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.

"'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.

Many factors determine the optimal way to deliver a baby. A vertex presentation is the ideal situation for a vaginal birth, however, occiput posterior positions tend to proceed more slowly, often requiring an intervention in the form of forceps, vacuum extraction, or Cesarean section. In a large study, a majority of brow presentations were delivered by Cesarean section, however, because of 'postmaturity', factors other than labour dynamics may have played a role. Most face presentations can be delivered vaginally as long as the chin is anterior; there is no increase in fetal or maternal mortality. Mento-posterior positions cannot be delivered vaginally in most cases (unless rotated) and are candidates for Cesarean section in contemporary management.

Coffin birth, also known as postmortem fetal extrusion, is the expulsion of a nonviable fetus through the vaginal opening of the decomposing body of a deceased pregnant woman as a result of the increasing pressure of intra-abdominal gases. This kind of postmortem delivery occurs very rarely during the decomposition of a body. The practice of chemical preservation, whereby chemical preservatives and disinfectant solutions are pumped into a body to replace natural body fluids (and the bacteria that reside therein), have made the occurrence of "coffin birth" so rare that the topic is rarely mentioned in international medical discourse.

Typically during the decomposition of a human body, naturally occurring bacteria in the organs of the abdominal cavity (such as the stomach and intestines) generate gases as by-products of metabolism, which causes the body to swell. In some cases, the confined pressure of the gases can squeeze the uterus (the womb), even forcing it downward, and it may and be forced out of the body through the vaginal opening (a process called "prolapse"). If a fetus is contained within the uterus, it could therefore be expelled from the mother's body through the vaginal opening when the uterus turns inside-out, in a process that, to outward appearances, mimics childbirth. The main differences lie in the state of the mother and fetus and the mechanism of delivery: in the event of natural, live childbirth, the mother's contractions encourage the infant to emerge from the womb; in a case of coffin birth, built-up gas pressure within the putrefied body of a pregnant woman pushes the dead fetus from the body of the mother.

Cases have been recorded by medical authorities since the 16th century, though some archaeological cases provide evidence for its occurrence in many periods of human history. While cases of postmortem fetal expulsion have always been rare, the phenomenon has been recorded under disparate circumstances and is occasionally seen in a modern forensic context when the body of a pregnant woman lies undisturbed and undiscovered for some time following death. There are also cases whereby a fetus may become separated from the body of the pregnant woman about the time of death or during decomposition, though because those cases are not consistent with the processes described here, they are not considered true cases of postmortem fetal extrusion.

The cause of postmortem fetal extrusion is not completely understood, as the event is neither predictable nor replicable under experimental conditions. Evidence has accumulated opportunistically and direct observation is serendipitous. While it is possible that more than one cause can produce the same result, there is an accepted hypothesis, based on established research in the fields of biochemistry and forensic taphonomy, and further supported by observational research, that accounts for the taphonomic mechanisms that would result in the most often encountered cases of postmortem extrusion of a non-viable fetus.

Typically, as a dead body decomposes, body tissues become depleted of oxygen and the body begins to putrefy; anaerobic bacteria in the gastrointestinal tract proliferate and as a result of increased metabolic activity, release gases such as carbon dioxide, methane, and hydrogen sulfide. These bacteria secrete exoenzymes to break down body cells and proteins for ingestion which thus weakens organ tissues. Increasing pressure forces the diffusion of excessive gases into the weakened tissues where they enter the circulatory system and spread to other parts of the body, causing both torso and limbs to become bloated. These decompositional processes weaken the structural integrity of organs by separating necrotizing tissue layers. Bloating usually begins from two to five days after death, depending on external temperature, humidity, and other environmental conditions. As the volume of gas increases, the pressure begins to force various body fluids to exude from all natural orifices. It is at this point during the decomposition of a pregnant body that amniotic membranes become stretched and separated, and intraabdominal gas pressure may force the and prolapse of the uterus, which would result in the expulsion of the fetus through the vaginal canal. It has been observed that the bodies of multiparous women are more likely to spontaneously expel the fetus during decomposition than those who died during their first pregnancy, because of the more elastic nature of the cervix.

Ganga and Jamuna Mondal (born Ayara and Jayara Ratun, 1969 or 1970), known professionally as The Spider Girls and The Spider Sisters, are conjoined twins from a Bengali family in Basirhat, West Bengal, India.

Ilona and Judit Gófitz (Hungarian: "Gófitz Ilona" és "Judit"; in contemporary publications: "Helen" and "Judith of Szőny"), also known as the Hungarian Sisters, were conjoined twins from Szőny, Hungary who lived from October 19, 1701 to February 8, 1723. In the image they appear to be pygopagus.

The twins are joined at the abdomen and pelvis, have four arms, and three legs, two of which are fused into a single, nine-toed limb, which remains concealed behind their body. They share a stomach, but have two hearts, two kidneys, one liver, and one reproductive tract.

The twins have been examined by doctors to determine whether or not they can be separated. However, they have no plans to do so because they feel it would be against God's will, carry too great a medical risk, and would rob them of their employment. They have stated ""We are happy as we are. The family will starve if we are separated.""

Ladan and Laleh Bijani (in Persian: ) (January 17, 1974 – July 8, 2003) were Iranian conjoined twin sisters. They were joined at the head and died immediately after their complicated surgical separation. Coincidentally, the twins were born 100 years to the day of the deaths of Chang and Eng Bunker, also conjoined twins, famously known as the "original" Siamese twins.

In France, Aymé, "et al." (1989) estimated the prevalence of Fryns syndrome to be 0.7 per 10,000 births based on the diagnosis of 6 cases in a series of 112,276 consecutive births (live births and perinatal deaths).

Environmental factors refer for example to maternal smoking and the maternal exposure to amine-containing drugs. Several research groups have found evidence that these environmental factors are responsible for an increase in the risk of craniosynostosis, likely through effects on fibroblast growth factor receptor genes.

On the other hand, a recent evaluation of valproic acid (an anti-epilepticum), which has been implicated as a causative agent, has shown no association with craniosynostosis.

Certain medication (like amine-containing drugs) can increase the risk of craniosynostosis when taken during pregnancy, these are so-called teratogenic factors.

The prevention of the complications mentioned above plays an important role in the discussion about the timing of the surgery. The general consensus is now to perform surgery in late infancy, i.e. between six and twelve months. In this time frame the efficacy of surgery is enhanced due to several reasons:

- The bone is still more malleable and can be remodelled relatively 'simply' by greenstick fractures of the bone. At approximately one year of age the bone has become more mineralized and brittle and needs to be fastened to the surrounding bone with sutures or an absorbable plate.

- Reshaping of the cranial vault most commonly means excision of the bones and adjustment of the shape. Replacement of the bones can leave 'gaps' which are readily re-ossified before the age of one year, but need bony filling thereafter.

The reason why most surgeons will not intervene until after the age of six months is the greater risk that blood loss poses before this age. If possible it is preferred to wait until after three months of age when the anaesthetic risks are decreased.

Surgery is not performed in early childhood in every country. In some countries surgical intervention can take place in the late teens.

It is important that families seek out a Pediatric Craniofacial Physician who has experience with craniosynostosis for proper diagnosis, surgical care, and followup.

Fryns syndrome is an autosomal recessive multiple congenital anomaly syndrome that is usually lethal in the neonatal period. Fryns (1987) reviewed the syndrome.

Stimmler syndrome is an autosomal recessive genetic disorder whose symptoms appear before birth or during infancy. In a study of two sisters born within a year of each other, both with Stimmler syndrome, it was found that high levels of alanine, pyruvate, and lactate were present in both the blood and urine. It was believed that the alanine was derived from the pyruvate.

The piriform (pear-shaped) morphology of the uterus has been given as the major cause for the finding that most singletons favor the cephalic presentation at term. The fundus is larger and thus a fetus will adapt its position so that the bulkier and more movable podalic pole makes use of it, while the head moves to the opposite site. Factors that influence this positioning include the gestational age (earlier in gestation breech presentations are more common as the head is relatively bigger), size of the head, malformations, amount of amniotic fluid, presence of multiple gestations, presence of tumors, and others.

Two-thirds of all vertex presentations are LOA, possibly because of the asymmetry created by the descending colon that is on the left side of the pelvis .

The Number of the Beast (, "Arithmos tou Thēriou") is a term in the Book of Revelation, of the New Testament, that is associated with the Beast of Revelation in chapter 13. In most manuscripts of the New Testament and in English translations of the Bible, the number of the beast is 666. Papyrus 115 (which is the oldest preserved manuscript of the "Revelation" ), as well as other ancient sources like "Codex Ephraemi Rescriptus", give the Number of the Beast as 616 (), not 666; critical editions of the Greek text, such as the Novum Testamentum Graece, note 616 as a variant.

The outcome of this disease is dependent on the severity of the cardiac defects. Approximately 1 in 3 children with this diagnosis require shunting for the hydrocephaly that is often a consequence. Some children require extra assistance or therapy for delayed psychomotor and speech development, including hypotonia.

Stimmler syndrome is a rare autosomal recessive congenital disorder first described by Stimmler et al. in 1970. It is characterized by dwarfism, diabetes, a small head, and high levels of alanine in the urine.

Because the cause of facial clefts still is unclear, it is difficult to say what may prevent children being born with facial clefts. It seems that folic acid contributes to lowering the risk of a child being born with a facial cleft.

Millie McCoy and Christine McCoy (July 11, 1851 – October 8, 1912) were American conjoined twins who went by the stage names "The Carolina Twins", "The Two-Headed Nightingale" and "The Eighth Wonder of the World". The Twins traveled throughout the world performing song and dance for entertainment.

3C syndrome is very rare, occurring in less than 1 birth per million. Because of consanguinity due to a founder effect, it is much more common in a remote First Nations village in Manitoba, where 1 in 9 people carries the recessive gene.

Chang (; ) and Eng (; ) Bunker (May 11, 1811 – January 17, 1874) were Thai-American conjoined twin brothers whose condition and birthplace became the basis for the term "Siamese twins".

There are several options for treatment of mouth anomalies like Tessier cleft number 2-3-7 . These clefts are also seen in various syndromes like Treacher Collins syndrome and hemifacial microsomia, which makes the treatment much more complicated. In this case, treatment of mouth anomalies is a part of the treatment of the syndrome.

Berdon syndrome, also called Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIH syndrome), is an autosomal recessive fatal genetic disorder affecting newborns. In a 2011 study of 227 children with the syndrome, "the oldest survivor [was] 24 years old." The Ann Arbor News reported a five year old survivor at the end of 2015.

It is more prevalent in females, 7 females to 3 males, and is characterized by constipation and urinary retention, microcolon, giant bladder (megacystis), intestinal hypoperistalis, hydronephrosis, and dilated small bowel. The pathological findings consist of an abundance of ganglion cells in both dilated and narrow areas of the intestine. It is a familial disturbance of unknown cause.

Walter Berdon "et al." in 1976 first described the condition in five female infants, two of whom were sisters. All had marked dilatation of the bladder and some had hydronephrosis and the external appearance of prune belly. The infants also had microcolon and dilated small intestines.

No treatment is available for most of these disorders. Mannose supplementation relieves the symptoms in PMI-CDG (CDG-Ib) for the most part, even though the hepatic fibrosis may persist. Fucose supplementation has had a partial effect on some SLC35C1-CDG (CDG-IIc or LAD-II) patients.