Whatever the origins of these parasites, it's unlikely that you'll find yourself surprised on your due date by an adorable baby bearing an acephalic twin on its breast or a stray arm erupting from its mouth. The first prenatal diagnosis of parasitic twins was made in 1988; since then, seven cases have been found using ultrasound, at an average gestational age of 19 weeks. Furthermore, the parasite may be unpleasant to look at, but it's typically harmless and can be removed. In the developing world, parasites are more likely to grow along with their autosites into adolescence or even adulthood. While on a joint military humanitarian mission in Tawi Tawi, a small island in the southern Philippines archipelago, physicians Elizabeth Satter and Sandra Tomita crossed paths with an otherwise healthy 10-year-old girl who had a rather odd-looking mass on her belly. On closer inspection, the authors observed a collection of dark hairs, bulbous protrusions, and a pair of "hyperpigmented mammilated plaques, reminiscent of vulva" that bordered "a small fistula that intermittently drained clear fluid with an electrolyte composition similar to urine." Surgeons were able to remove the parasite, and as of 2008, when the case was reported in the Journal of Pediatric Surgery, she was completely "cured" and living a happy, solitary life.
As a rule, the general prognosis for autosites—let's call them "children"—is favorable. Unless there are other congenital abnormalities, or the heart is burdened by an unusually aggressive parasite, the children in question tend to be perfectly normal. Psychologically speaking, there may be other effects—I'd imagine the whole affair would be quite traumatizing for all involved. But at least as far as I know, no one has investigated the mental well-being of those who once harbored parasitic twins or their immediate families. (I'm willing to go out on a twisted limb and say it's fairly unlikely that the characters in the 1982 cult horror movie Basket Case are realistic portrayals of autosite or parasite.)
In any case, the issue of excising parasites does make me wonder how a very religious person might regard all these multiple-birth anomalies. After all, your average parasite is entirely human, usually harmless to its sibling, and sometimes even has those precious little feet that pro-lifers are so fond of wearing on their lapels. Others have distinct faces, or they grow hair, or—as we've seen—they urinate. And post-op images of resected parasites can look like anti-abortion agitprop—children's arms and legs and abdominal viscera liberated crudely from the autosite's body and lying in surgeons' bins. (Is it wrong to describe the parasites as "children"? The whole is greater than the sum of its parts, I suppose ...)
Speaking of which, a former high-school classmate of mine is now raising three happy, healthy triplet girls, whom she believes God gave to her for being such a good Christian. She still lives not far from where we grew up together in Central Ohio, whose state motto, incidentally, is, "With God, all things are possible." Well, maybe not all things—21 malformed fetuses gestating inside a single baby's skull seems to be His limit for now. But there's actually some logic to that.
*Correction, Aug. 23, 2011: This article originally misspelled Mary-Kate and Ashley Olsen's surname.