Superfetation or Getting Pregnant While Already Pregnant

Superfetation is a rare phenomenon that can result in a woman becoming pregnant while already pregnant with another child. We'll explain why it may happen here.
Superfetation or Getting Pregnant While Already Pregnant

Last update: 19 September, 2021

Superfetation is a very rare and quite surprising phenomenon. It consists of getting pregnant while already pregnant.

The term comes from Latin. Essentially, it means ‘to conceive again.’ Few cases have been reported, although in 2016, what happened to Jessica Allen became quite relevant.

Assisted reproduction may have to do with the event and hence its incidence can be increasing. Therefore, in this article, we’ll explain everything you should know about superfetation and how it occurs.

What is superfetation?

Superfetation is the conception of a second fetus during pregnancy. That is, it occurs when a woman becomes pregnant with a second child while she’s already carrying another one. In short, she becomes pregnant while already pregnant.

The result is the development of two babies, twins, who have different gestational ages. Superfetation is relatively common in animals. For example, in rodents, sheep, or even horses. However, there are very few reports of cases in humans.

In fact, it’s very unusual. Most cases that have occurred in recent years had to do with assisted reproductive techniques. Similar to what happens with multiple pregnancies.

Superfecundation (or a multiple pregnancy) is the fertilization of two eggs simultaneously. It’s not the same. That is, it’s two eggs that have been released in the same menstrual cycle. Superfetation involves two eggs from different cycles.

A sperm fertilizing an egg.
Between superfetation and superfecundation, there are temporal differences and the involvement of different menstrual cycles.

How does it occur?

For superfetation to occur, a series of events must come into motion simultaneously. The first is that, during pregnancy, ovulation occurs again. This isn’t a common occurrence, as the female reproductive system has mechanisms to prevent this from happening.

As a publication in Assisted Reproduction explains, when a woman becomes pregnant, the concentration of progesterone increases. Progesterone is a hormone that prevents new follicles from maturing in the ovary.

In this way, it’s usually blocked and a new pregnancy is impossible. However, in superfetation, this event doesn’t take place and another egg appears that can be fertilized.

Progesterone also thickens the cervical mucus. When the cervical mucus thickens, sperm can’t pass through it and can’t reach the egg. However, this mechanism also fails in superfetation.

Fertilization of the new egg usually occurs 2 to 4 weeks after the first one. This is why there’s a difference in the gestational age of both fetuses. In addition, the sperm can come from different men.

Thus, the woman could be pregnant with two babies from different fathers at the same time. Most of the cases of superfetation, as we’ve pointed out, have had to do with assisted reproduction treatments. The reason is that all techniques require hormonal drugs that can affect the body’s normal procedures.

Differences between superfetation and multiple pregnancies

It’s easy to confuse superfetation with superfecundation, or a multiple pregnancy. However, it’s important not to interpret these types of pregnancy as the same.

In superfetation, the fertilized eggs come from different menstrual cycles. However, in a multiple pregnancy both eggs are released at the same time. That is, they come from the same menstrual cycle.

That’s why in a multiple pregnancy, it’s very difficult to have different fathers. Superfetation is an asynchronous multiple gestation that takes place at different times. Since there’s usually a time gap of 2 to 4 weeks between the two, one fetus will be smaller than the other. That’s the main reason why it’s important not to confuse it with a multiple pregnancy.

Some facts can help differentiate the two phenomena. In addition to fetal size, there are differences in heartbeat and other structures, such as cranial length or yolk vesicle.

How is it detected?

The diagnosis of superfetation usually takes place early on. That is, doctors usually detect it in the first-trimester ultrasound, which takes place at about 12 weeks gestation.

In this ultrasound, they can observe the presence of two embryos, each one in its own pouch and with its own placenta. In addition, there’s a difference in the development and size of both.

It’s common that, in cases of multiple pregnancy, there’s discordance between the weight and size of both babies. However, in superfetation, differences in organ development are also visible. This doesn’t usually occur in superfecundation.

You might be interested in: What Are the Causes of Prematurity?

Getting pregnant while already pregnant is a high-risk situation

The delivery of women who experience superfetation poses more risks than a single birth. In multiple gestations, it’s also high risk. The reason is that the mother must deliver both fetuses at the same time, whether vaginally or by cesarean section.

For vaginal delivery, both babies must be positioned appropriately. Their weight and size also play a role. We must take into account that, in superfetation, there’s a difference in gestational age, which complicates the procedure.

Therefore, it’s possible for one to be born at 35 weeks (premature, but with great chances of survival) and another at 32 weeks of gestation (making survival in the outside world difficult). The younger the baby is, the more likely it is to suffer from diseases, as its organs haven’t fully developed.

For this reason, in some cases, specialists make attempts to defer births. That’s to say, to keep the youngest fetus in the uterus as long as possible. This increases the probability of survival.

A premature baby in an incubator.
The prematurity of one of the babies is one of the most latent risks in superfetation. This is because it’s difficult to defer delivery or differentiate it for each fetus.

Cases of superfetation

As we’ve explained throughout the article, there have been few reports of cases of superfetation. The first dates back to 1932. Thereafter, about 10 more were discovered in the 20th century. Currently, scientists estimate 1 or 2 cases per year.

The most famous one took place in 2016. The mother’s name was Jessica Allen and she was a woman who was acting as a surrogate mother. That is, she was pregnant with a baby that she was going to give to a couple with fertility problems.

During that pregnancy, Jessica became pregnant by another man, her husband. Therefore, she gave birth to two babies who were biologically different. This led to ethical controversies and even reflections on the legal implications.

Getting pregnant while already pregnant is very uncommon

Superfetation is a phenomenon in which a woman becomes pregnant while already pregnant. The reason why it happens is a hormonal alteration. As a result, the body produces fertilizable eggs again although she’s already carrying a baby.

The result is twin babies of different gestational ages. This is a very rare situation, but it’s important to differentiate it from typical multiple pregnancies. However, both carry risks. Therefore, a specialist needs to follow this type of situation strictly.

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