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The chance of conceiving spontaneous triplets is as high as 1 in 8,000 (1 in 500,000 for identical triplets), there’s no question that multiples are somewhat of a rarity.
Multiple births are much more common today than they were in the past. According to the US Department of Health and Human Services, the twin birth rate has increased by over 75% since 1980, and triplet, quadruplet, and high-order multiple births have increased at an even higher rate. There are more multiple births today in part because more women are receiving infertility treatment, which carries a risk of multiple pregnancy.
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A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.[1] The independent twin is called the autosite.