"'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.

Embryo splitting in which zygote divide asexually,

to produce identical children, is blocked by mitosis inhibitor.

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.

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.

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.

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.

Biomechanical factors include fetal head constraint during pregnancy. It has been found by Jacob et al. that constraint inside the womb is associated with decreased expression of Indian Hedgehog protein and noggin. These last two are both important factors influencing bone development.

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.

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.

In a newborn boy thought to have Fryns syndrome, Clark and Fenner-Gonzales (1989) found mosaicism for a tandem duplication of 1q24-q31.2. They suggested that the gene for this disorder is located in that region. However, de Jong et al. (1989), Krassikoff and Sekhon (1990), and Dean et al. (1991) found possible Fryns syndrome associated with anomalies of chromosome 15, chromosome 6, chromosome 8(human)and chromosome 22, respectively. Thus, these cases may all represent mimics of the mendelian syndrome and have no significance as to the location of the gene for the recessive disorder.

By array CGH, Slavotinek et al. (2005) screened patients with DIH and additional phenotypic anomalies consistent with Fryns syndrome for cryptic chromosomal aberrations. They identified submicroscopic chromosome deletions in 3 probands who had previously been diagnosed with Fryns syndrome and had normal karyotyping with G-banded chromosome analysis. Two female infants were found to have microdeletions involving 15q26.2 (see 142340), and 1 male infant had a deletion in band 8p23.1 (see 222400).

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 .

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).

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.

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.

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.

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.

Symptoms for the disease include microcephaly, a low birth weight, dwarfism, small teeth, and diabetes. The symptoms of Stimmler syndrome are closely related to a disease studied by Haworth et al. in 1967 as well as Leigh subacute necrotizing encephalopathy with lactic acidosis

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, also known as CCC dysplasia, Craniocerebellocardiac dysplasia or Ritscher–Schinzel syndrome, is a rare condition, whose symptoms include heart defects, cerebellar hypoplasia, and cranial dysmorphism. It was first described in the medical literature in 1987 by Ritscher and Schinzel, for whom the disorder is sometimes named.

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.

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".

The estimated incidence of Wiskott–Aldrich syndrome in the United States is one in 250,000 live male births. No geographical factor is present.

Jalili syndrome is a genetic disorder characterized by the combination of cone-rod dystrophy of the retina and amelogenesis imperfecta. It was characterized in 1988 by Dr. I. K. Jalili and Dr. N. J. D. Smith, following the examination of 29 members of an inbred, Arab family living within the Gaza Strip.