Health Pregnancy

Superfetability or getting pregnant in pregnancy

Superfetation is a rare phenomenon that can lead to a woman becoming pregnant while she is already pregnant. We explain why it can happen.

Superfetability or getting pregnant in pregnancy

Superfetation is a very rare and quite surprising phenomenon. It consists in that a woman, being pregnant, becomes pregnant again in the same gestation.

The term comes from Latin. Super means ‘about’ and fetus, ‘brood’. That is, ‘conceive again’. Few cases have been described, although in 2016 what happened to Jessica Allen became relevant.

Assisted reproduction may be related to this event and hence its incidence may increase. Therefore, in this article we explain everything you need to know about superfetation and how it happens.

What is superfetation?

Superfetation, as explained by the Royal Spanish Academy (RAE), is defined as the conception of a second fetus during pregnancy. That is, it occurs when a woman becomes pregnant while already pregnant with a first baby.

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, very few cases have been described in humans.

In fact, it is considered very unusual. Most of the cases that have occurred in recent years were related to assisted reproductive techniques. Similar to what happens with multiple pregnancies.

Superfertilization (or multiple pregnancy) consists of the fertilization of two eggs simultaneously. Is not the same. That is, it is about two eggs that have been released in the same menstrual cycle. Superfetation involves two eggs from different cycles.

Superfetability or getting pregnant in pregnancy

Between superfetation and superfertilization there are temporal differences and the participation of different menstrual cycles.

How is it produced?

For superfetation to occur, a series of events must be set in motion simultaneously. The first thing is that, during pregnancy, ovulation occurs again. This does not happen regularly, since the reproductive system of women has mechanisms to prevent it from happening.

As an Assisted Reproduction publication 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, the usual thing is that it is blocked and a new pregnancy is impossible. However, in superfetation, this event does not take place and another ovum that can be fertilized appears.

Progesterone is also responsible for thickening the cervical mucus. When the mucus in the cervix thickens, sperm cannot pass through it and cannot reach the egg. However, this mechanism also fails in superfetation.

Fertilization of the new egg usually occurs between 2 and 4 weeks after the first. So there is a difference in the gestational age of both fetuses. Also, sperm can come from different men.

In this way, the woman could be pregnant with two babies from different parents at the same time. Most of the cases of superfetation, as we have pointed out, have appeared associated 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 pregnancy

Superfetation tends to be confused with superfertilization, also called multiple pregnancy. It is important not to interpret these types of gestation as the same.

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

That is why in multiple pregnancy it is very difficult for there to be different parents. Superfetation is an asynchronous multiple gestation that takes place at different times. As there is usually a period of time between 2 and 4 weeks between the two, one fetus will be smaller than the other. Here is the main reason why it should not be confused with a multiple pregnancy.

There are some data that can help differentiate both phenomena. In addition to fetal size, there are differences in heart beat and other structures, such as cranial length or yolk gallbladder.

How is it detected?

The diagnosis of superfetation is usually made early. It is usually detected in the first trimester ultrasound, which is carried out around 12 weeks of gestation.

In this ultrasound, the presence of two embryos can be observed, each one in its own bag and with its own placenta. Also, there is a difference in the development and size of both.

It is common for a multiple pregnancy to have a mismatch between the weight and size of both babies. However, in superfetation, differences in organ development can be seen. This does not usually happen in superfertilization.

Childbirth is high risk

The delivery of women who experience superfection is more risky than a single baby delivery. In multiple gestations it is also considered high risk. The reason is that both fetuses are born at the same time, whether the vaginal route is chosen or by cesarean section.

To be vaginally, both babies must be positioned properly. Their weight and size also have an influence. We must bear in mind that, in superfetation, there is a difference in gestational age, which complicates the procedure.

Therefore, it is possible that one is born with 35 weeks (premature, but with a great chance of survival) and another with 32 weeks of gestation (with complications to survive abroad). The younger the baby, the more likely it is to suffer from pathologies, since his organs are not fully developed.

For this reason, in some cases an attempt is made to delay deliveries. That is, keeping the youngest fetus in the womb for as long as possible. This increases the probability of survival.

Superfetability or getting pregnant in pregnancy

Prematurity of one of the babies is one of the most latent risks in superfetation, since it is difficult to delay delivery or differentiate it for each fetus.

Cases of superfetation

As we have explained throughout the article, few cases of superfetation have been described. The first was registered in 1932. From that moment on, about 10 more were discovered in the 20th century. Currently, it is estimated that there are 1 or 2 a year.

The most famous was in 2016. The mother was named Jessica Allen. She was a woman who was undergoing a surrogacy. That is, she was pregnant with a baby that she was going to deliver to a couple with fertility problems.

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

Superfetation is very rare

Superfetation is a phenomenon that consists of a woman becoming pregnant while she is already pregnant. The reason it happens is a hormonal disturbance. For this reason, fertilizable eggs are produced again despite carrying a baby.

The result is two twin babies who have different gestational ages. It is a very rare situation, but one that is important to differentiate from the usual multiple pregnancy. However, both carry risks. Therefore, it is important that these types of situations are strictly followed by a specialist.