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{{disambiginfo|Kembar}}
[[Berkas:TwinBoysInBathtub.jpg|thumb|250px|Anak kembar lelaki fraternal di bak]]
[[Berkas:TwinGirls.jpg|ka|jmpl|220px|Anak perempuan yang kembar monozigotik]]
 
Istilah'''Kembar''' atau '''anak kembar''' terutamaadalah menunjukdua kepadaatau dualebih individu yang membagi [[uterus]] yang sama dan biasanya, tapitetapi tidak selalu, [[kelahiran anak|dilahirkan]] dalam hari yang sama. SebuahPada [[fetus]]manusia, tunggalibu didengan uteruskandungan biasanyayang disebutmembawa ''singleton''.[[bayi]] Dikarenakankembar keterbatasandengan ukurandemikian uterusakan sang ibu,mengalami [[kelahiranpersalinan banyakberganda]] dan biasanya lebihmasa tidakmengandung membawayang sampailebih jangka waktu penuh dibanding dengan kelahiran tunggalsingkat (kembar biasanya sekitar 34 sampai 36 minggu) daripada kehamilan bayi tunggal. Karena [[kelahiran prematur]] biasanya memiliki konsekuensi kesehatan kepada bayi, kelahiran kembar seringkalisering kali ditangani dengansecara khusus yang proseduragak spesialberbeda daridaripada 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. [[Domestikasi hewan|Hewan-hewan domestik]] seperti [[kucing]], [[anjing]], [[tikus]], atau [[domba]] biasa memiliki anak lebih dari satu sekali melahirkan.
 
== Jenis kembar ==
[[Berkas:Identical-fraternal-sperm-egg.svg|jmpl|300px|Perbedaan kembar dizigotik (kanan) dan monozigotik (kiri).]]
=== Kembar fraternal ===
Dilihat dari asal usul [[zigot]], dikenal dua jenis persalinan kembar: fraternal (dizigotik) dan identik (monozigotik). Kembar dizigotik adalah hal yang umum terjadi pada [[vertebrata]], sementara kembar monozigotik merupakan hal yang jarang dijumpai. Manusia memiliki kemampuan ini. [[Armadillo]] bergaris-sembilan (''Dacypus novemcinctus'') jika melahirkan selalu memiliki kembar empat monozigotik.<ref name="Cyranosky">Cyranosky D. 2009. Two by two. ''Nature''. Vol. 458:826-829</ref>
'''Kembar fraternal''' (biasanya dikenal sebagai "kembar non-identik") biasanya terjadi ketika dua telur [[fertilisasi|terfertilisasi]] terimplan di tembok uterus pada saat bersamaan. Kedua telur ini membentuk dua "zigot", dan kembar ini juga dikenal sebagai '''dizigotik'''.
 
=== Kembar dizigotik atau fraternal (DZ) ===
Kembar dizigotik tidak lebih mirip secara genetik dari saudara biasa dan berkembang dalam [[amniotic sac|amnion]] yang terpisah, dengan [[plasenta]] terpisah. Mereka dapat memiliki jenis kelamin yang berbeda atau sama. Namun, kembar yang berbeda kelamin hampir selalu merupakan kembar fraternal.
'''Kembar dizigotik''' (dikenal sebagai "kembar non-identik") terjadi karena zigot-zigot yang terbentuk berasal dari sel telur yang berbeda. Terdapat lebih dari satu sel telur yang melekat pada dinding rahim yang terbuahi oleh sel-sel sperma pada saat yang bersamaan. Pada manusia, proses ovulasi kadang-kadang melepaskan lebih dari satu sel telur matang ke [[tuba fallopi]] yang apabila mereka terbuahi akan memunculkan lebih dari satu zigot.
 
Kembar dizigotik secara genetik tidak berbeda dari saudara biasa dan berkembang dalam [[amnion]] dan [[plasenta]] yang terpisah. Mereka dapat memiliki jenis kelamin yang berbeda atau sama, namun dengan konsekuensi memiliki tampilan fisik yang berbeda seperti wajah, postur tubuh, warna kulit, atau rambut
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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.
-->
 
Kajian juga menunjukkan bahwa bakat melahirkan kembar DZ diwariskan kepada keturunannya (bersifat [[genetika|genetik]]), namun hanya keturunan perempuan/betina yang mampu menunjukkannya (karena hanya perempuan/betina yang dapat mengatur pengeluaran sel telur).
=== Kembar identik ===
'''Kembar identik''' terjadi ketika [[ovum|telur]] tunggal difertilisasi untuk membentuk satu [[zigot]] ('''monozigotik''') namun kemudian zigot tersebut berpisah menjadi [[embrio]] yang berbeda. Kedua embrio berkembang menjadi fetus yang membagi rahim yang sama. Tergantung dari tahapan zigot terpisah, kembar identik dapat membagi amnion yang sama (dikenal dengan ''monoamniotik'') atau tidak (''diamnotik''). Kembar identik diamniotik dapat membagi [[plasenta]] yang sama (dikenal dengan ''monochorionic'') atau tidak (''diochorionic''). Seluruh kembar monoamniotic adalah monochorionic.
 
Istilah '''kembar dampit''' diberikan bagi anak kembar dengan kelamin berbeda.
Membagi amnion yang sama (atau amnion dan plasenta yang sama) dapat menyebabkan komplikasi dalam kelahiran. Contohnya, "umbilical cord" dari kembar monoamniotic dapat "entangled", mengurangi atau mengganggu penyaluran darah ke fetus yang berkembang.
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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]].
 
Istilah '''kembar campuran''' diberikan bagi anak kembar dengan warna kulit berbeda.
Monozygotic twins are genetically identical unless there has been a mutation in development, and they are always the same gender. They look alike, except that they are sometimes mirror images, but examination of details such as [[fingerprint]]s can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.
 
=== Kembar monozigotik atau identik (MZ) ===
It is thought that identical twins do not run in families, but occur more or less randomly.
'''Kembar monozigotik''' terjadi ketika [[sel telur]] tunggal terbuahi dan membentuk satu [[zigot]] (monozigotik). Dalam perkembangannya, zigot tersebut membelah menjadi [[embrio]] yang berbeda. Kedua embrio berkembang menjadi janin yang berbagi rahim yang sama. Tergantung dari tahapan pemisahan zigot, kembar identik dapat berbagi amnion yang sama (dikenal sebagai ''monoamniotik'') atau berbeda amnion. Lebih jauh lagi, kembar identik bukan monoamniotik dapat berbagi [[plasenta]] yang sama (dikenal dengan ''monokorionik'', ''monochorionic'') atau tidak. Semua kembar monoamniotik pasti monokorionik. Berbagi amnion yang sama (atau amnion dan plasenta yang sama) dapat menyebabkan komplikasi dalam kehamilan. Contohnya, tali pusar dari kembar monoamniotik dapat terbelit sehingga mengurangi atau mengganggu penyaluran darah ke janin yang berkembang.
 
Kembar MZ selalu berkelamin sama dan secara genetik adalah sama (klon) kecuali bila terjadi [[mutasi]] pada perkembangan salah satu individu. Tingkat kemiripan kembar ini sangat tinggi, dengan perbedaan kadang-kadang terjadi berupa keserupaan cerminan. Perbedaan terjadi pada hal detail, seperti sidik jari. Bila individu beranjak dewasa, tingkat kemiripan biasanya berkurang karena pengalaman pribadi atau gaya hidup yang berbeda. Penelitian dari Fraga [[et al.]] (2005) mengungkap adanya pengaruh [[regulasi epigenetik|epigenetik]] dalam proses yang membedakan individu-individu yang kembar MZ, akibat berbedanya gen-gen yang diaktifkan.<ref>{{Cite journal|author=Fraga MF, Ballestar E, Paz MF, Ropero S, Setien F, Ballestar ML, Heine-Suner D, Cigudosa JC, Urioste M, Benitez J, Boix-Chornet M, Sanchez-Aguilera A, Ling C, Carlsson E, Poulsen P, Vaag A, Stephan Z, Spector TD, Wu YZ, Plass C, Esteller M|year=2005|title=Epigenetic differences arise during the lifetime of monozygotic twins|journal=[[Proceedings of the National Academy of Sciences]]|volume=Epub ahead of print}} PMID 16009939</ref> Meskipun ada pengaruh kebiasaan atau pengalaman yang memengaruhi perbedaan-perbedaan itu, ilmuwan beranggapan proses acak lebih banyak berperan dalam perbedaan-perbedaan yang terjadi. Penelitian dengan tikus bahkan menunjukkan adanya perbedaan aktivitas pada histon (terkait dengan epigenetik) dari empat sel pertama yang terbentuk.<ref>Torres-Padilla, M.-E., Parfitt, D.-E., Kouzarides, T. & Zernicka-Goetz, M. 2007. ''Nature'' 445:214–218.</ref>
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.
 
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"/>
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).
 
=== Kembar lumayan mirip atau semi identik ===
==Complications of twin pregnancy==
Kembar Semi identik (disebut sesquizygotic) adalah anak kembar yang berasal dari satu sel telur namun dibuahi oleh dua sel sperma. Umumnya, 1 sel telur hanya mampu dibuahi 1 sel sperma. Mereka memiliki kemiripan fisik dengan beberapa perbedaan.<ref>{{Cite web|url=https://www.technologynetworks.com/genomics/news/semi-identical-twins-three-sets-of-chromosomes-two-babies-316090|title=Semi-identical Twins: Three Sets of Chromosomes, Two Babies|website=Technology Networks|access-date=27 Februari 2019}}</ref>
===Vanishing twins===
Main article: [[Vanishing twin]]
 
Contoh, Mawar dan Melati adalah anak kembar yang lumayan mirip. Mawar berwajah 100% mirip ibunya, Melati berwajah perpaduan dari 60% ibunya dan 40% ayahnya.
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.
Atau, misalkan Lili dan [[Orchidaceae|Anggrek]] adalah kembar yang memiliki wajah mirip hanya saja dengan warna rambut yang berbeda.
 
== Perbedaan kembar fraternal dan identik ==
===Conjoined twins===
{| class="wikitable sortable"
Main article: [[Conjoined twin]]
|-
! Fraternal !! Identik
|-
| DNA beda || DNA sama
|-
| jenis kelamin bisa beda ataupun sama || jenis kelamin sama
|-
| wajah/muka beda || wajah/muka sama
|-
| golongan darah beda || golongan darah sama
|}
 
== Persamaan kembar fraternal dan identik ==
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.
* Sidik jari beda.
* Anak urutan yang dikeluarkan (''dilahirkan'') dalam kandungan ibu.
* Hanya memiliki satu rahim.
 
== Perbedaan kembar biasa dan siam ==
===Parasitic twins===
Pada kembar biasa urutan anak dibedakan karena bayi dilahirkan ibu satu per satu sedangkan siam tetap.
Main article: [[Parasitic twin]]
 
== Jumlah anak kembar dalam kelahiran ==
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.
Umumnya, anak kembar terlahir sebagai dua orang. Namun dalam beberapa kasus, ditemukan kembar dengan jumlah tiga orang, empat orang, dan seterusnya, hanya saja tidak sesering dengan kembar dua orang. Kembar tiga disebut ''triplet'', kembar empat disebut ''quadruplet'', kembar lima disebut ''quintuplet'', kembar enam disebut ''sixptuplet'', kembar tujuh disebut ''septuplet'', kembar delapan disebut ''octuplet'', dan kembar sembilan disebut ''nonuplet'' </ref> {{cite web |url=https://www.verywellfamily.com/what-do-you-call-this-many-babies-2759259 |title=Special Names for Multiple Birth |date=31 Oktober 2021}}</ref>
 
===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.
-->
 
== Lihat pula ==
* [[Kembar siam]]
* [[Anak tunggal]]
* [[Kota kembar]]
* [[Daftar orang kembar di Indonesia]]
* [[Kloning]]
* [[Perbanyakan vegetatif]]
 
== Catatan kaki ==
*[[Daftar orang kembar]]
{{reflist}}
*[[Cloning]] with reference to identical twins.
*[[NICU]] (Newborn Intensive Care Unit)
 
== 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
* {{Journal reference issue | Author=Fraga MF, Ballestar E, Paz MF, Ropero S, Setien F, Ballestar ML, Heine-Suner D, Cigudosa JC, Urioste M, Benitez J, Boix-Chornet M, Sanchez-Aguilera A, Ling C, Carlsson E, Poulsen P, Vaag A, Stephan Z, Spector TD, Wu YZ, Plass C, Esteller M | Title=Epigenetic differences arise during the lifetime of monozygotic twins | Journal=[[Proceedings of the National Academy of Sciences]] | Volume=Epub ahead of print | Issue= | Year=2005 | Pages=}} PMID 16009939
* {{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
* {{Journal reference issue | 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=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
* {{Journal reference issue | Author=Wenk RE, Houtz T, Brooks M, Chiafari FA | Title=How frequent is heteropaternal superfecundation? |Journal=Acta geneticae medicae et gemellologiae | Year=1992 | Volume=41 | Issue=1 | Pages=43-7 }} PMID 1488855
 
* {{Journal reference issue | 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]]
* Makna Sehat: [http://www.maknasehat.com/ciri-ciri-orang-hamil-kembar/ Ciri Hamil Kembar] {{Webarchive|url=https://web.archive.org/web/20160815050717/http://maknasehat.com/ciri-ciri-orang-hamil-kembar/ |date=2016-08-15 }}
 
{{Kekerabatan}}
[[Kategori:Kehamilan]]
 
[[caKategori:BessóKeluarga]]
[[daKategori:TvillingKelahiran]]
[[deKategori:ZwillingeZoologi]]
[[en:Twin]]
[[eo:Ĝemeloj]]
[[es:Gemelos (biología)]]
[[fi:Ihmiskaksoset]]
[[fr:Jumeau]]
[[gn:Membykõi]]
[[he:תאומים]]
[[io:Jemelo]]
[[it:Gemelli (biologia)]]
[[ja:双生児]]
[[ko:쌍둥이]]
[[la:Gemini]]
[[lt:Dvyniai]]
[[lv:Dvīņi]]
[[ms:Kembar]]
[[no:Tvilling]]
[[pl:Ciąża bliźniacza]]
[[pt:Gêmeos]]
[[qu:Ch'ullayuq]]
[[ru:Близнецы]]
[[sr:Близанац]]
[[sv:Tvillingar]]
[[uk:Близнята]]
[[vi:Sinh đôi]]
[[yi:צווילינג]]
[[zh:雙胞胎]]
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