What is a vanishing twin?
Simply said, a vanishing twin is one that "disappears". We're talking about pregnancies that start out as being twin pregnancies, but one of the twins ceases to be visible on ultrasounds at some point during the pregnancy. A vanishing twin is essentially a situation in which one of the twins miscarries, while the other continues to thrive in utero. A vanishing twin can be completely reabsorbed by the mother's body or the twin itself, in which case the placenta offers the only physical clue that the other twin ever existed. In some cases, the vanishing twin isn't completely reabsorbed.
Rather, it becomes compressed and turns into a "mummified" disk of tissue. This is referred to as fetus papyraceus. Research indicates that vanishing twins may not be all that uncommon. Some even suggest that up to one in eight multiple pregnancies will end in this way, and that many cases remain unidentified because the reabsorption takes place before the mother's first ultrasound. More conservative estimates indicate that vanishing twin syndrome affects up to 30 percent of multiple pregnancies. Vanishing twins that are completely reabsorbed will not usually lead to any complications besides first-trimester bleeding.
However, a fetus papyraceus can block the cervix, necessitating a c-section. The demise of one twin during the second or third trimester can lead to premature labor, hemorrhage, and infection of the uterus caused by the presence of a non-reabsorbed twin. It is safe to assume that the reabsorption process associated with vanishing twin syndrome is a mechanism to maximize the survival chances of the remaining twin.
When was vanishing twin syndrome discovered, and why does it happen?
The syndrome was first recognized in 1945. Its diagnosis became more frequent with the use of ultrasound. This meant that twin pregnancies could be identified fairly easily and with them, also then disappearance of one of the fetuses. Vanishing twin syndrome occurs for much the same reasons as singleton miscarriages. The fetus may suffer from a chromosomal abnormality that is not compatible with life, may have severe birth defects, or may fall victim to placental problems.
In many cases, the second embryo may simply never have developed at all, something that is known as a blighted ovum. While the analysis of remaining fetal tissues or the placenta can sometimes reveal the exact cause behind individual cases of vanishing twin syndrome, the cause of individual cases also often remains unclear.
What happens afterwards?
The remaining twin is usually completely healthy. If vanishing twin syndrome occurred during the first trimester of pregnancy, chances are excellent that there will be no further complications for either the mother or the remaining twin. No special monitoring will be required during the rest of the pregnancy if the twin is completely reabsorbed. The demise of one twin during a later stage of pregnancy carries a much higher risk of complications, including cerebral palsy for the remaining twin and premature labor. Such pregnancies may be treated as high risk, and require additional monitoring.