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51-478: Twins are two offspring produced in the same pregnancy. Twins may also refer to: Twin In contrast, a fetus that develops alone in the womb (the much more common case in humans) is called a singleton , and the general term for one offspring of a multiple birth is a multiple . Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgänger . The human twin birth rate in

102-658: A bold assumption that mammals also reproduced via eggs. Karl Ernst von Baer discovered the mammalian ovum in 1827. The fusion of spermatozoa with ova (of a starfish) was observed by Oskar Hertwig in 1876. In animals, egg cells are also known as ova (singular ovum , from the Latin word ovum meaning ' egg '). The term ovule in animals is used for the young ovum of an animal. In vertebrates, ova are produced by female gonads (sex glands) called ovaries . A number of ova are present at birth in mammals and mature via oogenesis . Studies performed on humans, dogs, and cats in

153-490: A greater chance of dizygotic multiple births. In the United States it has been estimated that by 2011 36% of twin births resulted from conception by assisted reproductive technology . The risk of twin birth 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. Ovarian hyperstimulation without IVF has

204-404: A larger, female gamete and a smaller, male one). The term is used when the female gamete is not capable of movement (non- motile ). If the male gamete ( sperm ) is capable of movement, the type of sexual reproduction is also classified as oogamous . A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere . When fertilized, the oosphere becomes

255-427: A long neck with a wider base containing the egg cell. Upon maturation, the neck opens to allow sperm cells to swim into the archegonium and fertilize the egg. The resulting zygote then gives rise to an embryo, which will grow into a new diploid individual, known as a sporophyte . In seed plants , a structure called the ovule contains the female gametophyte. The gametophyte produces an egg cell. After fertilization ,

306-503: A more likely explanation. A high twinning rate has also been observed in other places of the world, including: 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 births ". In 2001, for the first time ever in the US , the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across

357-460: A pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same fingerprints however, because even within the confines of the womb, the fetuses touch different parts of their environment, giving rise to small variations in their corresponding prints and thus making them unique. Monozygotic twins always have the same genotype . Normally due to an environmental factor or

408-541: A rate of about 3 in every 1000 deliveries worldwide. The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world. This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India and up to over 20 in some Central African countries. The exact cause for

459-406: A separate fetus, rather than by the embryo splitting while hatching from the zona pellucida (the gelatinous protective coating around the blastocyst). Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of in vitro fertilization (IVF) to increase the number of available embryos for embryo transfer . Monozygotic twinning occurs in birthing at

510-541: A study in 2012 found that it is possible for a polar body to result in a healthy fetus. In 2003, a study argued that many cases of triploidy arise from sesquizygotic (semi-identical) twinning which happens when a single egg is fertilized by two sperm and splits the three sets of chromosomes into two separate cell sets. The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in

561-474: A very high risk of multiple birth. Reversal of anovulation with clomifene (trade names including Clomid ) has a relatively less but yet significant risk of multiple pregnancy. A 15-year German study of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes. The delivery interval between the twins was measured as follows: The study stated that

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612-557: Is a genetically linked tendency to hyper- ovulate . There is no known genetic link for identical twinning. Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births. Some women intentionally turn to fertility drugs in order to conceive twins. The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). In humans dizygotic twins occur more often than monozygotic twins. Less common variants are discussed further down

663-446: Is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos . The chances of having identical twins is relatively rare – around 3 or 4 in every 1,000 births. Regarding spontaneous or natural monozygotic twinning, a 2007 theory related to in vitro fertilization (IVF) proposes that monozygotic twins may be formed when a blastocyst contains two inner cell masses (ICM), each of which will lead to

714-419: Is the most common type of twin. Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to chromosomal crossover during meiosis . Dizygotic twins share on average 50 percent of each other's genes, the same as siblings that are conceived and born at different times. Like any other siblings , dizygotic twins may look similar , particularly as they are

765-668: The Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói . In Cândido Godói, one in five pregnancies has resulted in twins. The Argentine historian Jorge Camarasa has put forward the theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area. His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as

816-462: The blastocysts have fused. Monochorionic twins share the same placenta , and thus have a risk of twin-to-twin transfusion syndrome . Monoamniotic twins are always monozygotic . The survival rate for monoamniotic twins is somewhere between 50% and 60%. Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of twin-to-twin transfusion syndrome . Also, the two umbilical cords have an increased chance of being tangled around

867-418: The formative yolk ; and the nutritive yolk or deutoplasm , made of rounded granules of fatty and albuminoid substances imbedded in the cytoplasm. Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the embryo in the early stages of its development only. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation. In

918-400: The oospore . When egg and sperm fuse during fertilisation , a diploid cell (the zygote ) is formed, which rapidly grows into a new organism. While the non-mammalian animal egg was obvious, the doctrine ex ovo omne vivum ("every living [animal comes from] an egg"), associated with William Harvey (1578–1657), was a rejection of spontaneous generation and preformationism as well as

969-406: The oviparous animals (all birds , most fish , amphibians and reptiles ), the ova develop protective layers and pass through the oviduct to the outside of the body. They are fertilized by male sperm either inside the female body (as in birds), or outside (as in many fish). After fertilization, an embryo develops, nourished by nutrients contained in the egg. It then hatches from the egg, outside

1020-409: The 1870s suggested that the production of oocytes (immature egg cells) stops at or shortly after birth. A review of reports from 1900 to 1950 by zoologist Solomon Zuckerman cemented the belief that females have a finite number of oocytes that are formed before they are born. This dogma has been challenged by a number of studies since 2004. Several studies suggest that ovarian stem cells exist within

1071-560: The United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovarys to release an egg from each side. In Central Africa , there are 18–30 twin sets (or 36–60 twins) per 1,000 live births. In South America , South Asia , and Southeast Asia ,

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1122-445: The aid of a microscope or other magnification device. The human ovum measures approximately 120 μm (0.0047 in) in diameter. Ooplasm is like the yolk of the ovum, a cell substance at its center, which contains its nucleus , named the germinal vesicle , and the nucleolus , called the germinal disc . The ooplasm consists of the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm , often called

1173-507: The article. Fraternal twins can be any of the following: Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/female in Italy. However, males are also more susceptible than females to die in utero , and since

1224-473: The babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy due to lack of oxygen. When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. A 2006 study has found that insulin-like growth factor present in dairy products may increase

1275-501: The blue colour after GUS staining reveals. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo. In algae , the egg cell is often called oosphere. Drosophila oocytes develop in individual egg chambers that are supported by nurse cells and surrounded by somatic follicle cells. The nurse cells are large polyploid cells that synthesize and transfer RNA, proteins, and organelles to

1326-450: The chance of dizygotic twinning. Specifically, the study found that vegan mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation." From 1980 to 1997,

1377-406: The deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy , twins may express different sexual phenotypes , normally from an XXY Klinefelter syndrome zygote splitting unevenly. Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins

1428-413: The death rate in utero is higher for twins, it leads to female twins being more common than male twins. Zygosity is the degree of identity in the genome of twins. Dizygotic ( DZ ) or fraternal twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins") usually occur when two fertilized eggs are implanted in

1479-408: The differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells. Another cause of difference between monozygotic twins is epigenetic modification , caused by differing environmental influences throughout their lives. Epigenetics refers to

1530-405: The early stages of embryonic development. A 1981 study of a deceased XXX twin fetus without a heart showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning. However,

1581-463: The egg nucleus. The resulting zygote develops into an embryo inside the ovule. The ovule, in turn, develops into a seed and in many cases, the plant ovary develops into a fruit to facilitate the dispersal of the seeds. Upon germination , the embryo grows into a seedling . In the moss Physcomitrella patens , the Polycomb protein FIE is expressed in the unfertilised egg cell (Figure, right) as

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1632-682: The epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, certain characteristics become more alike as twins age, such as IQ and personality. In January 2021, new research from a team of researchers in Iceland was published in the journal Nature Genetics suggesting that identical twins may not be quite as identical as previously thought. The four-year study of monozygotic (identical) twins and their extended families revealed that these twins have genetic differences that begin in

1683-475: The globe. In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria , there were 40 twins and 2 triplets per 1000 births. Twenty-six percent of twins were monozygotic. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of

1734-536: The level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual. This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times

1785-406: The lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. Monochorionic twins generally have two amniotic sacs (called Monochorionic–Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins, and in 0.3% of all pregnancies. Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where

1836-459: The lowest rates are found; only 6 to 9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9 to 16 twin sets per 1,000 live births. Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting on average 37 weeks, three weeks less than full term. Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there

1887-406: The mammalian ovary. Whether or not mature mammals can actually create new egg cells remains uncertain and is an ongoing research question. In all mammals , the ovum is fertilized inside the female body. Human ova grow from primitive germ cells that are embedded in the substance of the ovaries . The ovum is one of the largest cells in the human body, typically visible to the naked eye without

1938-478: The mother's body shortly before birth, or just after the egg leaves the mother's body. Some fish, reptiles and many invertebrates use this technique. Nearly all land plants have alternating diploid and haploid generations. Gametes are produced by the haploid generation, which is known as the gametophyte . The female gametophyte produces structures called archegonia , and the egg cells form within them via mitosis . The typical bryophyte archegonium consists of

1989-400: The mother's body. See egg for a discussion of eggs of oviparous animals. The egg cell's cytoplasm and mitochondria are the sole means the egg can reproduce by mitosis and eventually form a blastocyst after fertilization. There is an intermediate form, the ovoviviparous animals: the embryo develops within and is nourished by an egg as in the oviparous case, but then it hatches inside

2040-525: The number of twin births in the United States rose 52%. This rise can at least partly be attributed to the increasing popularity of fertility drugs and procedures such as IVF, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food. About 1 in 90 human births (1.1%) results from a twin pregnancy. The rate of dizygotic twinning varies greatly among ethnic groups , ranging as high as about 45 per 1000 births (4.5%) for

2091-414: The occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering

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2142-403: The ovule develops into a seed containing the embryo. In flowering plants , the female gametophyte (sometimes referred to as the embryo sac) has been reduced to just eight cells inside the ovule . The gametophyte cell closest to the micropyle opening of the ovule develops into the egg cell. Upon pollination , a pollen tube delivers sperm into the gametophyte and one sperm nucleus fuses with

2193-473: The pregnancy. The timing of this separation determines the chorionicity (the number of placentae) and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week. In very rare cases, twins become conjoined twins . Non-conjoined monozygotic twins form up to day 14 of embryonic development, but when twinning occurs after 14 days,

2244-408: The rate of monozygotic twins) to 14 and more per thousand in some African countries. Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. Monozygotic ( MZ ) or identical twins occur when a single egg

2295-444: The same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes). Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum . Dizygotic twinning ranges from six per thousand births in Japan (similar to

2346-540: The second twin. There have also been cases in which twins are born a number of days apart. Possibly the worldwide record for the duration of the time gap between the first and the second delivery was the birth of twins 97 days apart in Cologne, Germany, the first of which was born on November 17, 2018. Ovum The egg cell or ovum ( pl. : ova ) is the female reproductive cell, or gamete , in most anisogamous organisms (organisms that reproduce sexually with

2397-455: The southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria . Twins are more common in people of African descent. The predisposing factors of monozygotic twinning are unknown. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman: Women undergoing certain fertility treatments may have

2448-414: The splitting of a zygote or embryo is unknown. IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000. Monozygotic twins are genetically nearly identical and they are the same chromosomal sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if

2499-436: The twins will likely be conjoined. Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications . It is a common misconception that two placentas automatically implies dizygotic twins, but if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is in fact indistinguishable from that of dizygotic twins. DiDi twins have

2550-466: The uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells , fraternal twins result. The two eggs, or ova , form two zygotes , hence the terms dizygotic and biovular . Fraternal twins are, essentially, two ordinary siblings who happen to develop in the womb together and who are born at the same time, since they arise from two separate eggs fertilized by two separate sperm , just like ordinary siblings. This

2601-420: Was analyzed for 506,786 single-nucleotide polymorphisms known to occur in human populations. Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 12 million nucleotides, which would imply hundreds of differences across the entire genome. The mutations producing

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