The type of placenta nurturing identical twins plays a significant role in the development of complications in multiple gestation. Identical twins may either have their own separate placentas or they may share a common placenta. The impetus for and the timing of the embryo to split into identical twins is unknown, but the later this occurs the more complications are seen.

The type of placenta is determined by when, in days, the embryo randomly splits into twins following the fertilization of the egg (conception). Twinning within the first four days results in dichorionic or separate placentas similar to those found in fraternal twins (see Figure 1). These identical twins have the lowest complication rates. Twinning four or more days after conception will lead to a shared or monochorionic (MC) placenta. Between four to eight days the MC twins will have separate sacs of water (diamnionic) despite a shared placenta, but after eight days they will also be in the same sac (monoamnionic). Diamnionic monochorionic (4 to 8 day split) twins are the most common placental type for identical twins, and most cases of TTTS occur in this group. It is important to note that if the twins have a MC placenta it is absolute proof that they are 'identical'.

The MC placenta contains two anatomic variables that are thought to develop randomly, which will contribute to and explain why, when and to what degree TTTS will affect the twins. The first is the presence of blood vessels in the placenta that connect the umbilical cords and circulations of the twins, and the second is the variations in the way the twins share their common placenta. These are discussed separately below, but in some MC twins both these abnormalities may be present.

Monochorionic Placental Blood Vessel Connections
Over 95% of MC twins have blood vessels in their shared placenta that connect their circulations (see Figure 2). These connecting vessels (also called chorioangiopagous vessels) probably occur by chance as the twins developed their individual placenta circulations in early pregnancy to provide them with vital nutrients from their mother. The twins were unaware of the presence of their co-twins as they claim arteries and veins as theirs and they wind up with both using some of the same placental vessels.

In each MC placenta the blood vessel connections vary in number, type and direction (from one twin to the other). There are three types of connections: artery-to-artery, artery-to-vein, and vein-to-vein. The connections are seen in and define the 'vascular equator' of the shared MC placenta. The equator is used by pathologists to determine the presence of equal or asymmetric sharing of the placenta by the twins (see below). In the majority of MC twins, the connecting vessels allow for the free flow or no flow of blood between the twins, but in 15% they lead to an imbalance of blood flow between the twins. This is the origin of the transfusion in TTTS.

Scientists have paid considerable attention to the study of the connecting vessels. There appear to be certain combinations of vessel type and number that are more common in TTTS, but factors such as unequal sharing of the placenta, umbilical cord insertion type (see Figures 3 and 4), and other unknown variables are also important. Although the placental type (i.e., mono- or dichorionic) can and should be determined by ultrasound in any multiple gestation, the placental vascular connections cannot be seen. When twins are determined to be MC, especially if there are signs of TTTS, vascular connections are assumed to be present. The connecting vessels can only be seen by inserting an endoscope into the uterus (as during fetoscopic laser surgery) or by examining the placenta after delivery.

The types of transfusion in MC twins are the chronic, acute or acute superimposed on a chronic transfusion. Chronic TTTS appears early, in the first or mid-trimester of pregnancy, and is usually a result of transfusion of blood products from a "donor" twin to a "recipient" through artery-to-vein connections. Acute transfusions can occur during labor or at any other time during pregnancy when a significant blood pressure difference occurs between the MC twins. An example of the latter is when one MC twin passes away for whatever reason and the live co-twin then bleeds suddenly through the connections back into his or her twin who has passed away (see Figure 4). Artery-to-artery and vein-to-vein connections are thought to be the likely type of connection causing the acute transfusion in such events. If a donor should pass away in chronic TTTS, an acute 'reverse" transfusion can occur from the recipient back to the donor, again this depends on the type of connecting vessels present.

The connecting vessels are ultimately the cause of most of the complications when MC twins are compared to twins with separate placentas. In addition to the effects described before that chronic and acute TTTS can have on the babies, the connecting vessels are thought to play a role in cases where one MC twin has a birth defect (e.g., heart, kidney, intestinal, etc.) not present in its 'identical' twin. Here a transfusion may have occurred very early on, in the embryonic period, when the different organs are developing. We even suspect that the connections and a significant transfusion may lead to the very early loss of a twin (vanishing twin on ultrasound), with the subsequent birth of a single baby.

Of all available therapies for TTTS, only fetoscopic placental laser surgery is directed at the vascular connections between the twins. By virtually disconnecting the twins, laser surgery can stop the chronic transfusion of blood from one twin to the other, and prevent the sudden, acute transfusion of blood should one twin pass away. This latter event is a particular concern when the twins do not share their common placenta equally, and one twin has a share too small to survive beyond early to mid-pregnancy.

Equal and Asymmetric Monochorionic Placenta Sharing
Abnormalities of MC placenta sharing have received considerably less attention by scientists than the connecting blood vessels, but are just as important. Monochorionic twin placental asymmetry has been variously called 'unequal sharing of venous return zones,' 'unequal allocation of parenchyma', or 'discordant vascular perfusion zones,' but a precise definition is lacking. The percentage of each twin's portion of the MC placenta cannot be directly determined by ultrasound scanning during pregnancy, but can be estimated during postpartum placental examination by noting the location of the vascular equator in the shared placenta.

The cause of MC placental asymmetry is also unknown, but the MC twin embryo (blastocyst) seems to have a problem when it implants into the lining of the mother's womb (see Figure 5). Because there are two different areas of cells in the blastocyst that will become babies, and have to develop their own healthy placenta, events that should lead to a normal placenta in a single baby cannot occur effectively when there are two future babies in the blastocyst.

The portions or shares of the MC twin placenta in TTTS are often unequal with the donor's typically the smaller. The threshold for significantly abnormal sharing (e.g., 60:40, 70:30, 80:20, 90:10, etc.) and placental insufficiency, which might lead to one twin being smaller than its co-twin or even jeopardize its normal growth and survival in the womb, may vary in each case and depend on the month of pregnancy, and type and number of connecting vessels. One study showed that if the placental asymmetry was 60:40 or more, a significant difference in twin birth weight could be expected in TTTS cases.

In cases without connecting vessels, unequal sharing is an important cause of size and growth differences in identical MC twins. The presence of connecting blood vessels can place a MC twin with an adequate placenta share at risk for abnormal events that may occur in the twin with placental insufficiency. Conversely, the connections may help a twin with a small share by supplementing nutrients that would otherwise be deficient. The clinical consequences of MC asymmetry depend on its degree, the type, direction and number of vascular anastomoses, and the gestational age (see Figure 7). Theoretically, there must be a placental share (perhaps less than 20%) that is incompatible with continued intrauterine survival of one twin.

In addition to the sharing differences, the asymmetric MC portions may differ qualitatively in placental circulation relative to the umbilical cord insertions, placental surface blood vessel pattern and the way blood flows deep within the substance of the placenta. Abnormal umbilical cord insertions and single umbilical cord artery (normally there are two) are more common in twins than single babies and are associated with smaller placentas. Not surprisingly there is a relationship of velamentous cord insertion (see Figures 3 and 4) to MC twins with TTTS (65% in TTTS versus 20% in MC twins without TTTS), and cases with velamentous cords have worse outcomes and earlier delivery despite attempts at treatment. Velamentous cord insertion and placental asymmetry were linked by one investigator who found a moderate to marked MC placental asymmetry and a small share for the twin with a velamentous insertion, particularly if the co-twin had central cord insertion (see Figure 4). Single umbilical artery occurs three to four times more frequently in twins, and when present in only one twin it is usually the smaller. Finally, when one looks at the twins' placental portions in TTTS cases with a microscope, the donor portions have blood vessels that are fewer in number, compared to the recipient who has more dilated and congested blood vessels.

Sadly, some MC twins may have shares of the common placenta that are unable to sustain their lives in the womb to a point where they can survive if delivered. In such cases additional placental tissue cannot be created. When this twin passes away, the other twin is at risk for death or birth defects because of the connecting vessels. The only therapy that can remove these risks to the other twin (with the larger or normal placental share) is laser occlusion of all the vessels because it will 'disconnect' the MC twins.

Three random events occur in identical (monozygous) twins that determine whether the twins develop TTTS: the timing of the twinning process which determines placental type, the number, type and direction of the connecting vessels, and the degree to which the twins share their common placenta equally or asymmetrically. Most identical twins have a monochorionic placenta and approximately 15% develop TTTS. The placenta findings determine when in pregnancy TTTS occurs, the degree and severity of the transfusion, and the outcomes (survival with and without abnormalities in the babies) with various treatments. In addition to TTTS, the placental abnormalities can cause the early loss of one twin, as well as developmental birth defects in one twin. Couples with monochorionic twins are at high risk and should be examined often by their physician, have frequent ultrasound scans (The TTTS Foundation advocates weekly ultrasounds from 16 weeks through delivery), and plan to have careful placental examination after delivery.

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