Pregnancies involving multiple embryos and eventually fetuses still remain a challenging area for the medical field. Numerous complications can arise during these pregnancies. Some are mild complications and others are much more severe and often, heartbreaking. Some are understood and others are still areas of research that baffle modern medical practitioners and researchers alike. One unique challenge that people pregnant with more than one embryo can face is referred to as “vanishing twin” or “vanishing twin syndrome.” The name of this syndrome reflects the mystery associated with it. As the name indicates, an entity that was seen in the mother’s womb somehow goes away. In general, what happens is that one of a set of twins or one of a set of multiple embryos either dies, disappears, or is resorbed entirely or partially. In some cases, the twin that does not survive becomes a fetus papyraceus (basically, paper fetus) and looks like a part of the womb. This reduces the number of embryos in a womb by one, so twins become a single pregnancy and multiples lose one. The number of embryos that were first visible by ultrasound early in a pregnancy does not match the number of babies delivered.
How often does this happen?
The vanishing twin syndrome sounds mysterious and perhaps like something that does not occur that often, but it turns out to be quite the opposite. For pregnancies involving three or more gestational sacs, nearly half of the pregnancies include a vanishing twin. For twin pregnancies, approximately 36% have a vanishing twin. For pregnancies that result from assisted reproductive technologies, approximately 20 to 30% include a vanishing twin. Vanishing twin syndrome occurs more commonly during the first trimester of the pregnancy.
What happens to the vanishing twin?
If a twin vanishes during a pregnancy, it may lead to a miscarriage that mothers are aware of. It may also occur as a form of vaginal bleeding or spotting that the mother is not fully aware of. The mother may not be aware that a twin is being lost because bleeding is a common complication during the first trimester of a pregnancy and may not be immediately associated with the loss of a twin. The loss is typically detected during an ultrasound with a loss of the additional gestational sac or womb for the twin that vanishes.
The cause of the vanishing twin is not known. It is still a research question. There are some factors that are connected to a vanishing twin. They include a mother being over thirty five years old – often referred to as advanced maternal age. Another factor is a disease related to the DNA of the twin – called a chromosomal abnormality. These can lead to early death in the womb or after birth during the first year or two of life in some severe cases. Another factor is the use of in vitro fertilization. Also, small placenta or other problems with structure of the placenta can affect whether there is a vanishing twin. There may also be certain toxins that a person can be exposed to that can lead to a vanishing twin.
The actual vanishing usually takes place early in the pregnancy. Often the vanishing twin is reabsorbed in the womb as it breaks down. When this occurs, the remaining twin usually has a high chance of surviving. The vanishing embryo may also be a gestational sac without an embryo along with a normal gestational sac containing an embryo. This situation can lead to vaginal bleeding when the gestational sac without the embryo is expelled from the body. The vanishing twin can also become flattened. This happens rarely and usually during pregnancies of multiple embryos. This can be observed on ultrasound. If it occurs later in the pregnancy it may cause complications for the surviving twin.
What should be done about a vanishing twin if anything?
Good prenatal care that involves a patient’s health care being coordinated well between the different clinicians involved in the care of a patient is important for detecting a vanishing twin. If one is detected during the second or third trimester of a pregnancy, more intense monitoring will likely be used to watch for signs of additional complications during the pregnancy. Clinicians work with patients to address their unique challenges regarding the impact of the vanishing twin on the pregnancy. After birth, the placenta should be examined in detail to understand what occurred with the other twin.