For the majority of the population, Vanishing Twin Syndrome will never be something of concern. Given that it only happens to pregnant women, and that pregnancy must start out as a twin pregnancy, you can see how narrow the margin of affected human beings is. If you were never pregnant with twins you have likely never even heard the term Vanishing Twin, or sometimes, Disappearing Twin Syndrome.
So what’s it all about? It occurs when one fetus out of a set of twin fetuses disappears in the uterus during early pregnancy, which is essentially a miscarriage of the fetus. The tissue from the fetus may then be absorbed by the surviving twin or by the mother, hence the appearance of a twin that has “vanished” or “disappeared”.
How is Vanishing Twin Syndrome identified? It can be diagnosed by use of ultrasound and has been diagnosed with more frequency and accuracy with the increasing use of ultrasound during pregnancy. Pregnant women will typically have an ultrasound by week 8 of gestation to verify the pregnancy. The doctor is also able to distinguish the number of fetuses within the womb. Sometimes, two fetuses with two distinct heartbeats, twins. On the next visit, the doctor may not find both heartbeats or sacs, which indicates one of the fetuses no longer exists.
What are the symptoms? Miscarriage symptoms may be identified even though there is still another baby present in the uterus. Sometimes bleeding, spotting, and/or cramping will occur. Still, in many cases, there will be no particular symptoms.
What causes a twin to disappear? For now, it is truly unknown. Some suggest that there may be abnormalities or chromosomal defects with the vanishing fetus. Another cause could be improper implantation. Statistics show that this happens more in woman over thirty whom are pregnant with twins, so age, too, may be a factor. If the loss happens in the first trimester, the prognosis of the surviving twin is very good, but it depends on the factors that caused the loss of the other twin. Physically, there are usually not any issues with the mother.
Interestingly, with more and more data to analyze as technology increases, scientists have concluded that the number of twin conceptions significantly outnumbers the number of actual twin births. This means that more people than we previously thought possible actually might have started out in life as a twin. Studies vary as far as percentages go. But, some conclude that up to 30% of all multiple pregnancies in the United States end up as a single birth. In any case, it is important to acknowledge the loss of the one baby. In some cases, it is helpful to seek counseling. For emotionally, this type of unexplainable loss can be difficult for parents to deal with.
You can view actual ultrasound images of twin pregnancies at various stages via this extensive Twin Ultrasound Gallery 
Originally published at Twin Pregnancy and Beyond