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SUPERFETATION: What women should know about getting pregnant while already pregnant


Superfetation is an extremely rare event that involves getting pregnant while you are already pregnant.

Your body goes through changes that make it nearly impossible for you to become pregnant while a fertilized egg (embryo) is already growing inside your uterus.

With superfetation, a new pregnancy bypasses your body’s natural barriers that prevent a new pregnancy while you’re pregnant.

These barriers are so effective that it’s improbable that superfetation could occur naturally. Almost all known cases of superfetation involve assistive reproductive technology (ART).

Are superfetation twins?

No, but superfetation pregnancies are similar to twin pregnancies in some ways. Like twins, two fetuses eventually share the same womb and develop alongside each other. Both babies are usually delivered at the same time.

Unlike twins, embryos from superfetation don’t form during the same menstrual cycle. As a result, they’re different gestational ages (in different stages of pregnancy). The embryo that was conceived first will mature ahead of the embryo that was conceived second.



How common is superfetation?

Superfetation is so rare in humans that there are only about 10 confirmed cases. They’re so unique that when they happen, they often make headlines. The chance that you’ll be affected by superfetation is close to zero.

What happens in superfetation?

For superfetation to take place without assistive reproductive technology (ART), you’d go through two menstrual cycles, back-to-back. Each cycle would result in a pregnancy. The scenario would go like this:

During your first cycle, your body releases an egg, you have intercourse, and then the egg gets fertilized and becomes an embryo. The embryo implants in your uterine lining and begins to grow. During your next cycle the same events take place, and the new embryo joins the other one in your uterus.

This scenario is highly unlikely because of the changes in your body that prevent a new pregnancy once you’re already pregnant.

What causes superfetation?

Superfetation is so rare that researchers don’t have enough data to confirm its causes. There are only a handful of these pregnancies documented. Most of them involve assistive reproductive technologies that bypass some of your body’s barriers to back-to-back pregnancies. Even with ART, superfetation isn’t likely

How is superfetation diagnosed?

Superfetation can be difficult to diagnose because it may resemble other conditions involving twins. Your provider will notice two or more fetuses during a routine imaging procedure to check on your pregnancy. One fetus will appear further along in its development (a different gestational age) than the other.

How is a superfetation pregnancy managed?

If one fetus matures more rapidly than the other, the younger fetus maybe be born prematurely. Your healthcare provider may recommend a planned C-section to increase the chances of a healthy delivery for both babies without complications.

How can I reduce my risk of superfetation?

Superfetation is an interesting phenomenon to know about, but it’s not a condition you should associate with risk or prevention. You probably won’t experience superfetation.

What is the difference between superfetation and superfecundation?

Superfetation involves two embryos that form during two separate menstrual cycles. With superfecundation, your body releases two or more eggs during the same menstrual cycle. Each egg is fertilized by sperm, which can be from the same or a different partner or donor.


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