Sunday, February 06, 2011

Chapter 8: "Vanishing" twins revealed

"Vanishing" twins - A new term for the death of one twin in the womb

At the Third International Congress on Twin Studies, held in Jerusalem in 1980, this phenomenon was discussed and the term “vanishing twins” was coined to describe it.   A “vanishing twin” pregnancy quickly became the accepted medical term to describe a particular situation - that is, when a pregnant woman is scanned by ultrasound, two sacs are seen at the first scan but one sac has “vanished” from view by the next scan and in the end only one baby is born.
The mystery of the vanishing twin at once captured the imagination of research scientists.  In 1982, an article entitled “The Vanishing Twin” quoted nine research studies.  Another article followed shortly with the same title and quoting even more studies. In April 1986 a new term, “The Disappearing Twin” was coined by the translator of an article in Czech but it never caught on. 
The term “vanishing twin” was not considered sufficiently scientific for some scientists.  In 1992 an effort was made to translate the name into Latin,  probably in order to put the whole idea onto a more academic footing. One writer adopted the term foetus vanescens, which means “vanishing foetus”  but by then the term “vanishing twin” was too beguiling and familiar to replace with any other term.
In 1998 a comprehensive article reviewing the existing literature, was published under the title, “The Vanishing Twin.” There were over 50 references referring to various other studies.  The term was by then in common usage even though it was known that none of the twins “vanished,” in fact.  Their bodies were no longer visible on the ultrasound screen but they did not totally disappear.  In fact, in many cases there was visible evidence at birth that the twin had once existed, such as an extra placenta; a foetus papyraceous (“papery” foetus) or an empty amniotic sac attached to the placenta of the survivor. 
In the late 1990s some highly imaginative people took the word “vanished” a little too seriously. They decided that “vanished” twins had been abducted by aliens and had been literally taken from the womb.  That idea faded along with the century, as report after report on the subject of the intrauterine death of one twin was published in reputable journals.  The phenomenon of the “vanished twin” today is widely discussed by obstetricians, gynaecologists and experts in multiple pregnancy.  Among the general public some residual scepticism still remains, but not among the pregnant women who witness the disappearance of one of their twins from the ultrasound screen and ask, “Where did it go?”   

How many twins die?

Obstetricians and midwives had known for a very long time that more twins are conceived than are born.  However, no one knew until the ultrasound studies of the 1980s just how many twins were lost.
Once a sufficient number of pregnancies had been monitored to be able to produce some reliable statistics, people began to discuss the “vanishing twin rate” as related to the general twinning rate.  Various painstaking ultrasound studies of twin pregnancies have been carried out in the USA and England to try and find out how many twins die before birth.  The results in each case seem to be consistent.  In about a third of twin pregnancies, one embryo is seen to disappear at some stage in the first twelve weeks.  
Unfortunately for researchers,  most routine scans on normally-conceived pregnancies are carried out after twelve weeks of pregnancy 
have passed, by which time many twin embryos have died and disintegrated, leaving an apparently “singleton” pregnancy.  However, scans are made earlier when the various assisted reproduction technologies (ART) are used to create a pregnancy.  As a result, we now know much more about the early loss of a twin. 

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