Facts About Multiples
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How Multiples are Formed How Multiples Are Formed ~ Twinning Rates ~ Various Statistics ~ Conjoined Twins Stats ~ Risks
How twins are formed: Identical twins (monozygotic or MZ) are caused by one egg being fertilized by one sperm. They are not caused by fertility drugs or other methods and are a spontaneous occurrence. Sometime after fertilization, the egg splits into two halves. Scientists have not determined what causes the splitting of the embryo. Identical twins are of the same sex (only 3 cases have been recorded of boy/girl identicals due to a genetic abnormality, see below). Identical twins have identical nuclear DNA (which is what most people hear about as DNA) but NOT exactly identical mitochondrial DNA. So the term "identical" twins is really not accurate, which is why scientists use the term monozygotic (which means arising from one egg). For the sake of simplicity here, I refer to them as identical twins. About 25% of all twins are monozygotic. Fraternal twins (dizygotic or DZ) are formed when two eggs are released by the mother, and each egg is fertilized by a different sperm. Fraternal twins can be of the same or opposite sex. They share up to 50% of their genes, and are no more alike or different than any two siblings would be. It is also possible for fraternals to be of different races or parentage, or to be conceived at different times, resulting in a large weight difference at birth, but this isn't common. (See Superfecundation and Superfetation below). About 75% of all twins are fraternal. Fraternal twinning can be hereditary through the maternal line. There is some belief that it may also carry through the paternal line, but this has not been proven. Fraternal twins don't necessarily skip a generation either and can occur with no family history of multiples as well. Conjoined twins form exactly like identical twins, but the egg begins to split very late, at least 12 days after conception. The process of division stops, and the twins develop attached to one another. This occurs in about 1 out every 40,000 births (but only once in every 100,000 to 200,000 live births), but 60% of conjoined twins are either stillborn or lost in utero. Females are conjoined more than males, at a ratio of 3:1 (73.95% are girls, while 26.05% are boys). This is because boys are more likely to be miscarried than girls. They are most likely to occur in India or Africa, but there has been a high rate of conjoined twins in Vietnam since the war. The birth rate of conjoined twins in Africa is reportedly 1 in 14,000. About 1 in every 400 monozygotic (identical) pregnancies will be conjoined twins. Other twin formations: Polar body twinning or Half Identical Twinning Polar body twinning, sometimes called half identical twinning, has not been proven to occur in humans. Scientists have speculated that it may occur when an egg (oocyte), before fertilization, has a small cell break off (the polar body). This cell does not function and will usually degenerate and die. It is thought that if the egg is older, the splitting off of the polar body could take place in an abnormal way and become larger, receiving more nourishment. In this case it might not die as it usually does. Instead, it could act as a second egg. The polar body and the egg would share identical genes from the mother, but could then be fertilized by two separate sperm from the father. This will result in twins who share about 75% of their genes. This could explain why some fraternal twins look as alike as some identicals, but it hasn't been proven that it occurs, or that it would produce viable twins. It is also nearly impossible to test for with DNA. If your doctor has told you that you are half-identical, he is mistaken. There has only been ONE case of "half-identical" twins proven, as one member of the set was hermaphroditic. The twins are identical on their mother's side but share only half the genes from their father's side. This was the result of two sperm cells fertilizing one egg and then dividing into two embryos. Only 1% of all conceptions are thought due to two sperm fertilizing one egg, but most of the resulting embryos are non-viable. Scientists believe this case of half-identical twinning is unique and are the only pair in the world. These twins are chimeras (see below). Superfetation - The development of a second fetus when one is already present in the uterus. It occurs when a women ovulates more than one egg but the eggs are released at different times, sometimes up to 24 days apart, and they are fertilized when they are released. The resulting twins would have different conception dates, so the babies may be very different in size and have different due dates. A woman can also conceive twins at different times when a woman conceives via IVF and also conceives naturally around the same time. Superfetation can be detected via ultrasound. Many cases are discovered due to the large differences in weight and size of the babies, and the exclusion of Twin to Twin Transfusion Syndrome. Superfetation is a fairly rare occurrence in humans (naturally), and only occurs when the mother has two uteri, or the menstrual cycle continues during pregnancy. Superfecundation - This happens when the mother ovulates more than one egg in one cycle and has more than one partner during her fertile period. One egg is fertilized with sperm from one partner, and the other egg from sperm of the second partner. These types of twins are always fraternal, and are genetically half siblings, despite being twins. There have been cases of this occurring, but it is still a fairly rare occurrence. Chimerism - A chimera is a person composed of TWO different types of cells. This can occur from a twin absorbing another in the womb, and will only be detectable if it is fraternal twins that merge. These individuals will sometimes have two different blood types (their own and that of their twin). Chimerism is quite rare, there have only been around 30 detected (but again, if identical twins were to merge, no one would be able to discover it). A true chimera is considered a person whose non-identical twin was merged with them, and they carry some genetic evidence of this twin, in their blood or other parts of their body. Chimerism can also occur between living twins. Chimerism is said to occur more frequently among IVF babies. Cases A woman in Boston, reported in New Scientist magazine - it was discovered when she needed a kidney transplant and her children's haplotypes (a block of genes) were different from hers (they should have been the same). The mother had 2 sets of haplotypes herself, one set of hers, the other a twin sisters. She had twice the chance to match haplotypes for her transplant, as she had 2 sets. Patricia McDonnell of England - discovered when she was pregnant and going through blood tests that she carried 2 blood types - O and A. About 93% of her blood was type O, and 7% type A. The scientists concluded after more than 3 years of tests, that the type A blood was her own, as the type O blood belonged to her twin brother. I believe this case was first reported in 1953. In British Columbia, one twin had cystic fibrosis (according to sweat tests), but her blood did not. Her assumed identical twin did not have it, and when blood tested NEITHER twin had CF and they were discovered to be genetically fraternal. In the womb, their placentas had fused and they had shared circulation, so much so that the healthy twin took over for the sick twin. So the sick twin had cystic fibrosis in her body, but not her blood. A hermaphrodite boy born in Britain with both male and female organs. Other parts of his body were tested as genetically female as well. In 1975, more than one blood type was found in 25 pairs of fraternal twins. By 1995 only 45 cases of chimerism had been detected. In 1996 Danish researchers found blood chimerism (where a twin will have two blood types) in 8% of fraternal twins and 21% of fraternal triplets.
How higher order multiples (triplets, quads, quints+) are formed: Identical (MZ): Identical triplets occur in almost the same way as identical twins, but the egg divides into twins and then one of those twins divides again to create three genetically identical babies. In the rare cases of identical quads, twins would form and both halves would divide again to form 4 identical babies. Only 1.58% of all quads are all identical. There have been sets of identical quintuplets in history, but there has never been higher all identicals than 5. Since the advent of fertility methods, all identical multiples are actually more rare (since the fertility methods will most likely produce all fraternal babies) than they were before the advent of fertility methods. All identical multiples are not the result of fertility drugs (although it is possible for someone on fertility drugs or IVF to have identicals). Fraternal (DZ): Fraternal triplets would occur if three eggs were released by the mother and each fertilized by a different sperm. The same would occur for fraternal quads if 4 eggs were released and fertilized, as well as 5 being released for quints. The chances of having spontaneous (not from IVF or fertility drugs) quads and quints isn't as common, spontaneous quints are quite rare, fraternal or otherwise. Fraternal twinning is determined by the mother, who has to release two eggs for it to occur. There is some discussion that double ovulating could be hereditary through the maternal line, and there is some belief that it could be passed paternally as well. Mixed: Higher multiples can often be a combination of identical and fraternal, due to some of the eggs dividing and others not. Triplets - The most common type of spontaneously conceived triplets are two identical and one fraternal. Two eggs would be released by the mother and fertilized, and only one would split to form identical twins. Since fertility methods became so widely used, all fraternal is most common, followed by 2 identical and 1 fraternal, then all identical. Quadruplets - With quadruplets, the most common combination is all fraternal, due to fertility treatments. Spontaneously, the most common would likely be 2 identical and 2 fraternal (three eggs released, one splits). Less common combinations are 3 identical and 1 fraternal (2 eggs released, one splits into two and one of those two splits again), 2 sets of identical (two eggs released, each one splits into twins) and all identical (one egg released, it splits into twins, then each twin splits again). 7.99% of all quadruplet sets will have identicals in them (Nov 04). Quintuplets - Almost all sets are from fertility treatments and are all fraternal, but the most common after that is 2 identical and 3 fraternal. There has also been a case of 3 identical boys and 2 identical girls in a set, one case of 3 identical and 2 fraternal, one case of 2 sets of identical and a fraternal, and 3 cases in history of all identical quints (only one survived). 4.35% of all quints will have identicals in them. (Nov 04) Sextuplets - About 99% are from fertility treatments and are all fraternal. There have been 4 cases of sextuplets with identicals - two sets born had 2 sets of identicals and 2 fraternals, another with 3 identical and 3 fraternal and another with 2 identical and 4 fraternal). 2.74% of all sextuplets sets have had identicals in them (Nov 04). Septuplets - Only one set of septuplets (none surviving) had identicals - 4 identical and 3 fraternal, in 1829 in England which translates to 2.33%. Octuplets - While I have no record of identicals occuring in any octuplet births, it may have happened. I have some sets listed before the advent of fertility methods that were natural conceptions, and some of those babies may have been identical, but there is no way to know.
Diagram
of twin combinations
Boy/Girl Identicals?: There has been very rare cases (3 I have found), of opposite sex identical twins because of genetic abnormality. In these very rare cases, identical boys would be conceived and one twin, during the twinning process, loses a Y chromosome (boys have chromosome type XY while girls have XX), creating a girl by default, but a girl with only one X chromosome (chromosome type XO). The girl will suffer from Turner's Syndrome, which is distinguishable by their short stature, folds of skin at the neck, abnormal development of secondary sexual characteristics, an intellectual deficit known as space-form blindness. The male co-twin would be unaffected. Turner's syndrome can occur in any birth, even that of fraternal female twins, or identical female twins. It also occurs in single births, so an individual with Turner's syndrome is not automatically a twin. Turner's Syndrome occurs in about 1 in 10,000 of all births. One of these cases occurred in 1974, when adolescent twins were tested and discovered to be genetically identical, yet one twin had a sex chromosome with the features of a female with Turner's Syndrome, while the other had the sex chromosome of a male. There has been one case of mixed sex identical triplets as well. An Italian set was born consisting of two identical males and the third was an identical female with Turner's Syndrome.
Finding Out About Multiples: The majority of mothers find out they are having multiples early on in the pregnancy. Here is a list of the percentage of multiples that were diagnosed before birth in various years: 1962 - 19.38% found out before birth Multiple pregnancies are being detected as early as 2 weeks post conception (in the cases of assisted reproduction). The average is anywhere from 12 weeks with twins to 4-5 weeks with sextuplets. The rule of thumb is that if you are using reproductive technology, you'll probably find out sooner because doctors will be looking for multiples, whereas spontaneous conceptions might not be noticed as quickly (by the mother or doctor).
Intrauterine Appearance of Twins: Identical Twins Identical twins can have their own, or share, both the outer sac (chorion) and the inner sac (amnion):
The chorion develops around day 4 - any twins that divide before day 4 will have their own chorion and develop their own amnion around day 9. These twins will have separate placentas, but these placentas may fuse and look like a single placenta. About 25-30% of identical twins will form this early. If the division takes place after day 4 and before day 9, the twins will share a chorion and each develop their own amnions. These twins will share 1 placenta. This is the most common time for the twinning process to occur, and 65-70% of identical twins will have 1 chorion and 2 amnions. If the cell division takes place after day 9 and before day 12, the twins will share both the chorion and the amnion. This is quite rare, with only 1-5% of twins being "late splitters". They are termed monochorionic/monoamniotic. About 23% of Mo/Mo twins are stillborn, 50% die due to cord entanglement and 1 in 10 will be delivered before 32 weeks. Many researchers believe that monochorionic/monoamniotic twins were very close to being conjoined. If the cell division takes place after day 12, the twins will be conjoined. Conjoined twins are also monochorionic/monoamniotic, which is another reason (aside from their joining) why many don't survive. It is also possible for identical triplets and higher to have different placentas/chorion makeup. If identical twins formed and separated between days 1-4, they would each have their own placenta, chorion and amnions. If one of those twins were to separate again between days 4-12, those ones would share a placenta/chorion and perhaps amnion depending on when the division occurred. This would give you identical triplets, one with it's own placenta, amnion and chorion, and two that share a placenta, chorion and may or may not share an amnion. This is yet another reason why going by placentas and chorion/amnion is not accurate in determining if multiples are identical or not. Fraternal Twins Fraternal twins will always have their own chorions and amnions, but may or may not share a (fused) placenta:
Fraternal twins always have their own chorions and amnions, as well as placentas, but the placentas may become fused and look like one placenta (Fused placentas are more common in African blacks than any other race), so it may not always be possible to tell if your twins are identical or fraternal from the placentas. Fraternal twins can't share a chorion or amnion. All Twins About 30% of all twins with fused placentas are identical twins. The other 70% are fraternal with fused placentas. Determining zygosity (identical/fraternal) is only possible during ultrasound if there is only 1 chorion visible. Only 20% will have one chorion and be identifiable through ultrasound as identical. Placentas that are fused may be nearly impossible to tell from one single placenta, so relying on placental examination to determine zygosity is not reliable. The only sure way to know if your twins are identical or fraternal (barring them being of different sexes or having different blood types) is a DNA test.
Position of Twins: Twins can appear as both head first (both vertex), both breech, one breech and one vertex, one vertex and one transverse (sideways) and one breech and one transverse. These are a few numbers on what is most common: Both vertex: Less than 50%
Who Has Twins and How Can I?: Here are some of the factors that affect having twins:
With the advent of fertility treatments, more and more women are having multiple births. There isn't anything proven that you can do to increase your chances of having twins, and there are no clear way of predicting whether an individual will have twins, regardless of family history, use of fertility drugs or any other factors.
Fertility Treatments: There are two basic kinds of fertility treatments:
There is more information on fertility treatments and statistics of the occurrence of multiples with their use in Section 3.
What are Mirror Image Twins?: Mirror imaging occurs in about 25% of identical twins. It's characterized by opposite features, such as one twin being right handed, and one being left handed, opposite hair whorl patterens, a mole being on the opposite side of one twin's face and other reversals. This feature is very common in conjoined twins, some to the extent that the internal organs of one twin are reversed (which is very rare in non-conjoined mirror twins). Scientists believe that mirror twinning is a characteristic of late separating identicals. Mirror imaging is nothing to worry about, and is basically a quirk in the twinning process. Identical triplets can have a mirror pair, as well as identical quads could contain mirror pairs. I've read that mirroring can also occur in fraternal twins.
Twins and Fingerprints: Identical twins do not have the same fingerprints. Their fingerprints may be similar, but never identical, probably due to environmental differences both inside and outside the womb. This is why the police can use fingerprinting to identify people, even identical twins.
Average Gestation Singleton - 39-40
weeks See MOST Online's surveys for ranges and more stats. They also found that 40% of triplets were born after 34 weeks, 40% of quads were born after 32 weeks and 48% of quints were born after 30 weeks.
Average Weight Singleton - 7lbs
7ozs The weight stats are from MOST Online's survey of families with multiples. (1119 triplet families, 178 quadruplet families, 40 quintuplet families and 7 sextuplet families)
Did You Know?: Over 50% of twins
are born before 37 weeks.
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