Twin

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 36 weeks). Since some premature births often have health consequence to the babies, twins birth are more often handled with special procedures than regular births.

Types of twins

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. The two eggs form two zygotes, and these twins are therefore also known as dizygotic.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Dizygotic twins are no more similar genetically than any siblings and develop in separate amnions, with separate placentae. They may have different genders or the same gender. However, twins of differing genders are always fraternal, as identical twins are always the same sex.

Related Topics:
Amnions - Placenta

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Identical twins

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. 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.

Related Topics:
Egg - Zygote - Embryo - Placenta

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 can develop where 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 always the same gender. They look alike, except that they are sometimes mirror images, but examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It is thought that identical twins do not run in families, but occur more or less randomly. The exact cause for the splitting of a zygote or embryo is unknown.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 can 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).

Related Topics:
DNA - Epigenetic modification

~ ~ ~ ~ ~ ~ ~ ~ ~ ~