Charles Boklage writes.....
7. TwinningAmong the most obvious of the more-or-less ‘unusual’ outcomes of human embryogenesis is twinning. According to the inferences about prenatal mortality and survival discussed above (Boklage 1990, 1995), twins born as members of live pairs represent no more than about one-in-fifty of all products of twin embryogenesis. Like singletons, over three-fourths of twin conceptions disappear completely (with loss of both conceptuses) before term birth.
Most of the remaining one-fourth of twinned embryos arrive at term alone, as sole survivors, outnumbering live-born twins apparently at least ten- to twelve-fold.
The strongest indications are that roughly one live birth in eight is a product of a twin
embryogenesis, with the great majority of them showing no easily recognized evidence of their origins in twinned embryos. These sole survivors have been totally ignored in all the various literatures about the epidemiology of twinning.
Developmental consequences of twin embryogenesis are not terribly hard to find in twins born in pairs (Boklage, 2009) and are therefore to be expected in the lives of those sole-survivor individuals.
We do not yet know any simple or inexpensive way to identify all of the sole survivors for an accurate count, but clearly a substantial fraction of all human embryogeneses are twinned and a similar fraction of all live births arise from twinned embryos. I remain satisfied with the estimate of one in eight, with the realization that it may vary considerably up or down with variations in overall efficiency of pregnancies in general.
That means that the figure I use of 1 in 10 is an underestimate. There are a billion womb twin survivors in the world. Our "inability to identify womb twin survivors" has decreased somewhat since the womb twin survivors project began. Since 2003 womb twin survivors have simply identified themselves to me, and continue to do so in their hundreds and thousands! Thank you, everyone!
Here is a list of most of the identifying physical signs that feature on the questionnaire: (we keep finding more and more....)
Read the list here on my site