A parasitic twin is very similar to a conjoined twin; it is essentially a conjoined twin that exists solely inside the main child's body.
I can't decide if Peruvian toddler Isbac Pacunda is a lucky kid, or an unlucky one. Let's consider him lucky: doctors discovered his parasitic twin while he was still young, and the surgery is set for tomorrow.
Pacunda's parasitic twin is situated in his stomach. It weighs about a pound and a half, is nine inches long, and has "some hair on the cranium, eyes, and some bones." A parasitic twin is very similar to a conjoined twin; it is essentially a conjoined twin that exists solely inside the main child's body.
Normally, a pair of identical twins is created when the single fertilized embryo splits into two separate halves. Each half develops into a whole, separate fetus. In the case of conjoined and parasitic twins, the embryo does not divide cleanly. With a parasitic twin, the larger portion of the embryo will envelop the smaller portion. The smaller portion becomes almost vestigial, typically just a few body parts, and is wholly dependent on the main twin for all blood flow and nourishment.
Parasitic twins are said to happen about once in every 500,000 births. In many cases, the only result is an unnerving pocket of developed tissue somewhere inside the main fetus's body. These frequently cause no problems, and may only be discovered at some much later date, during an unrelated surgery. Pockets of hair, teeth, and even the occasional developed eyeball are what is usually found inside these strange cysts.
Parasitic Twins Vs. Fetiform Teratomas
In 2010, a child in Trinidad was diagnosed as having a basketball-sized mass in his chest cavity. When doctors performed exploratory surgery they discovered "hair and teeth and what looked like eyes of an underdeveloped fetus." This was tentatively identified as a teratoma, which is not quite the same thing as a parasitic twin.
A teratoma is an encapsulated cyst, inside which strange structures (including eyes and hair) may be growing. The leading theory is that they occur very early in the fetus's development, when stem and mast cells still have the potential to become anything. For whatever reason, a cluster of cells in your chest cavity may decide to become hair.
There still exists some discussion regarding the boundary line between a fetiform teratoma and a parasitic twin. The general feeling is that a parasitic twin is one which has developed complex structures, like limbs and a rudimentary circulatory system. Whereas a fetiform teratoma is just a collection of random bits of things, none of which are remotely fetus-shaped.