The term twin most notably refers to two individuals (or one of two individuals) who have shared the same Uterus (womb) and are usually, but not necessarily, born on the same day. A Fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are more often handled with special procedures than regular births.Types of twins
Fraternal twins
Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilised eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic.
Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Dizygotic twins may be a different sex or the same sex, just as with any other siblings.
Studies show that there is a genetic basis for fraternal twinning-that is, non-identical twins do run in families. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum.
The later in pregnancy that twinning occurs, the more structures will be shared. Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning after 12 days post-fertilization will typically result in Conjoined twins.
Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to Twin-to-twin transfusion syndrome. About 50% of mono-mono twins die from umbilical cord entanglement.
Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. Examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.
While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. The exact cause for the splitting of a zygote or embryo is unknown.
Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). They develop their own individual personalities to enable themselves to be identified as individual persons. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. Twins are unique individuals that establish their own individual likes and dislikes. There are usually obvious signs of differences when the identical twins are observed separately or together.
Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. (Fraga, et al., 2005).Mirror Twins
Mirror twins, also called Mirror Image Twins, are a subset of identical twins and are identical twins with opposite features, that is one will be right handed and the other will be left handed. One quarter (1/4) of all identical twins may be mirror twins. These mirrors are literally reflections of each other and may possess matching or almost matching fingerprints and share the same DNA. They result from a late split of the fertilized egg at around 9-12 days. One mirror may or may not have Situs inversus. This is where some or all of the organs will be on the opposite side of the body, Such as the heart being on the right(Dextrocardia), etc.Complications of twin pregnancy
Vanishing twins
Main article: Vanishing twin
Researchers suspect that more pregnancies start out as multiples than come to term that way. Early Obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.
Conjoined twins are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. This condition occurs in about 1 in 100,000 pregnancies.
Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a Parasite towards the other.
Sometimes the parasitic twin just becomes an almost indistingishiable part of the other.
A chimera is a person who is a complete normal human with no extra parts, but some of the parts actually came from his or her twin. A chimera may be from either from identical twin fetuses (where it would be impossible to detect), or from dyzygotic fetuses, which could be identified by chromosonal comparisons from various parts of the body.
Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnacies resulting in identical twins occur randomly.Multiple births
Main article: Multiple birth
Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.
Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
Women undergoing certain fertility treatments may have a greater chance of multiple births. This can vary depending on what types of fertility treatments are used. With In vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelyhood of multiple births.
Twin studies are studies that assess identical (monozygotic) twins for medical, Genetic, or psychological characteristics to try to isolate Genetic influence from 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 invaluable in the exploration of Human nature.
Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of Sexual intercourse, either within one Menstrual cycle (Superfecundation) or, even more rarely, later on in the Pregnancy (Superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.
Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero'' multiples are not monozygotic. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.
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}}??2}}}} |. }} Epigenetic differences arise during the lifetime of monozygotic twins PMID 16009939
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}}}}}} |. }} 'Identical' twins with discordant karyotypes PMID 10073913
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}}}}}} |. }} How frequent is heteropaternal superfecundation? PMID 1488855
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}}}}}} |. }} Indisputable double paternity in dizygous twins PMID 9176461
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