A twin is one of two offspring produced in the same pregnancy.
Twins can either be monozygotic (MZ, colloquially "identical"), meaning that
they develop from one zygote that splits and forms two embryos, or dizygotic
(DZ, colloquially "fraternal") because they develop from two separate eggs
that are fertilized by two separate sperm.
Zygosity
Zygosity is the degree of identity in the genome of twins. There are five
common variations of twinning. The three most common variations are all
dizygotic:
Male–female twins are the most common result, 50 percent of DZ twins and
the most common grouping of twins.
Female DZ twins (sometimes called sororal twins)
Male DZ twins
The other two variations are monozygotic twins:
Female MZ twins
Male MZ twins (least common)
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 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.
Dizygotic twins
Dizygotic twins (commonly known as fraternal twins, but also referred to as
non-identical twins or biovular twins) usually occur when two fertilized eggs
are implanted in the uterine wall at the same time. When two eggs are
independently fertilized by two different sperm cells, DZ twins result. The
two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.
Dizygotic twins, like any other siblings, have an extremely small chance of
having the same chromosome profile. Like any other siblings, DZ twins may look
similar, particularly given that they are the same age. However, DZ twins may
also look very different from each other. They may be of different sexes or
the same sex. The same holds true for brothers and sisters from the same
parents, meaning that DZ twins are simply brothers and/or sisters who happen
to be the same age.
Studies show that there is a genetic basis for DZ twinning. However, it is
only their mother that has any effect on the chances of having DZ 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
the rate of monozygotic twins) to 14 and more per thousand in some African
countries.
DZ 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. For example, in New York City's Upper East Side there were 3,707
twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004.
Triplet births have also risen, from 60 in 1995 to 299 in 2004.
Monozygotic twins
In the uterus, a majority of monozygotic twins (60–70%) share the same
placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each
fetus has a separate placenta and a separate amniotic sac. A small number (1–
2%) of monozygotic twins share the same placenta and amniotic sac. Dizygotic
twins each have their own placenta and own amniotic sac.
Monozygotic twins, frequently referred to as identical twins, occur when a
single egg is fertilized to form one zygote (monozygotic) which then divides
into two separate embryos.
Mechanism
Regarding spontaneous or natural monozygotic twinning, a recent theory posits
that monozygotic twins are formed after a blastocyst essentially collapses,
splitting the progenitor cells (those that contain the body's fundamental
genetic material) in half, leaving the same genetic material divided in two on
opposite sides of the embryo. Eventually, two separate fetuses develop.
Spontaneous division of the zygote into two embryos is not considered to be a
hereditary trait, but rather a spontaneous or random event.
Monozygotic twins may also be created artificially by embryo splitting. It can
be used as an expansion of IVF to increase the number of available embryos for
embryo transfer.
Incidence
Monozygotic twinning occurs in birthing at a rate of about three in every 1000
deliveries worldwide, regardless of race.
The likelihood of a single fertilization resulting in MZ twins is uniformly
distributed in all populations around the world. This is in marked contrast to
DZ twinning, which ranges from about six per thousand births in Japan (almost
similar to the rate of MZ twins, which is around 4–5) to 15 and more per
thousand in some parts of India and up to 24 in the US, which might mainly be
due to IVF (in vitro fertilisation). The exact cause for the splitting of a
zygote or embryo is unknown.
In-vitro fertilization techniques are more likely to create twins. Only about
three pairs of twins per 1,000 deliveries occur as a result of natural
conception, while for IVF deliveries, there are nearly 21 pairs of twins for
every 1,000.
Genetic and epigenetic similarity
Monozygotic twins are genetically identical (unless there has been a mutation
during development) and they are almost always the same sex. On rare
occasions, monozygotic twins may express different phenotypes (normally due to
an environmental factor or the deactivation of different X chromosomes in
monozygotic female twins), and in some extremely rare cases, due to
aneuploidy, twins may express different sexual phenotypes, normally due to an
XXY Klinefelter's syndrome zygote splitting unevenly.
Monozygotic twins have nearly 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 human twins
ranging in age from three to 74 showed that the youngest twins have relatively
few epigenetic differences. The number of epigenetic differences between MZ
twins increases with age. Fifty-year-old twins had over three times 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. This phenomenon illustrates the
influence of genetics in many aspects of human characteristics and behaviour.
Phenotype similarity
Monozygotic twins are almost always the same sex and their traits and physical
appearances are very similar but not exactly the same.
Monozygotic twins look alike, although they do not have the same fingerprints
(which are environmental as well as genetic). As they mature, MZ twins often
become less alike because of lifestyle choices or external influences. The
children of MZ twins would test genetically as half-siblings rather than first
cousins.
Half twin
The extremely rare half twins or semi-identical twins are twins that inherit
exactly the same genes from their mother but different genes from their
father. The exact mechanism of their conception is not well understood, but
could theoretically occur in polar body twinning where sperm cells fertilize
both the ovum and the second polar body.
Degree of separation
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 the pregnancy. The
timing of this separation determines the chorionicity 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. 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 means twins are dizygotic
(non-identical). But if monozygotic twins separate early enough, the
arrangement of sacs and placentas in utero is indistinguishable from
dizygotic twins.
Predisposing factors
The predisposing factors of monozygotic twinning are unknown.
Dizygotic twin pregnancies are slightly more likely when the following factors
are present in the woman:
She is of West African descent (especially Yoruba)
She is between the age of 30 and 40 years
She is greater than average height and weight
She has had several previous pregnancies.
Women undergoing certain fertility treatments may have a greater chance of
dizygotic 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,
increasing the likelihood of multiples.
Delivery interval
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:
Within 15 minutes: 75.8%
16-30 minutes: 16.4%
31-45 minutes: 4.3%
46-60 minutes: 1.7%
Over 60 minutes: 1.8% (72 instances)
The study stated that 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 the second twin.
Complications of twin pregnancy
Conjoined twins (or the deprecated term "Siamese twins") are monozygotic twins
whose bodies are joined together during pregnancy. This occurs where the
single zygote of MZ twins fails to separate completely, and the zygote starts
to split after day 12 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 the brain, heart or liver.
Chimerism
A chimera is an ordinary person or animal except that some of their parts
actually came from their twin or from the mother. A chimera may arise either
from monozygotic 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 coloration in human chimeras. A chimera may be
intersex, composed of cells from a male twin and a female twin. In addition,
in certain cases the person or chimera may have two sets of DNA.
