With so few individuals actually surviving until birth, the only treatment option is surgery to try to remove the parasitic twin. Surgery, however, is very dangerous and has been successful only once. The problem with surgical intervention is that the arterial supplies of the head are so intertwined that it is very hard to control the bleeding, and it has been suggested that cutting off the parasitic twin's arterial supply might improve the odds of the developed twin's survival.

Two-headed people and animals, though rare, have long been known to exist and documented.

In humans, as in other animals, partial twinning can result in formation of two heads supported by a single torso. Two ways this can happen are dicephalus parapagus, where there are two heads side by side, and craniopagus parasiticus, where the heads are joined directly.

Only ten cases of craniopagus parasiticus have been reported in the medical research literature. Of those cases, only three have survived birth. The first case on record is that of Everard Home's Two-Headed Boy of Bengal, whose skull is preserved at the Hunterian Museum at the Royal Society of Surgeons.

Diprosopus (Greek , "two-faced", from , ', "two" and , ' [neuter], "face", "person"; with Latin ending), also known as craniofacial duplication (cranio- from Greek , "skull", the other parts Latin), is an extremely rare congenital disorder whereby parts (accessories) or all of the face are duplicated on the head.

Most human infants with diprosopus are stillborn. Known instances of humans with diprosopus surviving for longer than minutes to hours past birth are very rare; only a few are recorded. In 2002 and 2003, two living male infants with partial diprosopus were described in the medical literature in separate case reports. One infant was born with duplication of the nose and the cerebral frontal lobes, two widely spaced eyes, a small, underdeveloped central eye socket, and a large, asymmetric mouth. The other infant was born with duplication of the upper and lower jaw, two tongues ending in the same base, cleft palate, a slightly divided tip of the nose, and two widely spaced eyes, as well as absence of the corpus callosum, duplication of the pituitary gland and stalk, and abnormalities in the midbrain. Because they were born with a milder, partial form of diprosopus, both infants were considered candidates for surgical correction of their abnormal facial features.

"'Conjoined twins" are identical twins joined in utero. An extremely rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence in Southeast Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two contradicting theories exist to explain the origins of conjoined twins. The more generally accepted theory is "fission", in which the fertilized egg splits partially. The other theory, no longer believed to be the basis of conjoined twinning, is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins who also share these structures in utero.

The most famous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1811–1874), Thai brothers born in Siam, now Thailand. They traveled with P.T. Barnum's circus for many years and were labeled as the Siamese twins. Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. In modern times, they could have been easily separated. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be used as a synonym for conjoined twins.

If left untreated, the pump twin will die in 50–75% of cases.

After diagnosis, ultrasound and amniocentesis are used to rule out genetic abnormalities in the pump twin. A procedure may then be performed which will stop the abnormal blood flow. The acardiac twin may be selectively removed. The umbilical cord of the acardiac twin may be surgically cut, separating it from the pump twin, a procedure called fetoscopic cord occlusion. Or a radio-frequency ablation needle may be used to coagulate the blood in the acardiac twin's umbilical cord. This last procedure is the least invasive. These procedures greatly increase the survival chances of the pump twin, to about 80%.

The pump twin will be monitored for signs of heart failure with echocardiograms. If the pump twin's condition deteriorates, the obstetrician may recommend early delivery. Otherwise, the pregnancy continues normally. Vaginal birth is possible unless the fetus is in distress, although it is recommended that the delivery take place at a hospital with NICU capabilities.

Craniopagus twins are conjoined twins that are fused at the cranium. This condition occurs in about 10–20 babies in every million births in the United States. Among this small group, cephalic conjoining, or craniopagus twinning, represents the rarest of congenital abnormalities, accounting for 2–6% of all conjoined twins. Additionally, conjoined twins are genetically identical and always share the same sex. The union in craniopagus twins may occur on any portion of the Calvary, but does not include either the face or the foramen magnum. The thorax and abdomen are separate and each twin has its own umbilicus and umbilical cord. The union may involve the entire diameter of the head or only a small portion. This suggests that although there are many different kinds of vulnerabilities already known in the scientific community, there are an infinite number of variations that can occur. Most of these variations are based on the rotation of one twin's skull to the other and the different phenotype sub-groups of craniopagus twins are based on all these rotational conformations. Each of these factors (rotation, spot of union) affects the development of the brain, the vascular system within the brain and overall wellness of life both of the twins have outside the womb. Relatively few craniopagus twins survive the perinatal period – approximately 40% of conjoined twins are stillborn and an additional 33% die within the immediate perinatal period, usually from organ abnormalities and failure. However 25% of craniopagus twins survive and can be considered for a surgical separation and several attempts occur yearly worldwide. In the last-half century, many advances in medicine including brain imaging, neuro-anesthesia and neurosurgical techniques have proven that a successful outcome is possible following separation of total craniopagus twins.

Twin reversed arterial perfusion sequence—also called TRAP sequence, TRAPS, or acardiac twinning—is a rare complication of monochorionic twin pregnancies. It is a severe variant of twin-to-twin transfusion syndrome (TTTS). The twins' blood systems are connected instead of independent. One twin, called the "acardiac twin" or "TRAP fetus", is severely malformed. The heart is missing or deformed, hence the name acardiac, as are the upper structures of the body . The legs may be partially present or missing, and internal structures of the torso are often poorly formed. The other twin is usually normal in appearance. The normal twin, called the "pump twin", drives blood through both fetuses. It is called "reversed arterial perfusion" because in the acardiac twin the blood flows in a reversed direction.

TRAP sequence occurs in 1% of monochorionic twin pregnancies and in 1 in 35,000 pregnancies overall.

Surgery to separate conjoined twins may range from very easy to very difficult depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. Alice Dreger of Northwestern University found the quality of life of twins who remain conjoined to be higher than is commonly supposed. Lori and George Schappell and Abby and Brittany Hensel are notable examples.

In 1955, neurosurgeon Harold Voris (1902-1980) and his team at Mercy Hospital in Chicago performed the first successful operation to separate Siamese twins conjoined (Craniopagus twins) at the head, which resulted in long-term survival for both. The larger girl was reported in 1963 as developing normally, but the smaller was permanently impaired.

In 1957, Bertram Katz and his surgical team made international medical history performing the world's first successful separation of conjoined twins sharing a vital organ. Omphalopagus twins John Nelson and James Edward Freeman (Johnny and Jimmy) were born to Mr. and Mrs. William Freeman of Youngstown, Ohio, on April 27, 1956. The boys shared a liver but had separate hearts and were successfully separated at North Side Hospital in Youngstown, Ohio by Bertram Katz. The operation was funded by the Ohio Crippled Children's Service Society.

Recent successful separations of conjoined twins include that of the separation of Ganga & Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal, in 2000. The 197-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons Chumpon Chan and Keith Goh. The surgery left Ganga with brain damage and Jamuna unable to walk. Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in July 2009, at the age of 8, three days after being admitted for treatment of a severe chest infection.

A case of particular interest was that of infants Rose and Grace ("Mary" and "Jodie") Attard, conjoined twins from Malta who were separated in Great Britain by court order over the religious objections of their parents, Michaelangelo and Rina Attard. The surgery took place in November, 2000, at St Mary's Hospital in Manchester. The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Grace's. (The twins were attached at the lower abdomen and spine.) However, if the operation had not taken place, it was certain that both twins would die. Grace survived to enjoy a normal childhood.

In 2003 two 29-year-old women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains (craniopagus) were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Their complex case was accepted only because high tech graphical imagery and modelling would allow the medical team to plan the risky surgery. Unfortunately, an undetected major vein hidden from the scans was discovered during the operation. The separation was completed but both women died while still in surgery on July 8, 2003.

Hydatidiform moles should be treated by evacuating the uterus by uterine suction or by surgical curettage as soon as possible after diagnosis, in order to avoid the risks of choriocarcinoma. Patients are followed up until their serum human chorionic gonadotrophin (hCG) level has fallen to an undetectable level. Invasive or metastatic moles (cancer) may require chemotherapy and often respond well to methotrexate. As they contain paternal antigens, the response to treatment is nearly 100%. Patients are advised not to conceive for half a year after hCG levels have normalized. The chances of having another molar pregnancy are approximately 1%.

Management is more complicated when the mole occurs together with one or more normal fetuses.

The uterine curettage is generally done under the effect of anesthesia, preferably spinal anesthesia in hemodynamically stable patients. The advantages of spinal anesthesia over general anesthesia include ease of technique, favorable effects on the pulmonary system, safety in patients with hyperthyroidism and non-tocolytic pharmacological properties. Additionally, by maintaining patient’s consciousness one can diagnose the complications like uterine perforation, cardiopulmonary distress and thyroid storm at an earlier stage than when the patient is sedated or is under general anesthesia.

The exact nature of how conjoined twins develop inutero remains unclear. Embryologists have traditionally attributed identical twinning as "splitting or fission" of either the inner cell mass of pleuripotential cells or early embryonic disc at 13–14 days of gestation just before the primitive streak. Some theorists suggested that conjoined twins develop as a result of the failed fusion of a single fertilized ovum. However a new hypothesis suggests that cranial fusion occurs between two separate embryos prior to the end of the 4th week of gestation. This is able to happen because the cranial neuropore is still open which is responsible for the ultimate fusion and formation of the brain stem and central nervous system. Furthermore, this secondary fusion of embryonic discs could implicate that intact skin will not fuse to other intact skin, including the ectoderm of the embryo. This means that two embryonic discs could only unite in locations where the ectoderm is absent. Moreover, the fusion occurs from neural folds of two separate, dorsally oriented embryonic discs, and the union can occur only after the ectoderm is disrupted to allow the neural and surface ectodermal layers to separate from each other.

The union in craniopagus twins may happen at any portion of the calvarium. The juncture can involve either the entire diameter of the head or any portion of the head and can be positioned at a multitude of rotational angles. In fact, craniopagus twins are rarely found in a symmetrical union. Apart from this, the vertebral axes may have a straight line. Despite this, the angle of the vertebrae is the ultimate dictator in how the individuals heads actually face. The majority of twins face either the same way or the exact opposite direction.

Many reviews suggest a practical four-category system that breaks down the craniopagus twins on the basis of vertical or angular configuration or on the basis if there were shared dural venous sinuses. This scheme was applied to 64 cases and has adequately described sets of twins for over the last 86 years.

For malignant teratomas, usually, surgery is followed by chemotherapy.

Teratomas that are in surgically inaccessible locations, or are very complex, or are likely to be malignant (due to late discovery and/or treatment) sometimes are treated first with chemotherapy.

Embryonal teratomas most commonly occur in the sacrococcygeal region: sacrococcygeal teratoma is the single most common tumor found in newly born humans.

Of teratomas on the skull sutures, approximately 50% are found in or adjacent to the orbit. Limbal dermoid is a choristoma, not a teratoma.

Teratoma qualifies as a rare disease, but is not extremely rare. Sacrococcygeal teratoma alone is diagnosed at birth in one out of 40,000 humans. Given the current human population and birth-rate, this equals five per day or 1800 per year. Add to that number sacrococcygeal teratomas diagnosed later in life, and teratomas in other locales, and the incidence approaches ten thousand new diagnoses of teratoma per year.