placental abruption frequently results in stillbirth.
Placenta accreta spectrum (PAS), formally known as morbidly adherent placenta, describes the improper attachment and separation of the placenta to a defective decidual layer of the endometrium. Women with maternal age, multiparity, or history of C-section or uterine surgery are at high risk for abnormal placenta adherence. Discovery of this condition is on ultrasound before delivery; however, there is difficulty with placental separation after birth which causes postpartum bleeding. Therefore, prenatal diagnosis is important to prepare for difficult labor, which may involve surgery. If findings are unclear with ultrasound, magnetic resonance imaging (MRI) can clarify the diagnosis. There are three types of morbidly adherent placenta ranging from various depths of infiltration.
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