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.