In some cases, one fetus may be spontaneously lost rather than the entire gestation. As a result, the incidence of twins in the irst trimester is much greater than the incidence of twins at birth. It has been estimated that 1 in 80 births are multifetal, whereas 1 in 8 pregnancies begin multifetal but are spontane ously reduced (Corsello, 2010). Sonography studies in the first trimester have shown that one twin is spontaneously reduced or "vanishes" before the second trimester in up to 10 to 40 percent of all twin pregnancies (Brady, 20l3). The incidence is higher following ART conception. Also, monochorionic twins have a significantly greater risk of spontaneous reduction than dicho rionic twins (Sperling, 2006). Undoubtedly, some threatened abortions are the result of death and resorption of one embryo from an unrecognized twin gestation.
tion in 709 multifetal pregnancies. Before 12 weeks, one or more embryos died in 36 percent of twin pregnancies, in 53 percent of triplet pregnancies, and in 65 percent of quadru plet pregnancies. Interestingly, ultimate pregnancy duration and birthweight were inversely related to the initial gestational sac number regardless of the final number of fetuses at deliv ery. This efect was most pronounced in twins who started as quadruplets. Chasen and coworkers (2006) reported that spon taneous reduction of an IVF twin pregnancy to a singleton pregnancy was associated with perinatal outcomes intermediate did not undergo spontaneous reduction. Evidence for adverse immediate and long-term efects of twin spontaneous reduction on the remaining pregnancy is conflicting (MeN amara, 2016).
Notably, spontaneous reduction of a twin gestation may afect prenatal screening results. In one study of ART-conceived gestations, Gjerris and colleagues (2009) compared 56 twin pregnancies with a single early demise and 897 singleton gestations. They found no diferences in irst-trimester serum marker concentrations as long as the embryonic loss was identiied before 9 weeks' gestation. If diagnosed ater 9 weeks, the serum markers were higher and less precise in gestations with an early demise of one twin than in the singleton gestations. With a vanishing twin, first-trimester maternal serum levels of the pregnancy associated plasma protein-A (PAPP-A) can be elevated. Second-trimester MSAFP and dimeric inhibin A levels can also be higher (Huang, 2015). his phenomenon may also afect noninvasive prenatal testing using cell-free DNA (cDNA). In one report, this efect was thought to be responsible for 15 percent of the false-positive results from quantitative counting methods (Futch, 20l3). The recent development of single nucleotide polymorphism technology for cDNA testing appears to hold promise in better identiYing these cases (Curnow, 2015). Regardless, the diagnosis of a spontaneously reduced abortus is ideally excluded to help avoid confusion with results from aneuploidy and neural-tube defect screening.