By Steven Korzeniewski & Dr. Nigel Paneth, M.D.,M.S.
Only a little more than one in a hundred births is of a set of twins, but some doctors think that a lot of single births actually once had a twin, but that the co-twin died long before birth. In one study, among twin pregnancies diagnosed before the 10th week of pregnancy, more than 2/3 experienced a loss of one of the twins (Levi et al). This is sometimes called the Vanishing Twin Syndrome, and is diagnosed with much greater frequency because of the increased use of ultrasound to image fetuses early in pregnancy.
Why do so many twins die before birth? One reason is the Twin-Twin Transfusion syndrome, which some researchers think is the major cause of single twin deaths in the womb (Aslan et al). In this situation, the blood supply to the placenta is not shared equally between the twins. One twin does not have enough blood supply and dies, or sometimes, the twin getting too much blood is at risk of dying. What is important is that if a twin dies while still in the womb, certain chemicals produced by the breakdown of the twin’s body after death can harm the mother, and perhaps also harm the surviving twin. (Benirshke)
Recently, researchers have been investigating whether the death of a single twin in the womb could be contributing to Cerebral Palsy (CP). According to the well-known CP researcher, Dr. Peter Pharoah of Liverpool, England, the risk of CP is estimated to be 14 times higher in pregnancies where a twin has died in the womb. (Pharoah and Adi) (1) Even if as many twins die before birth as survive, twins would still contribute only a minority of CP cases, since twinning remains fairly rare. However, further investigation is ongoing to clarify the role of the vanishing twin in CP.
1. Pharoah POD, Adi Y. Consequences of in-utero death in a twin pregnancy. Lancet 2000;355:1597-1602.
2. Aslan H, Gul A, Cebeci A, Polat I, Ceylan Y. The outcome of twin pregnancies complicated by single fetal death after 20 weeks of gestation. Twin Research 2004;7:1-4.
3. Benirschke K. Intrauterine death of a twin: Mechanisms, implications for surviving twin, and placental pathology. Semin Diagn Pathol 1993;10:222.