When an ultrasound is performed to confirm pregnancy, it is time to also know the number of babies that are on the way, which will significantly influence pregnancy controls, childbirth and, of course, the lives of children and children. parents.
Not all multiple pregnancies are the same, but depending on the number of fertilized eggs and when the zygote divides, there are different types of twins .
Dizygotic or bivitelline twins (twins)
They represent 70% of twin pregnancies. They are the result of two ovules fertilized by two sperm that have formed two zygotes. They are two different pregnancies that develop at the same time. Each one implants independently, in its gestational sac with its own placenta and its own amniotic sac.
The development of dizygotic or bivitelline twins is usually greater than that of univitelline twins.
For this to happen, there must have been two ovulations in the same menstrual cycle, or two eggs from the same ovary, or one egg from each ovary.
Being the fertilization of two eggs and two sperm, the genetic load is different, therefore, children may or may not be of the same sex . For the same reason, they are not identical either, being what we know as twins. Of every 100 pregnancies of twins, half are of the different sex, and of those of the same sex, half are two boys and half two women.
Monozygotic or univitelline twins
It is less frequent than the previous one. They represent 30% of twin pregnancies and their incidence is approximately one pair of twins every 250 births. Unlike the previous ones, the zygote is formed by the union of an ovum and a sperm that after fertilization divides to create two embryos. The division of the zygote is considered as a fortuitous event, a “biological accident” that does not influence race, mother’s age or inheritance.
Identical twins
Babies that emerge from this fertilization carry the same genetic information, which is why they are always of the same sex, which is known as identical twins, which although not completely identical, their physical similarities are usually very large.
Depending on when the zygote is divided, we will have different types of monozygotic or univitelline twins .
- 1) Bichorial and biamniotic : when the zygote division occurs within three days after fertilization. Each embryo grows within its own amniotic sac and feeds on its own placenta.
- 2) Monochorial and biamniotic : when the zygote division occurs between the fourth and the seventh day after fertilization. They share the same placenta but each grows in its own amniotic sac. They represent 90% of monozygotic twin pregnancies.
- 3) Monochorial and monoamniotic : these are very rare cases. It happens when the zygote division occurs late, between the 7th and the 13th. day after fertilization. They share the placenta and the amniotic sac, the only thing that divides the embryo. Complications tend to be more frequent in this type of pregnancy.
This last group of monochorial and monoamniotic twins includes the pregnancies of Siamese or united twins, which occurs when the zygote division occurs after the 13th day after fertilization. The division is incomplete and they are united by some part of their bodies (thoracophagus, joined by the thorax; pyropagos, joined by the buttocks and craniopagus, joined by the head)
Triple multiple, quadruple, quintuple…
When more than two fetuses develop in pregnancy they are known as triplets, quadruplets, quintuplets, sextuplets and there are even exceptional cases such as the septuplets in Egypt and the famous octuplets in California.
Types of triplets
Within the triple pregnancy there are in turn three types depending on the number of fertilized eggs.
1) It may have been the fertilization of a single ovule that divided into two and one of them then divided again into two. They are known as identical triplets, because they share the same genetic material and of course, they are of the same sex.
2) Two ovules fertilized by two sperm and one of them divided into two, resulting in three children. The two children born to the same egg and sperm share genetic material and would have the same sex (what is usually called twins), while the other may be of a different sex (what is often called a twin).
3) Three ovules fertilized by two separate sperm (tricygotic twins). As in the case of dizygotic twins, each has its own genetic load, its own amniotic pouch and placenta. They look like any brother with the difference that they are born at the same time.
Types of quadruplets
The quadruple pregnancy is the result of fertilization of two eggs which in turn were divided into two, or 3 eggs, two independent and one which in turn is divided into two or more strange if we talk about natural fertilization, of four independent ovules fertilized by four sperm in the same cycle.
Thus, as the number of embryos resulting from the most exceptional divisions increases, they are pregnancies. We could continue with five, six or more.
The combinations can be many, but I hope that this classification of the types of twins can help to clarify some doubts about twin pregnancies and multiple births.