In a small village in India, not far from Mumbai, there is a young man upon whose head sits the malformed head of his own stone-faced twin. If one imagines, say, a woman balancing a jug of water perfectly upright on the top of her skull, this is the basic configuration of the man's gross anatomy—a cranial redundancy with both heads facing forward. "As the patient had grown up," observed physician Ritesh Kansal and his colleagues when the double-headed 20-year-old first stepped into their outpatient facility, "the additional head had [similarly] grown in size."
I've seen photos, and while the extra head is scarcely human in appearance, it does possess a fully developed nose, mouth, and ears. (Its eyes are closed.) In fact—and I'm loath to admit this, given the rampant superstition already surrounding the case—the second face bears a faint resemblance to a stereotypical smiling Buddha. The young man appears to have gotten the lion's share of the looks in the pair, but given the circumstances, even a handsome face isn't much consolation. Still, as the physicians reported in 2010, he refused to have an MRI to explore surgical removal of the extra head, not because he'd grown attached to it—ahem—or because he was afraid of disrupting the shared vasculature, but rather because of his religious convictions. (Without the benefit of imaging, it's unclear whether this particular extra head held any brain matter, as certain others do.)
Perhaps he's under the impression that God doesn't make mistakes. That may well be true; but if so, and as we're about to see, God has a rather vile sense of humor. What this young Indian man was exhibiting was a remarkable and extremely rare case of craniopagus parasiticus—so rare, in fact, that fewer than a dozen such cases have ever been recorded (the best-known example being Sir Everard Home's two-headed boy of Bengal, whose skull is on display at the Hunterian Museum in London). The unnerving craniopagus is a subtype of a broader developmental disorder known as parasitic, or "heteropagus," twinning (from the Greek heteros meaning "different," and pagus meaning "fixed"). From the perspective of human embryology, parasitic twinning is itself just one of many possible outcomes on the multiple-birth spectrum. Some such happenings produce Ashley and Mary-Kate Olsen*; a few others make for Cheng and Eng; and still fewer create small amorphous bodies that are hidden shamefully under people's shirts for their entire lives.
It's these unsung twins—the disarticulated, malformed, gelatinous or barnacle-like human beings—that are the real marvels of the multiple-birth world. (Actually, as we'll see, while they're certainly human, there's usually not much being to them at all.) Not only do these lesser, rarer couplings serve to highlight how naïve it is to boil down the world of twins to a simple distinction between fraternal and identical pairs, they also force us to re-examine our most commonsensical assumptions about, say, the "right to life."
Starting with the classical conjoined twins, most embryologists believe there is a natural progression of pathology. In a heteropagus pair, one severely defective individual (the "parasite") is born in an intimate, anatomical embrace with a healthy counterpart (the "autosite"). But believe it or not, it can get much, much worse. Next down the list is the fetus-in-fetu (in which a malformed fetus becomes encapsulated by a normal one, such that the parasite develops inside the autosite's body). Then there are the closely related teratomas (a chaotic, often cancerous, mix of mostly unidentifiable foreign tissues that grow inside the autosite). And the terrible acardiacs—malformed fetuses that come in their own amniotic sacs. Being born without a heart (as their name suggests) is among the least of these latter infants' physical problems. Some unfortunate women have given birth to one healthy twin and an acardiac sibling who consists of only a pair of legs, a head, or a torso.
" Would that these definitions were inviolate!" exclaims physician Rowena Spencer in her review of such cases in the journal Clinical Anatomy. What she's referring to, if somewhat exuberantly, is the fact that mothers may even give birth to multiple forms of these anomalies in a single delivery—a sort of mix-and-match of human nonstarters from a single conception. See, for example, the striking case of a stillborn Czech boy who was found to have 21 ill-formed embryos in his skull, ensconced in a brain teratoma.
There's also some confusion over the origins of the exoparasitic twins—the ones who materialize on the surface of their siblings' skin as extraneous body parts. Embryologists don't have much data to work with, as such cases occur just once in every million live births. But most believe they come about in more or less the same way as regular conjoined twins. (The incidence of those is closer to 1 in 200,000.) According to the most popular theory, a single embryoblast—a mass of cells that later give rise to the differentiated structures of the fetus—spontaneously divides about two weeks after fertilization, resulting in two connected centers of growth. That's exactly how identical twins form in the womb, except in that case the fission occurs a few days earlier. When an embryoblast splits at around Day 9, the two halves tend to form into discrete, functional units—like the Weasley brothers in Harry Potter. But if the process gets held up until Days 13 or 14, then you may end up with a " double monster." (That was a technical term, by the way, as of 1961.)
So conjoined and parasitic twins may represent two points on a spectrum of identical twinning gone wrong. At one end are the symmetrical siblings, like Krista and Tatiana Hogan—the Canadian toddlers whose bodies are fused at the brain. At the other end, one sibling has deteriorated in the womb, leaving little more than a couple of feet, a thorax with nipples, or a stray set of genitals. More developed parasites may have some viscera and bones, even whole organs, but only rarely is there any semblance of a heart or brain.
If parasitic twins are conjoined twins that happen to be very asymmetrical, then you'd expect to find certain similarities between these categories. Indeed, Spencer notes that both types tend to be connected at one of eight standard locations. Thus we have the craniopagus twins and parasites attached at the cranial vault, like Krista and Tatiana, or the Indian man we met earlier; the omphalopagus or thoracopagus variants, who stem from the thoracic region of their siblings, essentially growing out of the chest; the cephalopagus twins, connected at the mouth, ears or nose; the ischiopagus, originating in the lower abdomen; the pygopagus, joined at the sacrum, perhaps with a superfluous arm or leg that looks like a tail; the rachipagus, unfolding from the vertebral column; and parapagus, popping out from somewhere on the pelvis.
There are a few cases, however, where the model seems to falter. If identical twins, conjoined twins, and parasitic twins all start the same way—with the splitting of a single fertilized egg—then the pairs of each kind should have a shared genetic profile. When researchers have performed this analysis on parasites and autosites, the answer almost always comes up as expected: Their DNA is the same. But there are some outliers. In Santo Domingo, for example, a parasite and its host were found to have an "allelic discordance" suggesting they were a rare type of fraternal twins. There's also the case of a little girl in New Delhi who arrived in the world with an unexpected visitor attached to her back, a "lumbosacral parasitic rachipagus twin." According to Rajiv Chadha and his team from the Lady Hardinge Medical College,
The mass had a bony-hard feel underneath the soft tissues. Three globular structures resembling aborted digits were visible, along with a small mass covered with skin having the pigmentation and rugosity of the scrotum.
If you missed the important point of (on?) this strange mass on the baby's back, it's all in the scrotum. Again, the theory holds that both conjoined twins and parasitic twins are monozygotic, which means, of course, of the same biological sex. So a parasitic scrotum attached to a female autosite rattles the conventional wisdom.
Some scientists believe these flukes result from a different process in the womb. Instead of one embryoblast splitting late and dividing just part of the way, it could be that two embryoblasts come together unexpectedly. (In theory, this merging of fraternal twins could produce any of the abnormalities reviewed earlier.) The New Delhi physicians didn't have the resources to conduct genetic testing on their ostensibly mixed-gender pair, so it's certainly possible that the parasite's "scrotum" was in fact something else entirely. Still, the "fission vs. fusion" battle rages on in the world of abnormal embryology.
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.