types of stillbirth, including intrauterine death and intrapartum death. intrauterine death occurs when a baby dies during late pregnancy. intrapartum death, which is more common, occurs when a baby dies while the mother is giving birth. the cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time. heterotopic pregnancy heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy. ectopic pregnancies must be resolved because they can be life-threatening to the mother. however, in most cases, the intrauterine pregnancy can be salvaged. management of birth for otherwise healthy twin pregnancies where both twins are head down a trial of vaginal delivery is recommended at between 37 and 38 weeks. vaginal delivery in this case does not worsen the outcome for the infant as compared with caesarean section. there is controversy on the best method of delivery where the first twin is head first and the second is not. when the first twin is not head down a caesarean section is often recommended. it is estimated that 75% of twin pregnancies in the united states were delivered by caesarean section in 2008. in comparison, the rate of caesarean section for all pregnancies in the general population varies between 14% and 40%. in twins that share the same placenta, delivery may be considered at 36 weeks. for twins who are born early, there is insufficient evidence for or against placing preterm stable twins in the same cot or incubator (co-bedding). human twin studies twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. these studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. classical twin studies are now being supplemented with molecular genetic studies which identify individual genes. unusual twinnings bi-paternal twins this phenomenon is known as heteropaternal superfecundation. one 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%. mixed twins dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. one such pairing was born in london in 1993 to a white mother and caribbean father. monozygotic twins of different sexes among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). when monozygotic twins are born with different sexes it is because of chromosomal defects. the probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic. another abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the x chromosome is duplicated. this results in one normal male (xy) and one female with turner syndrome (45,x). in these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes. semi-identical (sesquizygotic) twins monozygotic twins can develop differently, due to their genes being differently activated. more unusual are "semi-identical twins", also known as "sesquizygotic". as of 2019, only two cases have been reported. these "half-identical twins" are hypothesized to occur when an ovum is fertilized by two sperm. the cell assorts the chromosomes by heterogonesis and the cell divides into two, with each daughter cell now containing the correct number of chromosomes. the cells continue to develop into a morula. if the morula then undergoes a twinning event, two embryos will be formed, with different paternal genes but identical maternal genes. twin calves of the hereford breed in miles city, montana in 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father's genetic makeup. the twins were both found to be chimeras. one was an intersex xx, and one a xy male. the exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning. the likely genetic basis of semi-identical twins was reported in 2019 by michael gabbett and nicholas fisk. in their seminal publication, gabbett, fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon. the reported twins shared 100% of their maternal chromosomes and 78% of their paternal genomic information. the authors presented evidence that two sperm from the same man fertilized an ovum simultaneously. the chromosomes assorted themselves through heterogonesis to form three cell lines. the purely paternal cell line died out due to genomic imprinting lethality, while the other two cell lines, each consisting of the same maternal dna but only 50% identical paternal dna, formed a morula which subsequently split into twins. mirror image twins mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. this type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus). if the split occurs later than this time period, the twins risk being conjoined. there is no dna-based zygosity test that can determine if twins are indeed mirror image. the term "mirror image" is used because the twins, when facing each other, appear as matching reflections. language development there have been many studies highlighting the development of language in twins compared to single-born children. these studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts. the reasons for this phenomenon are still in question; however, cryptophasia was thought to be the major cause. idioglossia is defined as a private language that is usually invented by young children, specifically twins. another term to describe what some people call "twin talk" is cryptophasia where a language is developed by twins that only they can understand. the increased focused communication between two twins may isolate them from the social environment surrounding them. idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. however, there are researchers and scientists that say cryptophasia or idioglossia is not a rare phenomenon. current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language. animals non-human dizygotic twinning is a common phenomenon in multiple animal species, including cats, dogs, cattle, bats, chimpanzees, and deer. this should not be confused with an animal's ability to produce a litter, because while litters are caused by the release of multiple eggs during an ovulation cycle, identical to the ovulation of dizygotic twins, they produce more than two offspring. species such as sheep, goats, and deer have a higher propensity for dizygotic twinning, meaning that they carry a higher frequency of the allele responsible for the likelihood of twins, rather than the likelihood of litters (whitcomb, 2021). cases of monozygotic twinning in the animal kingdom are rare but have been recorded on a number of occasions. in 2016, a c-section of an irish wolfhound revealed identical twin puppies sharing a singular placenta. south african scientists, who were called in to study the identical twins wrote that... “to the best of our knowledge, this is the first report of monozygotic twinning in the dog confirmed using dna profiling” (horton, 2016). additionally, armadillos have also been known to produce monozygotic twins, sometimes birthing two sets of identical twins during one reproductive cycle. monozygotic twinning in armadillos functions as an evolutionary adaptation preventing inbreeding. once an armadillo offspring enters its reproductive stage, the organism is forced to leave the nest in search of its mate, rather than mating with its siblings. not only does monozygotic twinning dissuade from armadillo siblings inbreeding, but by forcing migration from the nest, this adaptation ensures the increased genetic variation and geographical population diffusion of armadillo species. due to the increased parental investment provided for their offspring, larger mammals with longer life spans have slower reproductive cycles and tend to birth only one offspring at a time. this commonly repeated behavior in larger mammals evolved as a fixed, naturally-selected adaptation, resulting in a decreased twinning propensity in species such as giraffes, elephants, and hippopotami. despite this adaptation, a case of rare monozygotic twinning has been documented in two elephant calves at the bandipur tiger reserve in karnataka, india. chief veterinarian of the wildlife trust of india, nvk ashraf, in response to the twinning event, wrote that “in species that invest longer time in producing a baby, taking care of two twin calves will be difficult. therefore, the incidence of twinning will be comparatively less.”ashraf's insight not only illuminates the rarity of twinning among large mammals in the natural world, but directs our attention to the increased twinning propensity of animals under human care. this increased twinning propensity is thought to be either caused by random mutation facilitated by genetic drift, or the positive selection of the “twinning” trait in human-controlled conditions. due to the removal of natural predators and unpredictable environmental conditions with the increase of human-provided food and medical care, species residing in nature reserves, zoos, etc., carry an increased likelihood of reversing their naturally-selected traits that have been passed on for generations. when considering this phenomenon in relation to twinning, larger mammals not commonly associated with high twinning propensities can perhaps produce twins as an adaptive response to their human-controlled environment. additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment (rickard, 2022, p.2). thus if a species lives in a controlled environment with a low infant mortality rate, the frequency of the “twinning trait” could increase, leading to a higher likelihood of producing twin offspring. in the case of the monozygotic twin calves in india, their existence could be connected to a new, positively selected adaptation of twinning attributed to species living under human care (ward, 2014, p.7-11). species with small physicalities and quick reproductive cycles carry high twinning propensities as a result of increased predation and high mortality rates. as scientists continue to study the origin of dizygotic twinning in the animal kingdom, many have turned to species that demonstrated an increased output of twins during periods of evolutionary distress and natural selection. through their studies on vespertilionidae and cebidae species, scientists guilherme siniciato terra garbino (2021) and marco varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propensities. in his study on the evolution of litter size in bats, garbino discovered that the vespertilionidae genus has higher twinning propensities as a result of their high roosting habitats. when tracked phylogenetically, scientists determined that the common ancestor of bats carried a higher twinning propensity which was then lost, and picked up again, eighteen times in evolutionary history. while other bat genuses such as myotinae and murinae inevitably lost the twinning trait, the vespertilionidae genus retained a high trait frequency due to mutation and environmental conditions that triggered natural selection. the height and exposed nature of vespertilionidae's roosting locations resulted in a sharp increase in species mortality rate. natural selection offsets these dangers by positively selecting high twinning propensity, resulting in not only vespertilionidae's increased ability to produce twins but the increased likelihood of the genus's reproductive survival. this means that despite the genus's high exposure to factors that would seemingly