It is one of those very frequent confusions, and we may not know how to differentiate very well until it is touched. And, although they have the same origin, they are terms that have ended up distinguishing themselves. Twins and twins, are they the same? Well, although they are undoubtedly alike, they are not the same. What is the difference between them?
Etymologically both terms have the same origin (Latin "gemellicium") and although historically both have been used interchangeably to refer to children born from the same birth (with the difference that "twin" was more typical of popular speech and "twin" of the cult), the evolution of meaning of the words has been accompanied by a differentiation. What is the difference between twins and twins?
If we consult the latest edition of the RAE dictionary, these terms differ, although with the nuance of "especially" (that is, that the first part of the definition is common and one does not necessarily exclude the other in its second part):
Twin the. From the Latin 'gemellus'. 1. adj. Said of a person or an animal: Born from the same birth as another, especially when it was caused by fertilization of the same egg.
Twin, za. From the vulgar Latin 'gemellicius', from 'gemellus', twin. 1. adj. Said of a person or an animal: Born from the same birth as another, especially when it was caused by fertilization of a different egg.
In Babies and more The exceptional case of semi-identical twins, girl and boy, first detected in pregnancyThat is, we understand by "twins" those identical brothers or how much they look alike, of the same sex, because they come from the same ovule and the same sperm that fertilized it. Instead, the twins are brothers with a minor resemblance, because they have been developed from an ovule and a different sperm, which can be of the same sex or not.
Monozygotic or identical twins (univitelinos)
The word "monozygotic" that accompanies 'twin' gives us the key: 'mono' means 'one' and 'zygotic' refers to the zygote. This is: they are twins that come from a single zygote or from the same zygote, by zygote the cell that results from the union of male and female sex cells and from which the embryo develops.
That is, a single egg is fertilized by a sperm. Then there is a cellular bipartition and two different embryos originate, of the same sex, the identical twins (only in extremely rare cases there are twins of different sex, as we will see). Therefore, the separation into two organisms happens days after conception. It can be said that identical twins, at first, were one.
And why are they so alike? Why cell division originates two genetically equal cells They develop apart. However, there are twin brothers who have a minor resemblance and this is because, although they share a very similar genotype, their genes are not identical.
In Babies and more The curious case of the German twins who were born three months apart and in different yearsIn addition, there are always intrinsic features in each other's complexion: they receive uneven nutrition during pregnancy, epigenetics influences after birth ... and there are other variations in the development of both that can change their appearances during growth.
Types of monozygotic or identical twins
Let's now look at the different types of monozygotic or identical twins, since depending on when the cell division occurs, they will have a different development:
Typically, twins are monocorial and bi-biotic, that is, that each one develops in its own amniotic sac although they share corno (the outer membrane of the two that surround the embryo and that gives rise to the placenta). This is because the division usually occurs between the fourth and seventh day after fertilization, before the amniotic sacs are formed. These twins share the placenta, but have individual amniotic sacs. They are usually 90% of monozygotic twin pregnancies.
Bicorial and bi-biotic twins, less frequent: it is divided within the first three days after fertilization. Each embryo grows inside its own amniotic bag and feeds on its own placenta.
Monoamniotic twins are identical twins that develop within the same amniotic sacThey share the placenta inside the mother's womb, but they have two separate umbilical cords. The zygote division happens late, from the seventh day since conception. Having monoamniotic twins is rare and carries with it risks and usual complications (approximately 1 in 50,000 pregnancies happens). The latter group includes the strange phenomenon of "mirror twins."
In Babies and more A baby is born in Colombia with her twin sister inside her abdomen: a strange case of "fetus in fetu"In some exceptional cases of monozygotic twin brothers it happens that the brothers share some organ or different extensions of the same body. This happens when cell division occurs beyond twelve days after fertilization and they are known as Siamese brothers or Siamese twins.
Another rare case of twins is one that produces siblings of different gender, that is, they will be identical twins, but one male and one female. It occurs when one of the twins contributes all of their chromosomes (46), including chromosomes XX (female) and XY (male) while the other contributes only 45 missing the Y chromosome, or one of the X chromosomes. If there is only one X chromosome, the second twin will necessarily be a girl (if there is no Y chromosome) that carries a disease known as Turner Syndrome.
Twins or dizygotic twins (bivitelinos)
The word "twins", as we have said, refers especially to those brothers who come from two embryos that have been fertilized separately. That is why these brothers can be of different sexes. The word "dizygotic" refers to these brothers coming from two (or more) zygotes and not from one as in the previous case.
In these cases, pregnancy occurs simultaneously when two different ovules are fertilized by two different sperm cells (or more than two ovules by more than two sperm cells): this results in dizygotic or twin individuals.
Why does this multiplicity occur? Because during the ovulation period more than one egg is released, which can occur naturally (in this case, they are usually two ovules at the same time) and more frequently if the woman has undergone a fertility procedure (when the chances of having more than two ovules increase).
When these eggs are fertilized by different sperm, different zygotes begin to develop, independently. That is why the twins do not look so similar: they have a different physical and genetic constitution. In addition their placentas and membranes are different and separate.
It could even be the case that babies born in the same birth were from different parents, if one gamete is fertilized in one intercourse and the other is in a different sexual act.
In short, we see that a multiple pregnancy can be engendered in two ways. The most frequent, 70% -75% of cases, is the product of two ovules fertilized by two sperm cells: they are Twins or dizygotic twins. 25% -30% of cases of multiple births result from the division of the same zygote after fertilization: they are identical twins or monozygotic. In any case, double joy for parents!
In Babies and moreProbabilities of having twinsPhotos | iStock
In Babies and more | Probabilities of having twins, Types of twins
