Clones may in some respects be more identical to each other than naturally occurring identical twins. Twin fetuses can do a lot of harm to each other, and clones grown separately would not have to endure the competition in the womb. Also, there may be something about the splitting process that results in twins that may also cause birth defects. Mice embryos that have been artificially divided and allowed to develop tend to be weaker than normally developed mice. It is possible—in fact, likely—that clones could be created without the same liabilities that attend the twinning process.
During the broad public debate about the morality of cloning human beings, clones have repeatedly been described as nothing more than identical twins, as if that robbed the debate of its mystery. The truth is that the twinning process is much less well understood than cloning, which is a rather simple procedure, in spite of the burst of twin-based scholarship in recent years, much that is commonly believed to be true about twinning is either wrong or in dispute. It is not clear, for instance, whether twinning is a kind of birth defect or, contrarily, whether birth defects are caused by twinning (or if, indeed, either has anything to do with the other). We don't know what significance, if any, to attach to the elevated incidence of left-handedness among both kinds of twins. It is not even certain whether fraternal twins always come from two eggs or sometimes from one that has split before fertilization, creating a third kind of twin. We are only now learning that twins are different in particular ways from singletons (their teeth are less symmetrical, for example), but we don't know why or what that means. In sum, we don't know who twins are or how twins happen. We only presume to know what they tell us about who we are.
So much depends on a phenomenon about which we know so little. Even the prevalence of twins is a subject of puzzlement and controversy. With the increasing use of ultrasound to detect early pregnancies, we now know that twinning is a far more common occurrence than anyone had previously imagined. Although only about one out of eighty or ninety live births produces twins, at least one-eighth of all natural pregnancies begin as twins. Many of us singletons, in other words, began life as something more—as part of a pair.
The use of sonography to detect pregnancy dates from 1957, but it didn't become widespread until the late seventies. By that time, we already knew more about the ocean floor and the dark side of the moon than we did about embryogenesis. Before ultrasound, it was impossible to diagnose accurately twin pregnancies, and most multiple births came as a surprise. As ultrasound became more common and increasingly more sophisticated, however, doctors began having the unnerving experience of viewing twin embryos one month, only to find a singleton the next time they looked. What was happening? In 1980, at the Third International Congress on Twin Studies in Jerusalem, this question was raised, and one of the participants cried out, 'Vanishing twins!' thus giving a name to a phenomenon that has caused as much confusion as excitement.
Usually the only external sign of a vanished twin is vaginal bleeding. More advanced ultrasound equipment, including vaginal ultrasound, high-speed scanners, and color Doppler (the same kind of radar apparatus meteorologists use to track the movement of weather), has been able to detect multiple pregnancies as early as five weeks after conception. A 1992 Israeli study, using trans-vaginal sonography, diagnosed eighty-eight multiple gestations among women who had undergone ovulation induction. Of the fifty-four twin pregnancies detected in this group between five and six weeks, fifty-one resulted in the birth of singletons and three were spontaneously aborted; the rest had vanished. There were also twenty-six triplet gestations, producing twelve pairs of twins, twelve singletons, and two complete miscarriages; five quadruplet pregnancies, which produced three triplets, two twins, and a singleton; and three quintuplet pregnancies, two of which were deliberately reduced by doctors to a set of triplets and a set of twins, but there were three vanished fetuses as well. Two different studies found that the frequency of twins among abortions is three times higher than the frequency of twins at birth.
'People are picking up twin pregnancies the size of garden peas. They're seeing a lot more twins than they ever knew were there,' according to Charles E. Boklage, a geneticist at East Carolina University School of Medicine and a well-known maverick in the world of twin biology. Boklage has a knack for uncovering paradoxical data that undermine many of the commonplace assumptions people still harbor about twins. 'The so-called phenomenon of the "vanishing twin syndrome" is neither phenomenal nor a syndrome,' Boklage contends. 'It is much too common to be considered phenomenal, and it occurs for too many reasons to be considered any kind of syndrome.' He says that most pregnancies, whether multiple or singleton, fail in any case, so it is not as surprising as it seems that twins often disappear. 'Somewhere in the vicinity of twelve to fifteen percent of us—and that's a minimum estimate—are walking around thinking we're singletons when in fact we're only the big half,' says Boklage. He estimates that for every set of twins born alive, there are at least six singletons born who are sole survivors of twin conceptions. 'Now, the implications of this are profound. A huge fraction of the population are products of twin pregnancies, and therefore are presumably associated with all the things that known twin pregnancies are associated with in terms of malformations and extra risks.'
Twins are far more susceptible to the birth defects, spontaneous mutations, and vascular problems that threaten early life. Simply being a twin is stressful and raises the odds against survival because of the competition for space and nutrients. If an embryo disappears in the first trimester, it has probably been absorbed by the placenta or by the other twin, with little or no evidence that it ever existed, except for the tiny image it may once have left on ultrasound. A careful examination of the placenta after birth will sometimes reveal a nodule that turns out to be the remains of a vanished twin.
The death of one twin poses a real threat to the other. Identical twins share the same circulation in the womb, and the death of one could cause a blood clot to pass into the survivor, causing heart damage or developmental problems of exactly the sort twins are famous for. Cysts, called teratomas, composed of bits of hair and teeth and fetal bones, are sometimes discovered in adults. Either they are errant cells that have reverted to a primitive embryological state or they are the remnant of a vanished identical twin. Dead fetuses have also been found inside living children. There is a well-known case in 1949 of such a 'fetus-in-fetu,' in which five fetuses were removed from the brain of an infant girl in Philadelphia; several less spectacular cases have been reported since, including a six-pound fetus found during the autopsy of an elderly man.