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The superfetation is a little common phenomenon that occurs when the conception of an embryo in a woman who is already pregnant occurs. They are therefore two babies with different gestational ages who, despite being born in the same delivery, are not twins.

Once conception occurs, the female reproductive system stops producing eggs because they are not needed. However, in cases of superfetation, ovulation continues even though the gestation period has begun. As a consequence, if that other released ovum becomes fertilized, the woman carries two babies with different gestational ages; they are generally about three weeks apart.

The process is the following. First, the woman ovulates during pregnancy. Second, if the woman has sexual intercourse, the most normal thing during the gestation period is that the cervical mucus makes it difficult for the sperm to pass. However, in superfetation they manage to break through until they reach the fallopian tubes. And, third and last, the second blastocyst implants properly in the uterus, where it gets the nutrients it needs for its development.

Causes of superfetation

The direct cause of superfetation has to do with ovulation during pregnancy, which is due to a failure in the blocking system of the female reproductive system . When a woman becomes pregnant, levels of the hormone progesterone rise dramatically, causing what is called an ovarian blockage. In this way the maturation of new follicles is avoided. Thus, this phenomenon happens when this process fails.

In addition, for it to take place, it is essential that the sperm are able to pass through the cervical mucus of the cervix, which has a higher density during the gestation period. It is also essential that the endometrium is prepared for the implantation of a new embryo without affecting the one that is already implanted.

Superfetation is more frequent when assisted reproduction treatments are performed , which is why cases are more common today. It is due to stimulation of the ovaries or what is called hyperstimulation syndrome.

Diagnosis of superfetation

In almost 100% of cases the diagnosis is made during a routine ultrasound during pregnancy. It is the gynecologist who, when doing the ultrasound, detects the presence of two placentas and two embryos with different degrees of development. It is a relatively simple phenomenon to diagnose since there is a difference in the size of both embryos.

Childbirth in case of superfetation

One of the main doubts of women with this type of pregnancies has to do with what happens at the time of delivery. Well, the truth is that it does not differ too much from a normal delivery. The only medical condition is that both babies must be born at the same time . For safety reasons for both the babies and the mother, these types of deliveries are performed in the vast majority of cases by cesarean section.

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Depending on the degree of development of the smallest baby, it is the gynecologist who decides the date of delivery.

Superfetation and multiple pregnancy

In no case should superfetation be confused with a multiple pregnancy since they are different conditions, despite the fact that in both the woman gestates several embryos.

The most notable difference is that in superfetation the fertilized eggs belong to a different menstrual cycle . Meanwhile, in multiple pregnancy, the eggs belong to the same menstrual cycle.

In addition, in multiple pregnancy both embryos have the same gestational age , while in superfetation they are not, since they have been conceived at different moments in time.

Possible complications

This type of pregnancy does not lead to any negative consequences or complications. The gestation period is carried out in the same way as a twin pregnancy. Therefore, the most important thing is to enjoy this moment to the fullest and, above all, to follow medical recommendations.

Known Cases

The first known case of superfeta- tion dates from 1932. Also in 1960, a Baltimore couple had what at first appeared to be twins. However, doctors found that one of the babies appeared to be premature. After examining the bones of both babies using diagnostic X-ray tests, they determined that the youngest baby had been conceived two months after his brother.

However, the vast majority of cases that have occurred globally have taken place in the 21st century due to the rise of assisted reproduction systems.

A study carried out by a team of Canadian researchers published in 2005 pointed to the case of a 32-year-old woman who underwent assisted fertilization treatment, due to which two embryos developed successfully. During the fifth month of gestation, the doctors detected the presence of a third fetus in her uterus, whose gestational age was three weeks younger than the other two.

Other cases that have been widely disseminated internationally have taken place in 2007, 2008 and 2016 in the United Kingdom, United States and Australia, respectively.


Superfetation is a more frequent phenomenon among some animal species, such as rodents or horses, for example. However, in recent years there has been a notable increase in human cases, the vast majority of which are related to hormonal or assisted reproduction treatments .

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Hello Readers, I am Nikki Bella a Psychology student. I have always been concerned about human behavior and the mental processes that lead us to act and think the way we do. My collaboration as an editor in the psychology area of ​​Well Being Pole has allowed me to investigate further and expand my knowledge in the field of mental health; I have also acquired great knowledge about physical health and well-being, two fundamental bases that are directly related and are part of all mental health.

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