Univitelline or bivitelline twins: what is the difference?


The news of a pregnancy twin It can produce some initial surprise followed by a double dose of happiness and multiple emotions, but it also usually generate many doubts. An ultrasound will determine if there is one baby or several. When it comes to two babies, there can usually be confusion, since it could be identical or fraternal twins.

The difference between both types of multiple pregnancies lies in the number of fertilized eggs or when the same zygote that is formed at the time of pregnancy has proceeded to divide to become two embryos. We analyze what these types of twin pregnancies are like.

Univiteline twins

In univitelline twin pregnancies, the zygote forms after union of a single egg with a sperm and this divides after fertilization creating two identical embryos. The identical twins share their genetic makeup so they are almost identical physically. It is a fact that occurs in very few cases, between 25%.

There are different types of univiteline twins depending on when the zygote divides:

  • Bichorial and diamniotic: occurs 3 days after fertilization. The embryos are nourished by a single placenta and each one has its own amniotic sac.
  • Monochorionic and monoamniotic: It occurs between the seventh and the thirteenth day and where the embryos share the same placenta and the same amniotic sac. It is when there has been a division of exactly the same cells.

Univiteline or biviteline twins

Interesting facts about identical twins

In this type of division, the case of "vanishing twin" where one of the embryos does not finish developing and ends up being absorbed by the mother, by her own twin, or even by the placenta. It has been observed that in the second trimester two fetuses appear and in the following subsequent ultrasound one of them has disappeared.

Another curious and very rare fact is the so-called “situs inversus” where babies are born as twins and the organs have been formed in the opposite way. They can even do opposite things in their lives (reverse psychology), while one is left-handed, the other is right-handed, or they can even sleep in reverse.

Biviteline twins

A biviteline pregnancy occurs when two ovules are fertilized by two different sperm, forming two zygotes in two different sacs. It usually occurs in 70% of twin pregnancies. Each egg implants in the uterus independently, inside your gestational sac and your own amniotic sac and placenta. This type of twins is the most common. The biviteline twins are commonly called Twins.

The chance of a twin pregnancy increases when two zygotes are implanted in the mother's uterus at the same time. There is a curious fact in the fraternal twins, since they can be of different sex. Of every 100 pregnancies with this characteristic, they are usually of different sexes or may coincide in being two men or two women.

case of twins

Are biviteline twins the same or different?

The twins have different genetic information so they are not physically identical. They can even be of different sex as we have reviewed. Their physical resemblance is comparable to that of two ordinary siblings, although there are cases in which they can also be born very similar, but they will not be mirror twins. These twins are also called fraternal or dizygotic twins.

How to know if they are univitelinos or bivitelinos twins?

Sometimes it is difficult to distinguish whether twins are bivitelline or univitelline. If the babies are of different sex, they are biviteline twins since the univitelino twins they are always of the same sex.

If the babies are the same sex, they will be twins univitelinos if they share the outer bag or both bags. This data can be formalized with routine ultrasounds and analyzing all these data.

In case of doubt the doctor performs A blood test to get the blood group. If the blood group is different they will be twins. If the results of the analyzes are not conclusive, the last option is to carry out a experiment of DNA.

Differences between identical and fraternal twins

In order to provide more curiosities between these two types of twin pregnancy, we will conclude with some clear differences that will be much more significant and conclusive.

The sex of the babies

Before birth, the sex of unborn babies can be detected. If the sexes are different, there is no doubt that they are twins. or fraternal twins. But if they are of the same sex there will be doubts as to whether they could be identical, since the twins can also be of the same sex. We would have to analyze by ultrasound whether or not they share the amniotic sac.

Univiteline or biviteline twins

Amniotic sac and placenta

In twin pregnancies, since each of the embryos is created with its own egg and sperm, they will logically be implanted independently in the uterus. In this way each will create their own amniotic sac and placenta.

In the case of identical twins there are cases that can be very different:

  • It may happen that the embryos they usually share the same amniotic sac and placenta, this usually occurs between days 7 to 13 after fertilization. But even if it happens, the future may have repercussions on their birth, since there may be the case of Siamese twins (when they are born physically united to each other) or the transfusion-transfused syndrome may occur.
  • At the time of division of the zygote, the embryos are formed and each one can develop their own amniotic sac and their own placenta.
  • Between the fourth or seventh day the babies they can have their own amniotic sac, but they share the same placenta. This is a situation that usually occurs in 90% of cases.

Other data that can make them stand out is when they are born and their physical appearance is explored. If they are clearly identical, that is when they were born with the same genetic makeup and therefore they are identical twins. On the other hand, they can be twins, but in this case they share 50% of the DNA and are clearly distinguishable, although there are cases in which they can look a lot like each other. They also do not share the same blood group.

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