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[[Berkas:TwinGirls.jpg|right|thumb|220px|Anak perempuan yang kembar monozigotik]]
'''Kembar''' atau '''anak kembar''' adalah dua atau lebih individu yang membagi [[uterus]] yang sama dan biasanya, tapi tidak selalu, [[kelahiran anak|dilahirkan]] dalam hari yang sama. IbuPada manusia, ibu dengan kandungan yang membawa [[bayi]] kembar dengan demikian akan mengalami [[persalinan berganda]] dan biasanya masa mengandung yang lebih singkat (34 sampai 36 minggu) daripada kehamilan bayi tunggal. Karena [[kelahiran prematur]] biasanya memiliki konsekuensi kesehatan kepada bayi, kelahiran kembar seringkali ditangani secara khusus yang agak berbeda daripada kelahiran biasa.
Persalinan kembar adalah hal yang biasa pada hewan karena banyak hewan yang menghasilkan [[sel telur]] masak (siap [[pembuahan|dibuahi]]) pada saat yang sama. Hewan-hewan [[domestikasi|domestik]] seperti [[kucing]], [[anjing]], [[tikus]], atau [[domba]] biasa memiliki anak lebih dari satu sekali melahirkan.
Hingga sekarang ilmuwan belum bersepakat mengenai adanya pengaruh genetik untuk kejadian kembar MZ. Tetapi diketahui terdapat beberapa tempat di dunia yang memiliki frekuensi kembar MZ yang lebih tinggi daripada tempat lainnya.<ref name="Cyranosky"/>
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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 foetus. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. In rare cases, blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to [[twin-to-twin transfusion syndrome]].
Identical twins can behave as differently as any other siblings (a matter of much interest to [[psychology|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 can assume that they will behave alike just as they look alike; however, this is not necessarily the case. Twins establish their own individual likes and dislikes. This is not to say they are totally different, but 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 [[Epigenetics|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).
==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===
Main article: [[Conjoined twin]]
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.
===Parasitic twins===
Main article: [[Parasitic twin]]
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.
===Miscarried twin===
Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. Similar to [[vanishing twin]].
==Human twins==
Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The rate of twinning varies greatly among [[ethnic group]]s, ranging as high as about 6% for the [[Yoruba]] or 10% for a tiny Brazilian village (see [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=9181177&dopt=Abstract]). The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of [[multiple birth]]s". In [[2001]], for the first time ever in the [[United States|US]], the twinning rate exceeded 3% of all births. Thus, approximately 6% of children born in the US in 2001 were twins.
Nevertheless, the rate of ''identical'' twins remains at about 1 in 250 across the globe.
===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 37 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.
===Predisposing factors===
The cause of monozygotic twinning is unknown. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). Some evidence suggests that the environment of the womb causes the zygote to split in most cases.
Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
*She is of [[Africa]]n descent
*Between the age of 30 and 40 years
*Greater than average height and weight
*Conception is soon after the cessation of [[oral contraceptive]]s
*Several previous pregnancies.
Women who have been taking fertility drugs have the greatest chance of multiple births, ranging from 10 to 20 percent. This is primarily down to the passing of multiple embryos into the uterus in order to increase the success rate of [[in vitro fertilisation]] (IVF).
===Twin studies===
''See main article at [[twin study]]''
Twin studies are studies that assess identical (monozygotic) twins for [[medical]], [[genetic]], or [[psychology|psychological]] characteristics to try to isolate [[genetic]] influence from [[natural environment|environment]]al 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]].
===Twin Studies currently recruiting subjects===
The following Twin Studies are ongoing studies that are recruiting subjects:
*[http://www.iop.kcl.ac.uk/iopweb/departments/home/default.aspx?locator=557 The Maudsley Bipolar Twin Study]
===Unusual Twinnings===
There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are sufficiently unusual that most obstetricians or midwives may go their entire careers without encountering a single case.
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 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 [[karyotype]]s.
==Twinning in animals==
Multiple births are common in many animal species, such as [[cat]]s, [[sheep]], and [[ferret]]s. The incidence of twinning among [[cattle]] is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed.
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== Lihat pula ==
* [[Kembar siam]]
* [[Kloning]]
== Catatan kaki ==
{{reflist}}
== Referensi ==
*
* {{Cite journal | author=Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. | title='Identical' twins with discordant karyotypes | journal=Prenatal Diagnosis | volume=19 | issue=1 |year=1999 | pages=72-6}} PMID 10073913
* {{Cite journal | author=Wenk RE, Houtz T, Brooks M, Chiafari FA | title=How frequent is heteropaternal superfecundation? |journal=Acta geneticae medicae et gemellologiae | uear=1992 | volume=41 | issue=1 | pages=43-7 }} PMID 1488855
* {{Cite journal | author=Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E | journal=Fertility and Sterility | title=Indisputable double paternity in dizygous twins |year=1997 | volume=67 | issue=6 | pages=1159-61 }} PMID 9176461
== Pranala luar ==
* {{kamus|kembar}} di [[KBBI]]
{{Kekerabatan}}
[[Kategori:Keluarga]]
[[Kategori:Kehamilan dan kelahiran]]
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