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 section on Placental Analysis), 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.