When a twin pregnancy is found at a very early scan but only one baby is seen at a later one, this is known as the disappearing twin syndrome or vanishing twin phenomenon.
Disappearing Twin Syndrome happens much more frequently than you would think.
Though one in 80 of all live births are twins, many more than this are conceived. However, sometimes, at a very early stage one of the embryos does not develop and grow. Because ultrasound technology is now so advanced, it is possible to see two sacs as early as five weeks gestation with a vaginal scan. At this stage the sacs appear empty but by five and a half weeks, you may just see heartbeats and yolk sacs. Tiny embryos can be seen from about six weeks when they are just 3mm long.
Once a twin pregnancy has been diagnosed by an early scan, about one in five of these will subsequently disappear before 12 weeks. When one of a twin conception fails to survive at this early stage, the remaining twin continues to develop normally and hormone levels stay high which prevents a miscarriage, though sometimes there is some vaginal bleeding. As the remaining twin grows, its sac spreads to fill the uterus and the contents of the other sac are absorbed.
You have probably spent several weeks between the two scans imagining what it would be like to have twins. You have probably been getting excited about the idea and may now feel sadness at the loss. If your first scan had not been done so early, you would never have known about the twin conception. It's therefore advisable not to get too excited about the idea of a twin pregnancy if you find out about it in the early weeks - it's better to wait until the 12 week scan when both twins are fully formed and much more likely to survive.
March 2008
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