Twins and twins, what is the difference?

It is one of those very frequent confusions, and it may be that until it touches you closely we will not know how to differentiate very well. 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 undoubtedly look alike, they are not the same. What difference is there 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” de la culta), the evolution of the meaning of 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, 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 has originated from the fertilization of the same ovum.
  • 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 has originated from the fertilization of a different ovum.

In other words, we mean by “twins” those identical or similar siblings of the same sex, because they come from the same ovum and the same sperm that fertilized it. On the other hand, twins are siblings with a lesser resemblance , because they have been gestated from a different egg and a different sperm, with which they can be of the same sex or not.

Monozygotic or identical twins (univitheline)

The word “monozygotic” that accompanies ‘twin’ gives us the clue: ‘mono’ means ‘one’ and ‘zygotic’ refers to the zygote. That is: they are twins that come from a single zygote or the same zygote, understanding 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. Afterwards, a cellular bipartition takes place and two different embryos originate, of the same sex, identical twins (only in extremely rare cases are there twins of different sex, as we will see). Therefore, the separation into two organisms occurs days after conception. It can be said that the identical twins, at first, were one.

And why are they so similar? Because cell division originates two genetically identical cells that develop apart. However, there are twin brothers who have a lesser resemblance and this is because, although they share a very similar genotype, their genes are not identical.

In addition, there are always intrinsic traits in the complexion of each: they receive unequal nutrition during pregnancy, epigenetics influence after birth … and there are other variations in the development of both that can change their appearances during growth.

Monozygotic or Identical Twin Types

Let’s now look at the different types of monozygotic or identical twins, since depending on the moment in which cell division occurs, they will have a different development:

  • The usual thing is that twins are monochorionic and bi-amniotic , that is, that each one develops in its own amniotic sac even though 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 form. These twins share the placenta but have individual amniotic sacs. They are usually 90% of monozygotic twin pregnancies.
  • Bichorial and bi-amniotic twins , less frequent: it divides within the first three days after fertilization. Each embryo grows within its own amniotic sac and feeds on its own placenta.
  • Monoamniotic twins are identical twins that develop within the same amniotic sac , share the placenta within the mother’s uterus, but have two separate umbilical cords. The division of the zygote happens late, from the seventh day after conception. Having monoamniotic twins is rare and carries common risks and complications (it happens about 1 in 50,000 pregnancies). This last group includes the strange phenomenon of “mirror twins” .

In some exceptional cases of monozygotic twin brothers it happens that the siblings share some organ or different extensions of the same body. This happens when cell division occurs beyond twelve days after fertilization and are known as Siamese twins 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 its chromosomes (46), including the XX (female) and XY (male) chromosomes while the other contributes only 45 with the Y chromosome or one of the X chromosomes missing. having a single X chromosome, the second twin will necessarily be a girl (as there is no Y chromosome) carrying a disease known as Turner Syndrome.

Twins or dizygotic twins (bivitelinos)

The word “twins”, as we have said, refers especially to those siblings that come from two embryos that have been fertilized separately That is why these siblings can be of different sexes. The word “dizygotic” refers to the fact that these siblings come from two (or more) zygotes and not from just one as in the previous case.

In these cases, gestation occurs simultaneously when two different eggs are fertilized by two different sperm (or more than two eggs per more than two sperm): this results in dizygotic individuals or twins.

Why does this multiplicity occur? Because during the ovulation period more than one egg is released, which can occur naturally (in this case, it is usually two eggs at the same time) and more frequently if the woman has undergone a fertility procedure (when the chances of more than two eggs are increased).

When these eggs are fertilized by different sperm, different zygotes begin to develop independently. That is why twins are not so much alike: they have different physical and genetic makeup. Furthermore, their placentas and membranes are different and separate.

It could even be the case that babies born in the same birth were to different parents, if one gamete was fertilized in intercourse and the other in a different sexual act.

Ultimately, we see that a multiple pregnancy can be engendered in two ways . The most common, 70% -75% of cases, is the product of two ovules fertilized by two sperm: they are dizygotic twins or twins. 25% -30% of cases of multiple births result from the division of the same zygote after fertilization: they are identical or monozygotic twins. In any case, double joy for dads!

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