Two children at once –

The emergence of life is one of the wonders of nature. In order for a new person to develop, it needs the mother’s eggs and a paternal sperm. The egg cells mature in the mother’s ovaries until they reach the fallopian tube through ovulation, where they can be fertilized by a male sperm. This creates the so-called zygote, a cell that now contains the father’s genome in addition to the mother’s DNA in its cell nucleus.

The fertilized egg moves down through the fallopian tube into the uterus to implant and grow. During this migration, the zygote begins to divide to form more and more connected cells, a composition of two, then four, eight, sixteen and more cells grow. After about seven days, the cells begin to specialize in different cell types. At this stage it is so-called blastocystmoves the group of cells from the fallopian tube to the uterus and settles there to continue growing.

Cellular color play

Fraternal twins are formed by two different fertilized eggs, while monozygotic twins are formed by only one fertilized egg. National Human Genome Research Institute / public domain

… or even two

Coincidentally, this group of cells can sometimes not “hold together properly” on the way to the uterus and divide into two different clumps of cells. This can happen right at the beginning, after the first cell division of the zygote, up to the blastocyst stage. In very rare cases also later. After this division, the two new cell clusters continue to migrate and grow separately from each other. In this way, two living beings develop from a fertilized egg cell, which therefore has identical genetic information. The result is identical, identical twins.

Bizygotic or dizygotic twins, on the other hand, occur when two different ova mature at the same time, are fertilized in the fallopian tube by different sperm, and then migrate independently to the uterus. Due to the different egg cells and sperm cells, these twins do not have identical genetic material, but are only as closely related as normal siblings.

Of Siamese and semi-identical twins

In very rare cases, when identical twins are created, the cell structure may divide “too late”, about twelve days after fertilization, and therefore can no longer be completely separated. As a result, these twins continue to develop together and are born “grown together”. In most cases, such Siamese twins are connected at the sternum, but there are also numerous other options such as a connection via the head, hips or tailbone.

As long as they do not share any vital organs, joined twins can be separated through surgery. But sometimes they share only one heart, lungs or brain, making separation impossible. Siamese twins have been around for as long as humans have been around. But they only got their name from a couple of twins born in Siam in 1811, which became known worldwide as a circus attraction.

Even rarer and an absolutely special case among the identical twins are the so-called semi-identical twins. Usually, due to a protective mechanism, egg cells are designed to be fertilized by only one sperm cell. This is the only way to ensure that the chromosome set is correct and that the zygote is viable. In semi-identical twins, on the other hand, one egg cell is fertilized by two sperm cells.

The zygote can only survive if it happens to divide in such a way that the chromosome set is correct again. Therefore, this twin form is extremely rare: so far, only two cases are known worldwide. These twin pairs have 100 percent the same genetic information from the mother when they were created from the same egg cell as normal identical twins, but have inherited different genetic information from the father due to the two sperm cells.

placental conditions
Twins can share the fetus and placenta in different ways. © Kevin Dufendach (2008) / CC-by-sa 3.0

A “WG” in the womb

When twins grow up in the womb, they need to share their protective and nurturing home. Various forms of prenatal “living community” may occur. Usually a baby grows in a fetal sac consisting of two layers of skin and is provided with a placenta. In contrast, Geminis can sometimes share these spaces and resources in different ways. However, this depends on their origin and development.

When twins each have their placenta and amniotic sac, this condition is called dichorion-diamniosis because the placenta arises from what is called chorion and amnion is the inner skin layer of the amniotic sac. This is how all fraternal twins and a third of identical twins grow up.

However, this is different for identical twins who later separate. By the time the separation occurs, the chorion, and rarely the amnion, has already begun to form. Depending on when the separation takes place, these components no longer occur twice, but only once for both twins before the separation. The most common of these are monocorion-diamniotic twins, which separate four to seven days after fertilization. These then share a placenta, but each develop their own amnion and thus their own amniotic sac. In one percent of cases, twins separate so late that the chorion and amnion are already developed, and the twins therefore have to share placenta and amniotic sac. Monokorionic-monoamniotic twins are formed.

These sharing relationships between the fetus and the placenta are examined in each twin pregnancy as they can carry different risks. A split placenta can lead to undersupply and poorer development of a twin. In the case of a monochorionic monoamniotic pregnancy, there may also be a risk of Siamese twins or umbilical cord wraps. Last but not least, this study can also provide information on whether the twins are monozygotic or dizygotic.

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