We have already known the different types of twins that may exist and what are the chances of having twins. As we have learned, the creation of twins is the result of a complex process of cell division, which in case of failure can lead to rare cases of twins.
In the first place we will talk about rare cases of monozygotic or univithelial twins, that is, those that result from the fertilization of a single ovum and a sperm forming a zygote that multiplies its genetic load to divide into two fetuses.
Although it is very rare, when the segmentation is not correct, genetic errors such as those described below occur.
Among monozygotic or univiteline twins (identical twins):
Parasitic twin or "Fetus in fetu"
It occurs in one of every 500 thousand births and occurs due to a genetic error during fertilization of the ovule. The segmentation of the two cells that will give rise to fetuses does not occur as it should, causing one embryo absorbs the other during growth.
A stunted twin that has not been formed is found inside the other twin, benefiting from fetal circulation, like a parasite. It is extremely rare to occur, but in most cases it is diagnosed in the first years of life (89% before 18 months) and even during pregnancy by ultrasound.
Generally, it is diagnosed as a tumor (called teratoma) composed of embryonic tissues that is inside the body of its twin brother.
Evanescent Twin
The evanescent or missing twin syndrome has already been explained in the blog a few months ago. It is a twin pregnancy in which one of the fetuses does not develop properly during the early stages of pregnancy and ends up disappearing inside the uterus.
The causes for which one of the fetuses does not develop and is absorbed by the uterus without a trace are unknown, but research has shown that the phenomenon certainly occurs.
Twin trapped or suspended (fetal transfusion syndrome)
Also known as transfused twin syndrome occurs in twins that share placenta (univiteline or monochorionic).
Is a great hemodynamic imbalance and occurs when the blood of one of the twins (donor) passes to the other twin (recipient). They have an unequal size. One is smaller than the other, suffers delayed intrauterine growth, dehydration and anemia, sometimes being wrapped in its amniotic membrane as if it were "trapped."
For its part, the larger one suffers from higher blood pressure and may develop heart failure.
Siamese or United Twins
It occurs in one of every 200 thousand births when twins are born united Somewhere in their bodies. Normally, embryo division occurs within the first 13 days after fertilization, but when it occurs late, after the thirteenth day, division is incomplete causing babies to develop coupled or fused. When the division later occurs, more organs will share.
Depending on the part of the body that joins them, they are classified into: thoracophages (joined by the sternal region, pyrophogous (joined by the back), ischiopagos (joined by the pelvis) and craniopagos (joined by the head).
The cause that causes an incomplete division is foreign to the genes, that is, it is not inherited nor will it be transmitted to the children. Depending on the connection between the two brothers, the possibility of separating them through surgical intervention is assessed. The operation is very complex but we have already seen in the news several cases of Siamese brothers who have been successfully separated.
Identical twins but of different sex
Monozygotic or univiteline twins (identical twins) are the product of the union of an ovum and a sperm that is then divided into two, so they share the genetic load and are of the same sex.
A rare exception to this rule is 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.
Since 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 (It occurs only in women and is characterized by the total or partial absence of the X chromosome). Girls who suffer from this syndrome, which affects one in 2,500, are much shorter than normal, do not reach sexual maturity and cannot have children.
So far we have talked about rare cases of monozygotic or univiteline twins, but there are also rare cases when we talk about bicogotic or biviteline twins, that is, when two ovules are fertilized by two spermatozoa independently, but they develop at the same time inside the mother's womb. Rare cases may occur depending on how and when fertilization occurs.
Among bicigotic or biviteline twins:
Superfetation
It occurs when an egg is released by the woman while she is already pregnant (something that is very rare). If the woman has sexual intercourse and it turns out that that ovum is fertilized by a sperm, it results in two gestations at the same time, that is twins of different gestational age.
Superfecundation
When two ovules are fertilized by two sperm from two different men. To make it clearer, the woman releases an egg, has sex with a man and one of her sperm fertilizes the woman's egg. In the same cycle he returns to ovulate, has sexual relations with another man and one of his sperm fertilizes the second ovum of the woman giving rise to a pregnancy of twins of different father They share the womb.
As you can see, although it is not usual, much less, sometimes twin gestations with exceptional characteristics can occur giving rise to rare cases of twins like the ones we have described.
