Some experts estimate that 1/5 to 1/3 of all multiple pregnancies involve a vanishing twin. While this number can seem high, it should be noted that many pregnancies that result in a vanishing twin are never diagnosed as a multiple pregnancy; indeed, the mother many never know she was carrying more than one baby.
Since the advent of ultrasound, many more doctors and women are aware that a multiple pregnancy exists much earlier than in the past. For example, during my first pregnancy, my OB monitored my hormone levels and noted that my HCG levels were very high. She suggested I was most likely carrying twins. During the eighth week of my pregnancy, I experienced some bleeding and came in for an ultrasound. After reviewing my ultrasound, my OB said that she saw one healthy fetus, and that she thought I had lost another. The rest of the pregnancy was uneventful, and I gave birth to a healthy son the day after his due date.
When vanishing twin syndrome occurs in the first trimester, there is little effect on the mother or the surviving fetus. However, when the twin vanishes in the second or third trimester, there is a higher likelihood of effect on the other fetus, such as a higher incidence of cerebral palsy, and a possibly a threat to the viability of the pregnancy in general.
Most vanishing twins are reabsorbed into the placenta or be compressed by the pressure of the remaining fetus and its amniotic sac. If the placenta is analyzed, evidence can sometimes be found of another DNA profile, which indicates the vanishing twin existed.
Some experts believe that there are possible physical and psychological effects on the surviving twin. While there has been much speculation as to the psychological effects of losing a twin in utero, little scientific research had been done in this area. My son seems to have suffered no ill effects, physically or psychologically, at least not at this point in time. I can honestly say that though I regret losing the twin who vanished, I believe that this twin disappeared for a reason, though it may be one I don't understand.
When I found myself pregnant again, five years after giving birth to my son, I was astounded to find I was again pregnant with twins. After doing some climbing on the family tree, I found that twins did indeed run in the family on my mother's side--but only amongst women who gave birth after age 30 (my mother and grandmother stopped having children at about 30, so only had singletons). Happily, the second time around, both twins survived and are happy and healthy.
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eMedicine - Vanishing Twin Syndrome http://www.emedicine.com/med/topic3411.htm
The Vanishing Twin Website http://www.vanishingtwin.com/
Vanishing Twin Syndrome - Wikipedia http://en.wikipedia.org/wiki/Vanishing_twin
What is a Vanishing Twin? - LaborofLove articles http://www.thelaboroflove.com/articles/what-is-a-vanishing-twin/