Incidence
Based on 2005 USA National Center for Health Statistics (4,138,349 total births), the rate of multiple births per year is now 1:30 (3.4%), or approximately 139,816 twins or higher multiples. The majority of identical twins share a common (monochorionic) placenta, and of these approximately 15% go on to develop TTTS. By extrapolating the number of expected identical twins (about one-third) from annual multiple births, and the number of twins with monochorionic placentas (about two-thirds), and from these the number thought to develop TTTS (about 15%), there are at least 4,500 TTTS cases per year in the U.S. alone: 139,816 X .33 X .66 X .15 = 4,568 cases of TTTS per year in U.S. (involving 9,500 or more babies) Since spontaneous pregnancy loss (spontaneous abortion) and pregnancy terminations (elective abortions) that occur prior to 20 weeks go uncounted by the C.D.C., our estimate of TTTS cases may be very conservative. Although infertility treatments have increased the rate of multiple birth, they have not diluted the expected incidence of identical twins even though multiple embryos are often produced and implanted. Studies show a higher rate of identical twins (up to 20 times with IVF) in women having these treatments than occur naturally.
There is approximately a 1 in 1000 chance of having TTTS in a given pregnancy.
One Australian study, however, noted an occurrence of only 1 in 4,170 pregnancies or 1 in 58 twin gestations. This distinction could be partly explained by the "hidden mortality" associated with MC multifetal pregnancies—instances lost due to premature rupture of membrane (PROM) or intrauterine fetal demise before a thorough diagnosis of TTTS can be made.
Read more about this topic: Twin-to-twin Transfusion Syndrome
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