What Is a Vanishing Twin?

Beginning around 1945, doctors realized that some single births were originally twin pregnancies. One baby was miscarried during the pregnancy without the mothers or doctors knowing. Doctors called these cases vanishing twins or vanishing twin syndrome (VTS). 

The tissue from a vanishing twin is mostly reabsorbed by the mother's body and the remaining baby. Sometimes some evidence remains. Before ultrasounds, doctors found proof of vanishing twins by examining the placenta after the birth of the surviving twin.  

Today, doctors diagnose vanishing twin syndrome by using ultrasounds. An early ultrasound may show two babies, and a later ultrasound may show only one. Doctors can see vanishing twins that they once would not have found. 

Likelihood of Vanishing Twin Syndrome

According to one study, about 36% of twin pregnancies experience vanishing twin syndrome. It also occurs in around half of multiple pregnancies, or pregnancies where a woman carries more than one baby.

Some researchers think the number of women who experience this syndrome may be increasing. The increase could be because procedures like in vitro fertilization (IVF) are becoming more common. During IVF, doctors may transfer more than one fertilized egg to the mother. Often, one or more of the eggs may not survive to birth.

In another study, researchers looked at nine studies of vanishing twin syndrome. They found that reported rates of VTS varied a lot. Researchers also found that poor ultrasound equipment and incomplete scans can cause doctors to miss cases of vanishing twin syndrome. Also, finding a vanishing twin depends upon when ultrasounds are performed. Without an early ultrasound, doctors may not realize that a woman is carrying twins.

Causes of Vanishing Twin Syndrome

Abnormal chromosomes can cause one twin to die early in a pregnancy. These problems are usually there beginning at conception. It's unusual for them to occur later in pregnancy.

Some factors increase the risk of vanishing twin syndrome. These include:

  • Advanced age of the mother (over 30 years old)
  • Use of IVF and other assisted reproduction technology (technology that helps couples conceive)
  • Abnormalities of the placenta
  • Other factors that affect babies in the womb, such as rubella infection
  • Genetic factors

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Risks to the Mother

Mothers who experience vanishing twin syndrome often have no symptoms. You may have spotting and cramps, which are common in pregnancy, especially during the first trimester.  

If vanishing twin syndrome occurs in the first trimester, doctors rarely take any special steps. If it occurs in the second or third trimester, doctors often call the pregnancy high risk. They may watch the mother and baby carefully.  

Most pregnancies where VTS happens don't have other problems. Mothers in these pregnancies do have a higher risk of gestational diabetes, a special type of diabetes that occurs during pregnancy. They also have a higher chance of going into labor early and of having induced labor. And, they are at a greater risk of having low amniotic fluid

Risks to the Surviving Twin

In general, twin pregnancies are at higher risk than single pregnancies. Doctors look at whether the twins share a single placenta or whether each baby has its own. 

The placenta supplies blood and nutrients to babies in the womb. When twins share one, they sometimes share unequally. This puts one twin at risk. 

Babies who survive vanishing twin syndrome may have health problems if they shared a placenta with their twin. The death of one baby can affect the blood supply of the other. In these cases, doctors use frequent ultrasounds to check the health of the surviving twin. 

A 1997 study found that babies who lived through vanishing twin syndrome could be at higher risk of cerebral palsy. A later study didn't confirm this risk. It found that the risk for these babies was similar to the risk for other twins and babies born from multiple pregnancies. The researchers concluded that further study is needed. At this time, a link between cerebral palsy and VTS hasn't been proven.

Twins who live through vanishing twin syndrome often have health problems. They are likely to be smaller than expected while in the womb. They also have lower birth weights. They may have lower scores on the Apgar test, which rates a baby's general condition at birth. They also have a greater risk of dying during the first week of life.

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Psychological Impact of VTS

The loss of a twin in the womb is a type of miscarriage and can cause grief for the parents. Sometimes the twin who lives through vanishing twin syndrome develops feelings of guilt. The whole family may need help from counseling and other mental health resources. 

WebMD Medical Reference Reviewed by Dan Brennan, MD on June 08, 2021

Sources

SOURCES:

Acta Geneticae Medicae et Gemellologiae: "The vanishing twin." 

American Pregnancy Association: "Vanishing Twin Syndrome." 

Fertility and Sterility: "Vanishing twin syndrome: is it associated with adverse perinatal outcome?"

Hong Kong Medical Journal: "Single foetal death in twin pregnancies: review of the maternal and neonatal outcomes and management." 

Johns Hopkins Medicine: "Complicated Monochorionic Twins." 

Journal of Clinical Medicine Research: "Single Fetal Demise at 10 - 14 Weeks of Monochorionic and Dichorionic Twin Pregnancy." 

Twin Research: "A Case-Control Study of Vanishing Twin as a Risk Factor for Cerebral Palsy." 

Zamani, Z. and Parekh, U. Vanishing Twin Syndrome. StatPearls Publishing, 2021. 

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