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).

Figure 1. The type of placenta in identical twins is determined by when the twinning process occurs in the first two weeks of pregnancy. Twinning within the first four days results in separate (dichorionic) placentas, after four days a shared (monochorionic) placenta with separate (diamnionic) bags of water, and after eight days the twins will share the placenta and water bag (monoamnionic). TTTS occurs in monochorionic twins.

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.

Figure 2. This is a placenta from a TTTS pregnancy treated with reduction amniocenteses that delivered at 27 weeks’ gestation. The donor and recipient sides of the placenta are labeled, and their umbilical cords are seen with plastic clamps attached. This placenta was equally shared by the twins and contained three artery-to-vein connections (arrows) between the twins which joined in the region of the vascular ‘equator.’                                (Photo courtesy of Dr. Julian E. De Lia)

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, section on Placental Analysis), and other unknown variables are also important.

Figure 3. This is a placenta from a TTTS pregnancy that was lost at 21 weeks’ gestation before any treatment could be started. Both twins had umbilical cords that were velamentous and attached at opposite ends of the placenta. The twins had multiple vascular connections between them (mostly artery-to-vein), but these connected in a vascular equator (arrows) that was off-center resulting in asymmetrical sharing (80% vs. 20%) of the placenta by the twins. (Photo courtesy of Dr. Julian E. De Lia)

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).

Figure 4. This is a placenta from a TTTS pregnancy that was lost at 20 weeks’ gestation before treatment could be started. The twins had all three types of anastomoses between them (artery-to-artery, artery-to-vein, vein-to-vein). The vascular equator (arrows) divided the placenta into two portions that were significantly asymmetrical (95% vs. 5%). The twin with the smaller share (donor) has an umbilical cord attachment in the membranes (velamentous) outside the body of the placenta, while the other twin’s cord inserted into the center of the placenta. The donor’s placenta share was too small to provide its nutrients beyond 20 weeks and was the cause of its death. The larger twin then passed away from an acute transfusion into the other twin through the large artery-to-artery connection seen near the tip of the upper arrow. (Photo courtesy of Dr. Kurt Benirschke)

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.

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.

Figure 5. This diagram shows the difficulty that identical MC twins encounter when the blastocyst (a ball like stage of embryonic development, see 6-A, TC) enters the mother’s womb and imbeds in the nutrient lining (DB). At this stage, each twin is represented by a few cells (the two inner cell masses – ICM). In a single pregnancy (not pictured) the placenta will most likely develop normally (large, with an umbilical cord in the middle of the placenta) if the pole containing the inner cell mass enters the uterine lining first. Any rotation of the blastocyst pole containing the inner cell mass will lead to a smaller defective placenta. Since the cells that develop into the MC twins sit at opposite ends (poles) of the blastocyst, as one twin’s cells imbed perfectly (6 – B), the other twin’s cells move farther away from the lining that provides nutrients from the mother. All MC twin blastocyst implantations, therefore, represent a compromise and are prone to develop placentas that are smaller and unequally shared by the twins. EE = endometrial gland; UV = uterine blood vessel                                (Diagram courtesy of Dr. Emanual P. Gaziano)

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 (see Figure 6). 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.

 
Figure 6. This black and white photograph shows the areas of degeneration in the former vascular equator produced by the fetoscopic laser occlusion of the connecting (anastomotic) blood vessels. The placenta was symmetrically shared by the twins. The laser surgery was performed at 20 weeks, with the 4lb. 5oz. and 3lb. 14oz. twin boys born 12 weeks later.    (Photo courtesy of Dr. Julian E. De Lia)

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.