Types of Twins
Identical or monozygotic twins are formed from one fertilized egg (one ovum and one sperm) which splits after conception into two identical halves which develop separately. Each identical child (always the same sex) will have the same chromosones and many of the same physical attributes. Identical twins will also share the same blood type, eye and hair color. According to research done by the National Organization of the Mothers of Twins Clubs, slightly more than half of all twins born are male and approximately one in four pairs of identical twins are mirror-image twins. About 1/3 of all twin births are identical.
Fraternal or dizygotic twins are formed from two fertilized eggs (two ovums and two sperms) and can result in two siblings of the same or opposite sexes. 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 borne. One-third of all twin births result in same sex fraternal twins and one-third are different sex fraternal twins.
And despite there being just the two types of twins, there is a growing scientific belief of a possible third type of twinning, Polar-Body Twinning. 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.
There are five variations of twinning that commonly occur. The three most common variations are all fraternal: (1) male-female twins are the most common result, at about 40% of all twins born; (2) female fraternal twins (sometimes called sororal twins); (3) male fraternal twins. The last two are identical: (4) female identical twins and (5) (least common) male identical twins. Male singletons are slightly, about 5%, more common than female singletons. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.
Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. Although their traits and physical appearances are not exactly the same due to environmental conditions both in and outside the womb, they do have identical DNA. This is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide,[2] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first 2 days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or "di/di") twins, which occurs 20-30% of the time. Most of the time in identical twins the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ("mono/di") twins. In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac called; monochorionic, monoamniotic ("mono/mono") twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 60%, principally due to cord entanglement prior to 32 weeks gestation. Mono/di twins have about a 25% mortality due to twin-twin transfusion. Di/di twins have the lowest mortality risk at about 9%, although that is still significantly higher than that of singletons.
Complications of twin pregnancy
Vanishing twins
Researchers suspect[attribution needed] that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and not been detected or recorded. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome.
Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".
Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".
Conjoined twins
Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely, and the zygote starts to split after day 13 following fertilization. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as brain, heart or liver.
Chimerism
A chimera is an ordinary person or animal except that some of his or her parts actually came from his or her twin or from the mother. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin colouration in human chimeras. A chimera may be a hermaphrodite, composed of cells from a male twin and a female twin.
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 becomes an almost indistinguishable part of the other.
Partial molar twins
A very rare type of parasitic twinning is where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.
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. This occurrence is similar to the vanishing twin syndrome.
Low birth weight
Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies. Throughout their lives twins tend to be smaller than singletons on average.
Twin-to-twin transfusion syndrome
Identical twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the other, the 'recipient' twin, is large and polycythemic. The lives of both twins are endangered by this condition.