Understanding Pregnant with twins

 What is the difference between twins and twins? This is perhaps the most repeated question when talking about multiple pregnancies, which are more and more frequent.

Although many people use the two terms interchangeably, twins and twins are not the same. In this article we tell you how they differ and we clear up another very common question: is having twins or twins hereditary? 

Am I pregnant with twins or twins?  

Understanding the difference between twins and twins is very simple. It depends on how the embryos that will give rise to each individual are formed. Fertilization takes place when a sperm (which contains genetic information from the father) joins an egg (which contains genetic information from the mother). When this happens, a zygote is created, which, after numerous cell divisions, gives rise to an embryo, whose genetic information comes half from the mother and half from the father. 

When two different eggs are fertilized at the same time by two different sperm, we speak of twins. Therefore, we find two separate zygotes that will give rise to two different embryos, with unique genetic information in each of them. Sometimes other terms such as non-identical twins or dizygotic twins are used to refer to twins. 

In the case of twins, a single ovum is fertilized by a single sperm and, very early in embryogenesis, the zygote divides, giving rise to two embryos. As both come from the same zygote, that is, from the same sperm and the same egg, their genetic information is identical, therefore they are also known as identical twins or monozygotic twins. Within the twins we differentiate:

Monoplacental and biamniotic monozygotic twins: Twins have a single placenta with two independent amniotic sacs.

Mono-placental mono-amniotic twins: Twins have a single placenta and pouch.

Monozygotic diplacental biamniotic twins: Twins have two placentas and two independent bursae.

On rare occasions it may happen that the division of the zygote that gives rise to the two identical embryos is not completely completed, giving rise to Siamese brothers, that is, twins joined by some part of their body. This happens very rarely. The incidence is estimated to be 1: 50,000 to 1: 100,000 pregnancies and, in most cases, they are not compatible with life.  

Twins and twins, are they genetically the same?

As we have just seen, twins and twins originate in different ways, the main difference being that twins are genetically identical and twins are not. 

Twins: genetically distinct

Twins, formed from a different egg and sperm, do not share the same genetic information. Therefore, we can say that twins are like two brothers that have originated at the same time and will not be identical. The twins can be of the same sex or not , since as you will know from other articles on our blog, the chromosome that determines sex is provided by the sperm. If a twin has two X chromosomes (XX), it will be a girl, while if it has one X and one Y chromosome (XY), it will be a boy. Are you interested in reading more about this topic? Then we recommend our article The probability of having a boy or a girl according to genetics.

Twins: genetically identical

Twins, by forming from a single egg and a single sperm, do share the same genetic information. That is why there is talk of identical twins. Although it is true that, with the passage of time, the twins are differentiating themselves more and more. How is it possible if they share the same genetic information? Well, the environment comes into play here. Although genes determine things like height, freckles, eye color, and even predisposition to some inherited genetic diseases , genes are also regulated by the environment. That is, the expression of genes is modified depending on our lifestyle. So that,Although twins share identical genetic information, the expression of their genes may be different, which makes them increasingly different over time. 

The exceptional case of semi-identical twins

Did you know that there is a type of semi-identical twins? These are the so-called sesquizygo twins, discovered a few years ago.

As we have seen, identical twins are the result of the zygote bipartition that forms after a single sperm fertilizes an egg. But why don't more sperm get into the egg?

The answer lies in the genetic endowment. As we have mentioned, all of our cells have two copies of each chromosome, with the exception of the gametes, that is, the egg and the sperm. In this way, when the zygote is formed, it will have the two necessary copies of each chromosome. If more than one sperm entered the egg, the zygote would have too much genetic material and would be biologically unviable. For this reason, once a sperm manages to fuse with the egg, the entry of the rest of the sperm is immediately blocked.

But as in everything, sometimes there are exceptions. A few years ago, the existence of twins was demonstrated in which a single egg had been fertilized by two sperm. These twins share the same genetic information as the mother (since they come from a single egg) but the father's is not exactly the same (since they come from two different sperm). For this reason, they are not identical as in the case of twins, but they share a higher percentage of genetic information than twins.

Is it true that having a multiple pregnancy is hereditary?

It is popularly thought that if a woman has a history of twins or twins in her family, the probability of a multiple pregnancy is higher. But what is the reality in this belief? 

In the case of twins, several families have been described where cases of identical twins are more frequent than expected, although there are not enough studies in the field of genetics to establish a hereditary factor. 

As for twins, there are certain factors that increase the probability of having them:

• Advanced age of the mother: due to hormonal imbalances associated with age, a woman can release two eggs in the same cycle instead of just one, which increases the likelihood that two eggs will be fertilized at the same time.
• Fertility treatments: delaying the age at which women have children means that in many cases they must resort to in vitro fertilization, since after the age of 35 the chances of becoming pregnant begin to decrease. When this type of treatment is used, more than one embryo is frequently implanted in the mother's uterus to increase the chances of a pregnancy, although this practice is less and less frequent. For this reason, the numbers of dizygotic twin pregnancies have increased in recent years.
• Genetics: a study published in the American Journal of Human Genetics revealed the existence of two polymorphisms that increase a woman's chances of having twins. 

We hope this article has helped you clear up your doubts about the differences between twins and twins. If you are pregnant, single or twin, congratulations!