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Resources Centre / Twins Tips Introduction / Twins Tips / Types of Twins : Identical, Fraternal & Unusual Twinning
Types of Twins : Identical, Fraternal & Unusual Twinning
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 37 weeks).
Since some premature births often have health consequence to the babies. twin births are more often handled with special procedures than regular births.

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 that is when the mother releases two eggs and both become fertilized by two different sperms. 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 chromosone profile. but most likely have a number of different chromosones that distinguish them. Dizygotic twins may be a different sex or the same sex. just as with any other siblings. Like singleton siblings. they share 50% of their DNA.
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. Your likelihood of having fraternal twins is dependent upon the woman carrying a fraternal twin gene and can also be affected by heredity. race. marital age and number of children previously born. Two-thirds of all twin births result in same sex fraternal twins and one-third are different sex fraternal twins. About two-thirds of all twin births are fraternal.

Identical Twins

Identical Twins occur when a single egg is fertilized by a single sperm to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The biological mechanisms that prompt the single fertilized egg to split in two remain a mystery. The two embryos develop into foetuses sharing the same womb. Depending on the stage at which the zygote divides. identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic.
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.
  • Dichorionic/Diamniotic : each twin has his/her own placenta, chorion and amniotic sac
  • Monochorionic/Diamniotic: twins share placenta and chorionic sac but have their own amniotic sac
  • Monoamniotic/Monochorionic : twins share placenta, chorionic and amniotic sac

Monoamniotic / Monochorionic 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 foetus. 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. Identical twins will also share the same blood type. eye and hair colour. Examination of details such as fingerprints and teeth marks 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.

Behaviour & Environmental Influences

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 form a late split of the fertilized egg at around 9-12 days. The one mirror may or may not have situs reverus or situs inversus. This is where the organs will be on the opposite side of the body. heart being on the right. etc.

Half Identical Twins (Polar Body Twinning)

Polar body twinning occurs when an egg splits prior to fertilization and each half receives a separate sperm. The twins share 75% of their DNA. This type (although not fully accepted as an "official" type of twins by scientists yet) theorizes that twins aren't exactly identical and aren't exactly fraternal. but half identical/half fraternal. It can happen when the oocyte (primary egg cell) divides twice on its way to maturity. yielding egg cells and polar bodies at different stages along the way. Usually these smaller polar bodies don't play a meaningful role in reproduction. But now scientists believe that some twins could be the result of two of these egg cells and larger polar bodies being pregnated by two sperm. The twins would most likely share all of their mother's genes but only half of their father's genes.

Twins of Two

Two eggs are released by the ovaries and each egg is fertilized by a different father. These fraternal twins are genetically half-siblings and share 25% of their DNA.

Unusual Twinnings

There are some patterns of twinning that are exceedingly rare: while they have been reported to happen. they are so 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 suits was approximately 2.4%.

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.

Complications of Twin Pregnancy
Vanishing twins

Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasound exams sometimes reveal an "extra" fetus. which fails to develop and instead disintegrates and vanishes. This topic is discussed in more detail under a separate article.

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.

Conjoined twins / Siamese twins
Conjoined twins are monozygotic twins (Monoamniotic-Monochorionic) whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. perhaps because they split very late in development (12 days or later since conception). Most conjoined twins are also mirror twins. This condition occurs in about 1 in 100.000 pregnancies.

Parasitic twins

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.

Human Twins - Twinning Rate

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 groups. ranging as high as about 6% for the Yoruba (Nigeria) or 10% for a tiny Brazilian village. 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. Thus. approximately 6% of children born in the US in 2001 were twins. In the UK approx 1.5% of all multiple births are twins (3% of the total UK population). this has increased by almost 50% over the last 20 years. The primary reasons for this are increase in maternal age. assisted conception techniques eg IVF and the improvement in neo-natal care.
Nevertheless. the rate of identical twins remains at about 1 in 250 across the globe. further suggesting that pregnancies resulting in identical twins occur randomly.

Multiple births

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 African descent
  • Between the age of 30 and 40 years
  • Greater than average height and weight
  • Conception is soon after the cessation of oral contraceptives
  • Several previous pregnancies.
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. Currently around 25% of IVF treatment results in a multiple birth.

Twin Studies

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.

Zygosity Testing:

It is difficult to tell if twins are identical or fraternal at birth. Some identical twins may be born with individual sets of membranes, which may lead to the mistaken assumption that the babies are fraternal. One way to tell the difference is to have the twins DNA tested. Identical twins share the same genetic information. while fraternal twins share around half. The test can be done with a sample of cheek cells. collected painlessly. Contact The Multiple Births Foundation for more information on DNA testing.

Article by: Delyth Raffell, Founder of TwinsUK and mum to fraternal twins.

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