A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed. In some instances, the dead twin will be compressed into a flattened, parchment-like state known as "fetus papyraceus".

Vanishing twins occur in up to one out of every eight multifetus pregnancies and may not even be known in most cases. "High resorption rates, which cannot be explained on the basis of the expected abortion rate...suggest intense fetal competition for space, nutrition, or other factors during early gestation, with frequent loss or resorption of the other twin(s)."

In pregnancies achieved by IVF, "it frequently happens that more than one amniotic sac can be seen in early pregnancy, whereas a few weeks later there is only one to be seen and the other has 'vanished'."

The common clinical features are smaller symphysis fundal height, fetal malpresentation, undue prominence of fetal parts and reduced amount of amniotic fluid.

Complications may include cord compression, musculoskeletal abnormalities such as facial distortion and clubfoot, pulmonary hypoplasia and intrauterine growth restriction. Amnion nodosum is frequently also present (nodules on the fetal surface of the amnion).

The use of oligohydramnios as a predictor of gestational complications is controversial.

Potter syndrome is a condition caused by oligohydramnios. Affected fetuses develop pulmonary hypoplasia, limb deformities, and characteristic facies. Bilateral agenesis of the fetal kidneys is the most common cause due to the lack of fetal urine.

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.

In medicine (obstetrics), the term fetal distress refers to the presence of signs in a pregnant woman—before or during childbirth—that suggest that the fetus may not be well. Because of its lack of precision, the term is eschewed in modern American obstetrics.

Confined placental mosaicism (CPM) represents a discrepancy between the chromosomal makeup of the cells in the placenta and the cells in the baby. CPM was first described by Kalousek and Dill in 1983. CPM is diagnosed when some trisomic cells are detected on chorionic villus sampling and only normal cells are found on a subsequent prenatal test, such as amniocentesis or fetal blood sampling. In theory, CPM is when the trisomic cells are found only in the placenta. CPM is detected in approximately 1-2% of ongoing pregnancies that are studied by chorionic villus sampling (CVS) at 10 to 12 weeks of pregnancy. Chorionic villus sampling is a prenatal procedure which involves a placental biopsy. Most commonly when CPM is found it represents a trisomic cell line in the placenta and a normal diploid chromosome complement in the baby. However, the fetus is involved in about 10% of cases.

Generally it is preferable to describe specific signs in lieu of declaring "fetal distress" that include:

- Decreased movement felt by the mother

- Meconium in the amniotic fluid ("meconium stained fluid")

- Non-reassuring patterns seen on cardiotocography:

- increased or decreased fetal heart rate (tachycardia and bradycardia), especially during and after a contraction

- decreased variability in the fetal heart rate

- late decelerations

- Biochemical signs, assessed by collecting a small sample of baby's blood from a scalp prick through the open cervix in labor

- fetal metabolic acidosis

- elevated fetal blood lactate levels (from fetal scalp blood testing) indicating the baby has a lactic acidosis

Some of these signs are more reliable predictors of fetal compromise than others. For example, cardiotocography can give high false positive rates, even when interpreted by highly experienced medical personnel. Metabolic acidosis is a more reliable predictor, but is not always available.

Circumvallate placenta is a placental morphological abnormalitiy, a subtype of placenta extrachorialis in which the fetal membranes (chorion and amnion) "double back" on the fetal side around the edge of the placenta. After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface.

The fetal surface is divided into a central depressed zone surrounded by a thickened white ring which is incomplete the ring is situated at varying distance from the margin of the placenta. The ring is composed of a double fold of amnion and chorion with degenerated decidua vera and fibrin in between. Vessels radiate from the cord insertion as far as the ring and then disappear from the view.

Complete circumvallate placenta occurs in approximately 1% of pregnancies. It is diagnosed prenatally by medical ultrasonography, although one 1997 study of prenatal ultrasounds found that "of the normal placentas, 35% were graded as probably or definitely circumvallate by at least one sonologist," and "all sonologists misgraded the case of complete circumvallation as normal." The condition is associated with perinatal complications such as placental abruption, oligohydramnios, abnormal cardiotocography, preterm birth, and miscarriage.

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.

Hydrops fetalis is a condition in the fetus characterized by an accumulation of fluid, or edema, in at least two fetal compartments. By comparison, hydrops allantois or hydrops amnion is an accumulation of excessive fluid in the allantoic or amniotic space, respectively.

Locations can include:

- subcutaneous tissue/scalp

- pleura (pleural effusion)

- pericardium (pericardial effusion)

- abdomen (ascites)

The edema is usually seen in the fetal subcutaneous tissue, sometimes leading to spontaneous abortion. It is a prenatal form of heart failure, in which the heart is unable to satisfy its demand for a high amount of blood flow.

The differential diagnosis includes;

- Symbrachydactyly

- Chorionic villus sampling

- Congenital amputations

- Hypoplasias of hand, digit, thumb

- Adams-Oliver syndrome

- ADAM complex

ADAM Complex; CRS is sometimes mislabeled as ADAM complex. ADAM is an abbreviation for Amniotic Deformity, Adhesions Mutilations. CRS is the malformation due to a constriction ring around mostly a limb. ADAM-complex is the association of limb defects (caused by constriction rings) and certain craniofacial clefts

“Adams-Oliver syndrome is often mislabeled as CRS and consists of cutis aplasia of the scalp in which a longitudinal defect can vary in size and can often be associated with full-thickness skullcap loss. The distal digital or toe hypoplasia-aplasia is often confused with CRS. Constriction rings with or without edema are not present. The digital or toe hypoplasia-aplasia usually contains diminutive nails or nail folds”.

Mirror syndrome or triple oedema or Ballantyne syndrome is a rare disorder affecting pregnant women. It describes the unusual association of fetal

and placental hydrops with maternal preeclampsia.

The name "mirror syndrome" refers to the similarity between maternal oedema and fetal hydrops. It was first described in 1892 by John William Ballantyne.

In most cases Ballantyne syndrome causes fetal or neonatal death and in contrast, maternal involvement is limited at the most to preeclampsia.

The constriction ring syndrome is a complex collection of asymmetric congenital anomalies, in which no two cases are exactly alike. This is why a classification is difficult to make.

The most widely used classification system was proposed by Patterson. This classification system is based on the severity of the syndrome and is useful because, the different types require different treatments. Other clinicians have amended this scheme by separating the depth of the ring into mild, moderate, severe and amputation and by further defining the presence or absence of lymphedema or soft tissue loss distal to the ring. Expanding over subdivision in depth of the clefts for every classification is not necessary because the principles of treatment and technique for correction are the same

There are four categories:

1. simply constriction rings

2. constriction rings associated with deformity of the distal part with or without lymphedema

3. constriction rings associated with acrosyndactyly

4. uterine amputation

Patterson divided the constriction ring associated with acrosyndactyly into three types:

- Type I: conjoined fingertips with well-formed webs of the proper depth

- Type II: the tips of the digits are joined, but web formation is not complete

- Type III: joined tips, sinus tracts between digits and absent webs

Patterson I;

There are simple constriction rings which are strands most commonly around the distal extremities as fingers and toes. In general, the thumb is not likely to be affected by a constriction ring because the fetus typically holds the thumb in tight adduction flexion, making entanglement with strands less likely. These malformations need to be surgically removed which must be executed in different stages and can done by different techniques (see also treatment).

Patterson II;

The CRS involves strands which obstruct the lymphatic vessels and thus causing fluid retention, distal of the affected extremity. This utters itself with swollen parts distal of the constriction.

Patterson III;

In this form there is a complex form of syndactyly named acrosyndactyly, the fingers (or toes) were initially separated but due to the constriction they are formed back together. Sometimes multiple fingers can be involved. The distal fusion between digits or toes never initially involves a skeletal coalition. The digits are usually hypoplastic if multiple digits are involved. When the constriction cuts of the blood supply to the fingers, the fingers can form a peak with the most palmar digit being the index finger. Normal neurovascular bundles are not present in the distal parts. Hands with fused fingers need to be released in phases to preserve the distal blood supply.

Paterson IV;

One of the most severe consequence of constriction strains is probably intrauterine amputations, this is where the constriction goes as deep as the bone and cuts of the blood supply of the proximal extremity. The result will be that the developing toe or finger will become ischemic and will fall of. Because the end result is a transverse amputation that cuts off the vascular supply to the developing extremity, the actual constriction ring is not seen

This can result in different outcomes;

- complete resorption; this is the most common form of amputation. In this form the amputated digit is completely resorbed during intrauterine development and therefore the amputated digit can’t be found.

- recovery of the digit; the digit is recovered with the placenta during delivery.

- engraftment elsewhere; the amputated digit can be, in rare cases, engrafted somewhere on the fetus.

Intrauterine death;

In extremely rare cases a strain can form around the umbilical cord and cut off the blood supply to the fetus which will result in intrauterine death.

Malformation associated with constriction ring syndrome;

The percentage of associated anomalies varies from 40% to as high as 80%

Constriction ring deformities are as common on the lower extremity as on the upper, almost all of these involve the musculoskeletal system, with clubbed feet being the most common in up to 30% of reported cases

Large reported series reveal an incidence between 5% and 15% of craniofacial malformations with clefting of the lip or palate.

Fetal trimethadione syndrome (also known as paramethadione syndrome, German syndrome, tridione syndrome, among others) is a set of birth defects caused by the administration of the anticonvulsants trimethadione (also known as Tridione) or paramethadione to epileptic mothers during pregnancy.

Fetal trimethadione syndrome is classified as a rare disease by the National Institute of Health's Office of Rare Diseases, meaning it affects less than 200,000 individuals in the United States.

The fetal loss rate while using trimethadione has been reported to be as high as 87%.

Fetal trimethadione syndrome is characterized by the following major symptoms as a result of the teratogenic characteristics of trimethadione.

- Cranial and facial abnormalities which include; microcephaly, midfacial flattening, V-shaped eyebrows and a short nose

- Cardiovascular abnormalities

- Absent kidney and ureter

- Meningocele, a birth defect of the spine

- Omphalocele, a birth defect where portions of the abdominal contents project into the umbilical cord

- A in mental and physical development

MRLS was initially characterized by four syndromes: (1) EFLs, (2) LFLs and the nonreproductive syndromes, (3) unilateral uveitis, (4) pericarditis, and later (5) "Actinobacillus" encephalitis. MRLS was observed in mares of all breeds and ages. Early and late fetal losses were observed within the first and last trimesters of pregnancy, respectively. For EFLs, clinical signs from the sick mares included pus-like discharge from the vulva and fetal membranes protruding from the vulva, as the fetuses were located in either the vagina or vulva. One to three days prior to the EFLs, several mares showed mild colic symptoms, abdominal straining, or low-grade fever. Within a week of abortion, inspections indicated moderate to severe inflammation within the uterine region. Performing ultrasounds revealed either dead fetuses or live fetuses with slow heart rates and lethargic movements. All the fetuses, both alive and dead, were surrounded by cloudy amniotic fluid.

Clinical signs for LFLs included explosive parturition, dystocia, foaling while standing, premature placenta separation, and foals either stillborn or born weak. Placentas were observed to carry a pale brown hue as opposed to their usual dark reddish-brown color. The umbilical cords were thick, dull, yellowish, and inflamed. The weak foals were often incapable of breathing on their own and required resuscitation. These foals were also observed to be dehydrated and hypothermic, with irregular heartbeat and respiration. The majority of these foals did not survive past four days.

One finding observed exclusively in MRLS was unilateral uveitis; initially, affected horses expressed inflammation around a single eye, along with fluid accumulation on the corneas, anterior and posterior chambers due to the inflammation. The fluid in the anterior chambers exhibited tan to yellow hues, and often was accompanied by hemorrhaging around the surface of the iris.

Children with amyoplasia often suffer from internally rotated shoulders, extended elbows, ulnar flexed wrists. The type of displacement of the hips and knees is more variable, and they often have club feet. Most children have symmetrical limb involvement.

About 10% of children with amyoplasia have evidence of vascular compromise including Intestinal atresia, abdominal wall defects, and gastroschisis.

Umbilical cord prolapse occurs when the umbilical cord comes out of the uterus with or before the presenting part of the fetus. It is a relatively rare condition and occurs in fewer than 1% of pregnancies. Cord prolapse is more common in women who have had rupture of their amniotic sac. Other risk factors include maternal or fetal factors that prevent the fetus from occupying a normal position in the maternal pelvis, such as abnormal fetal lie, too much amniotic fluid, or a premature or small fetus. The concern with cord prolapse is that pressure on the cord from the fetus will cause cord compression that compromises blood flow to the fetus. Whenever there is a sudden decrease in fetal heart rate or abnormal fetal heart tracing, umbilical cord prolapse should be considered. Due to the possibility for fetal death and other complications, umbilical cord prolapse is considered an obstetric emergency during pregnancy or labor. Current management guidelines focus on quick delivery, which usually entails a cesarean section. With appropriate management, the majority of cases have good neonatal outcomes.

There are three types of confined placental mosaicism depending on the cells involved at the time of the error:

- "Type 1 CPM"– The error occurs in a trophoblast cell, and thus only trophoblast cells are affected. This type of mosaicism is most often associated with normal pregnancy outcome.

- "Type 2 CPM"– The error occurs in a non-fetal cell of the inner cell mass. This trisomy is confined to the chorionic villus stroma. This type of mosaicism is described in normal pregnancies and is sometimes associated with delayed growth of the fetus.

- "Type 3 CPM"– Trisomic cells are seen in trophoblast cells and the villus stroma, but are absent in the embryo. This type of mosaicism is more commonly associated with delayed growth in the fetus.

Perinatal mortality (PNM), also perinatal death, refers to the death of a fetus or neonate and is the basis to calculate the perinatal mortality rate. Variations in the precise definition of the perinatal mortality exist specifically concerning the issue of inclusion or exclusion of early fetal and late neonatal fatalities. The World Health Organization defines perinatal mortality as the "number of stillbirths and deaths in the first week of life per 1,000 total births, the perinatal period commences at 22 completed weeks (154 days) of gestation and ends seven completed days after birth", but other definitions have been used.

The UK figure is about 8 per 1,000 and varies markedly by social class with the highest rates seen in Asian women. Globally about 2.6 million neonates died in 2013 before the first month of age down from 4.5 million in 1990.

The first sign of umbilical cord prolapse is usually a sudden decrease in fetal heart rate that is severe and does not immediately resolve. On a fetal heart tracing, this would usually look like moderate to severe variable decelerations. Occasionally, the cord can be seen or felt on vaginal examination, particularly with overt cord prolapse.

It is the most common form of arthrogryposis multiplex congenita (AMC), where multiple joint contractures are present at birth. Arthrogryposis is derived from the Greek word meaning "with crooking of joints." It occurs in about one out of every 3,000 live births. There are more than 150 types of AMC. Amyoplasia accounts for 40% of AMC cases.

Microchimerism is the presence of a small number of cells that originate from another individual and are therefore genetically distinct from the cells of the host individual. This phenomenon may be related to certain types of autoimmune diseases; however, the mechanisms responsible for this relationship are unclear.