Multiple births is a condition that describes a woman’s pregnancy more than a fetus in the same pregnancy. Different names are given to the multiplicity process according to the number of offspring resulting from them. The birth of two or three children is the most common pattern of multiple births, the two that are called sequentially: bipolar twins and trigeminal twins. These two cases of multiple births and other cases occur in varying degrees in most animal species, although the term is often used with placental species.
Brothers are the product of multiple births, either monogamous or bisexual. The first type is produced from one fertilized egg or one zygote that is split into two or more embryos, each carrying the same genetic material. The identical twins are often called twin siblings. Because in the case of multiple identical births, the same genetic material is shared, most of which are mostly of the same sex.
However – and in rare cases – the fertilized egg carries an additional sex chromosome. A fertilized egg or egg can have one of the following chromosomal structures: xxx, xxy or xyy. When the zygote is divided into xxy, it leads to the birth of twins, one of which carries the chromosome xx, the other carries the chromosome xy, and the twin is identical in all but sex. (Fraternal twins), they are the product of multiple eggs that reached maturity and was released by the ovulation of women in the menstrual cycle itself. They are then fertilized and then transformed into multiple embryos that are no more than genetic similarities, such as the similarity between ordinary siblings.
Multiplicity of polygamous births is a combination of two types of twins: non-identical and identical. For example, triplets can be twins that result from a single egg and a third sibling from a second egg. Bilateral twins can be considered the most common forms of multiple births. Most placental species give birth to a multiplicity of normal and familiar births, resulting in the phenomenon known as litter or pupal belly.