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They are called Gemini two or more siblings born in a multiple birth, that is, from the same gestation of the mother, may or may not be identical.
By extension, children born from triple, quadruple or more births are also called twins. Although there is no accurate statistics, it is estimated that one in 85 pregnancies is twin. There are two ways to be born twin brothers.
The bivitelino twins are dizygotic or multiviteline, ie, they are formed from two eggs. In this case two oocytes II are produced and both are fertilized, thus forming two embryos. They are almost always formed in different placentas and do not divide the amniotic sac.
Fraternal twins are not very similar to each other, may or may not have the same blood factor, and may or may not be of the same sex. They are also known as different twins. In fact they are two ordinary brothers who had coincident gestation. They represent 66% of all twin pregnancies, and in this type of pregnancy, 1/3 have different sexes, while 2/3 the same sex.
One in a million twins of this type have different colors even from the same parent. It is possible for fraternal twins to have completely different parents.
Brothers born from the same pregnancy and developed from two eggs that were released from the ovary simultaneously and fertilized in the same sexual relation (rule), but they can be conceived of different copulations, but of that same double ovulation. They may or may not have the same sex, differ both physically and in their genetic constitution and have two placentas and two independent and well differentiated membranes.
The frequency of dizygotic twins varies according to ethnic origin (maximum incidence in black, minimal in Asian and intermediate in white), maternal age (maximum when mother is 35 to 39 years old) and genetics, with a higher incidence of maternal and paternal genetic lines, even though parents may transmit predisposition to double ovulation to their daughters. In general, the overall ratio is two-thirds of dizygotic twins to one of monozygotic twins (ie, identical twins).
When an egg is produced and fertilized by a single sperm and splits into two complete cell cultures, it gives rise to identical, or monozygotic, or univiteline twins. Always have the same sex. Identical twins have the same genome, and are clones of each other. Only 1/3 of pregnancies are from univiteline twins.
Pregnancy is difficult because only 10% to 15% of identical twins have different placentas, usually have the same placenta.
Xipophagous Twins (Siamese)
Xipophagous twins, or Siamese, are monozygotic, ie formed from the same zygote.
However, in this case, the embryonic disc is not fully divided, producing twins that will be linked by a body part, or have a body part common to both. The embryo of xipophagous twins is then constituted of only one cell mass, being developed in the same placenta, with the same amniotic sac.
It is estimated that among 40 monozygotic twin pregnancies, one results in twins interconnected by complete non-separation.
In another type of xipophagous twins (now known to be more common), the union happens later, that is, they are identical identical twins that join at some stage of gestation by similar parts: head to head; abdomen with abdomen; buttocks with buttocks, etc. When we see news of twins who have been "separated" by surgery, it is almost always a case like this.
The term "siamese" it originated from a famous recorded occurrence of this phenomenon: the twins Chang and Eng, born in Siam, Thailand, in 1811, glued to the shoulder. They married, had 22 children and remained united until the end of their days, having died within 3 hours of each other.