The Monochorionic Placenta

TTTS. The Monochorionic Placenta

The monochorionic twin placenta is usually shaped as a single disk (Figure 5.2). However, the chorionicity cannot be assigned by the number of placental masses (Figure 5.3). 

Dichorionic placentas that are in close proximity may abut one another and become fused into a single placental mass.14 This occurs in approximately 50% of dichorionic placentas. As detailed above, a monochorionic placenta may rarely possess separate lobes that are connected together via a diminutive bridge of placental tissue. 

For these reasons, establishing chorionicity may pose as a diagnostic dilemma both prenatally via ultrasound and postnatally at time of gross placental evaluation. The prenatal sonographic markers of chorionicity are reviewed in later chapters of this book. 

Postnatal gross and histological markers of monochorionic vs dichorionic are detailed below. Detailed documentation of the placental anatomy in twins is useful in that it may provide clinicopathological correlation to outcome. Evaluation of the monochorionic twin placenta, particularly if the pregnancy was complicated by TTTS, requires additional scrutiny. The first step is to establish chorionicity. Pathological ascertainment of chorionicity may be performed via several methods. Analysis of the ‘dividing fetal membranes’ by an experienced examiner is the surest method for assigning placental chorionicity. 

Dividing membranes are defined as the portion of the fetal membranes in which the two separate twin gestational sacs are in contact. All twins, except monochorionic–monoamniotics, have a dividing membrane. In the case of a monochorionic–diamniotic twin gestation, the dividing membrane is composed of only two abutting amnions, whereas the dividing membranes of dichorionic–diamniotic twins have intervening chorions between the two amnions (Figure 5.4). 

Thus, the dividing membranes of a monochorionic placenta are thin and have no blood vessel remnants within them. Because the amnion is composed of epithelium and connective tissue only, the dividing membrane is translucent. This is particularly useful during endoscopic fetal surgery for the treatment of TTTS, for the translucency of the membranes of monochorionic– diamniotic twins allows for visualization of the vascular anatomy on the opposite side of the dividing membrane. Lastly, manual separation of the dividing membranes of a monochorionic placenta to their insertion on the placental surface may be continued such that the amnion is stripped from the surface of the placenta.16 This is in contrast to the fused dichorionic placenta. 

The dividing membrane of fused dichorionic placentas is thick, considerably more opaque, has remnants of villi and atrophied vessels in their four layers, and separation of the fetal membranes cannot be continued once the chorionic plate is reached, with further dissection resulting in placental disruption. The presence of vascular anastomoses is another placental finding seen nearly exclusively in monochorionic placentas. The next step in the characterization of the monochorionic placenta is to identify the location of the dividing membrane on the surface of the placenta relative to the vascular equator. 

The vascular equator is defined as the region of the placenta where anastomotic vessels cross. It may be identified grossly by the relative avascular zone (aside from the anastomoses), measuring a few centimeters in width on the chorionic plate between the two placental cord insertions. The line of insertion of the dividing membrane into the placenta may not be in the region of the vascular equator. In fact, we have not uncommonly noted during fetoscopic laser surgery that the dividing membrane may run nearly perpendicular to the vascular equator (Figure 5.5). 

Because the amniotic membranes may be moved freely over the chorionic surface in utero, one can envisage that the polyhydramnios noted in the recipient sac in TTTS may displace the dividing membrane insertion site towards the donor fetus (Figure 5.6). This may explain our observation that approximately two-thirds of all vascular anastomoses are located exclusively in the recipient’s gestational sac. The placental mass that supported each fetus may be qualitatively assigned by partitioning the placenta along the vascular equator. 

Umbilical cord description and position should be defined. Assignment of the umbilical cord to the appropriate fetus is obviously important. Accurate tagging and recording of the correct umbilical cord should be done at the time of delivery. Also, the location of the placental cord insertion sites is recorded relative to the nearest placental margin and to each other, with a special note for marginal or velamentous cord insertions. 

Finally, the vascular anatomy should be concisely characterized. The methodology of placental vascular evaluation is described in the next section. Briefly, the origin and type of each vessel should be documented. All vascular anastomoses should be described relative to type, size, and location, and the overall vascular pattern should be assessed. A color photograph is most helpful in providing a visual record (Figure 5.2). Lastly, histological findings, particularly as they relate to TTTS, should be reported. To better understand the role of the monochorionic placenta as it pertains to TTTS, the following placental factors will be reviewed in detail: (1) vascular anatomy; (2) partitioning of the placental mass; and (3) placental cord insertions.