Placenta types and grading

USG Findings of Normal and abnormal types of placenta.

1. Placenta Types and Grading BY DR NISHANT RAJ

2. DEVELOPMENT OF PLACENTA The human placenta develops from the trophectoderm , the outer layer of the pre-implantation embryo, which forms at ∼5 days post fertilisation . At this stage, the pre-implantation embryo termed as blastocyst has two lineages: the inner cell mass(embryoblast) and the trophoblast. The embryonic end of Trophoblast which consists of cytotrophoblast and syncytiotrophoblast attaches to the endometrium. Following attachment to the endometrium at 6-7 days post fertilization, the Trophoblast fuses to form a primary syncytium. This is the prelacunar phase of placental development.

3. PLACENTA DEVELOPMENT CONTD… Following implantation, The primary syncytium quickly invades through the surface epithelium into the underlying endometrium, which is transformed during pregnancy into a specialised tissue known as decidua. By the time of the first missed menstrual period (∼14 days post fertilization), the blastocyst is completely embedded in the decidua and is covered by the surface epithelium. Fluid-filled spaces (lacunae) then appear within the syncytial mass that enlarge and merge, partitioning it into a system of trabeculae. This is the lacunar stage. The syncytium also erodes into decidual glands, allowing secretions to bathe the syncytial mass.

4. PLACENTA DEVELOPMENT CONTD.. The trophoblast cells beneath the syncytium (termed cytotrophoblast cells) are initially not in direct contact with maternal tissue but rapidly proliferate to form projections that push through the primary syncytium to form primary villi (a cytotrophoblast core with an outer layer of syncytiotrophoblast , ); this is the villous stage of development (The villous trees are formed by further proliferation and branching, and the lacunae become the intervillous space. Cytotrophoblast cells eventually penetrate through the primary syncytium and merge laterally to surround the conceptus in a continuous cytotrophoblast shell between the villi and the decidua The blastocyst is now covered by three layers: the inner chorionic plate in contact with the original cavity; the villi separated by the intervillous space; and the cytotrophoblast shell in contact with the decidua.

5. PLACENTA DEVELOPMENT CONTD.. Soon afterwards, around day 17-18, extraembryonic mesenchymal cells penetrate through the villous core to form secondary villi. By day 18  dpf , fetal capillaries appear within the core, marking the development of tertiary villi. The villous tree continues to rapidly enlarge by progressive branching from the chorionic plate to form a system of villous trees. Where the cytotrophoblast shell is in contact with the decidua (the maternal-fetal interface), individual cytotrophoblast cells leave the shell to invade into decidua as extravillous trophoblast in a process closely resembling epithelial-mesenchymal transition. In this way, by the end of the first trimester, the blueprint of the placenta is established.

6. STAGES OF DEVELOPMENT OF PLACENTA The early stages of human placental development. Diagram depicting the early steps in placenta formation following blastocyst implantation. (A,B) The pre-lacunar stages. (C) The lacunar stage. (D) The primary villous stage. 1° ys , primary yolk sac; ac, amniotic cavity; cs, cytotrophoblastic shell; eec , extra-embryonic coelom; exm , extra-embryonic mesoderm; GE, glandular epithelium; ICM, inner cell mass; lac, lacunae; LE, luminal epithelium; mn . tr, mononuclear trophoblast; pr. syn , primary syncytium; TE, trophectoderm; vs, blood vessels.

7. NORMAL ANATOMY OF PLACENTA At term: Weight: 470 grams, round to oval in shape Diameter: 22cm and central thickness of 2.5 cm. Composed of a Placental Disk, Extra-Placental membranes and a three vessel umbilical cord. Maternal Surface is Basal Plate Fetal Surface is the chorionic plate into which umbilical cord inserts into the center. Chorionic plate and its vessels are covered by amnion.

8. ULTRASOUND APPEARANCE In first and early second trimester placenta appears homogenous in echotexture and mildly hyperechoic compared to underlying myometrium. It becomes more isoechoic with advancing gestation. After midpregnancy , it is common to identify small placental lucencies . In the third trimester, placenta appears more heterogenous with visible calcifications. Placental Thickness in mm roughly approximates to gestational age in weeks. Does not normally exceed 4 mm in second trimester or 6 cm in the third trimester. Retroplacental clear space normally measures less than 1 to 2 cm and is hypoechoic. Retroplacental clear space common location for hematoma development.

9. Normal placenta at 10 weeks gestation. Transverse gray-scale Ultrasounf d image shows the chorion laeve (right arrow) and chorion frondosum (left arrows) of the placenta.

10. Normal placenta at 12 weeks gestation. Transverse color Doppler image shows intervillous flow (arrow).  M  = myometrium,  P  = placenta.

11. Normal placenta at 18 weeks gestation. Longitudinal gray-scale US image shows a homogeneous placenta  (P)  with central placental cord insertion  (CI)  and the hypoechoic retroplacental complex (arrows) behind the placenta.

12. USG APPEARANCE OF NORMAL PLACENTA

13. Placental grading (Grannum classification) The grading system is as follows: grade 0:  <18 weeks uniform echogenicity Smooth chorionic plate grade I:  18-29 weeks occasional parenchymal calcification/hyperechoic areas subtle indentations of chorionic plate grade II:  ​30-38 weeks occasional basal calcification/hyperechoic areas deeper indentations of the chorionic plate: seen as comma type densities at the chorionic plate. grade III:  ​​≥ 39 weeks significant basal plate calcification chorionic plate interrupted by indentations that reach up to the basal plate: cotyledons an early progression to a grade III placenta is concerning and is termed as hyper mature placenta and sometimes associated with placental insufficiency. associated with smoking, chronic hypertension, SLE, diabetes  

14. ABNORMALITIES OF PLACENTAL SHAPE

15. BILOBED PLACENTA Bilobed placenta is a placenta with two roughly equal-sized lobes separated by a membrane. The umbilical cord may insert in either lobe, in velamentous fashion, or in between the lobes. B ilobed placentas can be associated with first-trimester bleeding, polyhydramnios, abruption and retained placenta. A placenta with more than two lobes is rare and is termed a multilobate placenta. 

16. These are two views of a bilobate placenta, where each of the lobes (one on the anterior and one on the posterior aspects of the uterine cavity) are marked with an *

17. US image shows a bilobed placenta. The two lobes of the placenta (P1 and P2) are separated by a thin bridge of placental tissue that covers the internal os. In this case, the umbilical cord (arrowhead) inserts into the bridge of tissue.

18. CIRCUMVALLATE PLACENTA Circumvallate placenta is a placenta that is an annularly-shaped placenta with raised edges composed of a double fold of chorion, amnion, degenerated decidua, and fibrin deposits.  In this condition, the chorionic plate is smaller than the basal plate, resulting in hematoma retention in the placental margin. Within the ring, the fetal surface has the usual appearance except that the large vessels terminate abruptly at the margin of the ring. Circumvallate placenta is associated with poor pregnancy outcomes due to increased risk of vaginal bleeding beginning in the first trimester, premature rupture of the membranes , preterm delivery, placental insufficiency, and placental abruption.

19. CIRCUMVALLATE PLACENTA USG Longitudinal gray-scale US image at 21 weeks gestation shows the raised edge of the placenta  (P)  as a linear band of tissue or shelf-like structure (arrow) that may mimic a uterine synechia. 

20. US image shows a circumvallate placenta. The chorionic plate (the fetal surface of the placenta) (black arrowheads) is smaller than the basal plate (the surface interfacing with the uterus), with rolling and shouldering of the placental margins (white arrowheads). F = fetus.

21. Succenturiate placenta S uccenturiate placenta is a condition in which one or more accessory lobes develop in the membranes apart from the main placental body to which vessels of fetal origin usually connect them. It is a smaller variant of a bilobed placenta. The vessels are supported only by communicating membranes. If the communicating membranes do not have vessels, it is called placenta supuria . .Advanced maternal age and in vitro fertilization are risk factors for the succenturiate placenta. Other factors leading to succenturiate placentas include implantation over leiomyomas, in areas of previous surgery, in the cornu or over the cervical os . Ultrasound, particularly color Doppler, can be used to identify this condition . The risks of vasa previa and retained placenta are increased with this condition like with bilobed and multilobate placentas.

22. US image shows a placenta (P) with a succenturiate lobe (S). The main body of the placenta is located along the posterior uterine wall. A second soft-tissue structure of the same echogenicity but located anteriorly is the succenturiate lobe.

23. PLACENTA MEMBRANACEA Placenta membranacea or placenta diffusa is a rare abnormality in which all or most fetal membranes remain covered by chorionic villi, because the chorion has failed to differentiate into chorion leave and chorion frondosum.  Clinically the abnormality can present with vaginal bleeding in the second or third trimester which is often painless or during labor. Other placenta abnormalities, such as placenta previa and placenta accreta , can be associated with this condition.  Ultrasound has been reported as a diagnostic tool for this condition, but due to the condition’s rarity, there is no data on its sensitivity and specificity.

24. Ring SHAPED PLACENTA The ring-shaped placenta is an annularly-shaped placenta that is a variant of placenta membranacea . It can sometimes be a complete ring of placental tissue, but more often tissue atrophy in a portion of the ring results in a horseshoe shape. The ring-shaped placenta can cause antepartum and postpartum bleeding as well as fetal growth restriction.

25. ABNORMAL PLACENTAL THICKNESS Placentomegally , if placental thickness exceeds 4cm in 2 nd trimester and 6 cm in 3rd trimester. Causes: maternal diabetes Severe maternal anemia, Severe fetal growth restriction Aneuploidy Congenital Infections: Syphillis,cmv,parvovirus , toxoplasmosis,herpesvirus , rubella,schistosomiasis . Is a component of hydrops fetalis (Immune or Non-Immune Hydrops) In rare cases it may be due to: Collection of blood or fibrin within placneta . Neoplasia GTD creates a thick cystic appearing placenta , cystic vesicles are also seen with placental mesenchymal dysplasia.

26. PLACENTAL THICKNESS ON USG Longitudinal US image of a normal placenta at 18 weeks gestation shows the measurement calipers appropriately positioned at the anterior and posterior margins of the placenta  (P) , perpendicular to the long axis of the placenta near the umbilical cord origin

27. ABNORMALITIES OF PLACENTAL LOCATION Placental location is determined according to the main placental body position from the uterine equator. It can be anterior or posterior, fundal, or left or right. The term low-lying placenta is used when the placental edge is located in the lower uterine segment within 2 cm or less of the internal cervical os . The term placenta previa is used when the placental edge covers the internal cervical os Historically, terms such as complete, marginal, and partial were used to describe how much of the placenta was covering the internal os . Transvaginal ultrasonography has improved sonographic description of the precise location of the placenta, and these terms have been abandoned.

28. PLACENTA PREVIA USG US is the gold standard for diagnosing placenta previa Complete placenta previa. Longitudinal gray-scale US image at 28 weeks gestation shows the placenta  (P)  completely covering the internal os   (x)

29. PLACENTA PREVIA USG Longitudinal gray-scale US image of another patient at 16 weeks gestation shows a low-lying placenta  (P)  extending to, but not covering, the internal os   (x).

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