Superfetation is a rare event that involves getting pregnant a second time while you’re already pregnant. It’s so uncommon that cases of superfetation often make headlines. Your body does a good job preventing subsequent pregnancies once an embryo is developing inside your uterus. Your chance of experiencing superfetation is near zero.
Superfetation is an extremely rare event that involves getting pregnant while you’re 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).
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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.
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.
Superfetation is more common in other species, including rodents, small mammals and fish.
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.
Once you’re pregnant:
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.
One confirmed case of superfetation involves someone who became pregnant with twins through in vitro fertilization. With in vitro fertilization, your provider collects eggs from your ovaries, fertilizes them with a partner or donor’s sperm outside of your body, and then transfers the embryos to your uterine lining. Three weeks after the procedure, her provider discovered a third embryo. This one was conceived at a different time, without IVF.
Another case involves someone who received artificial insemination treatment while taking medications to stimulate her ovaries to produce more eggs. After they became pregnant through insemination, doctors discovered they were already pregnant. The first pregnancy was ectopic. An ectopic pregnancy is when an embryo doesn’t implant in your uterine lining. As a result, it isn’t a viable pregnancy.
A surrogate who was impregnated with another couple’s embryo through IVF discovered six months later that they were carrying a second embryo as well. The second embryo turned out to be their own biological child, conceived naturally after the first embryo was implanted.
There aren’t any pregnancy symptoms unique to superfetation.
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.
Some researchers believe that superfetation is a misdiagnosis of other conditions, including:
It’s also possible that an ultrasound error may lead to misdiagnosing a pregnancy involving twins as one involving a single fetus. Later, the error could be chalked up to superfetation when the second embryo is discovered.
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.
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.
The odds that you’ll have a healthy pregnancy are good. Most confirmed cases of superfetation involve twins that are only a few weeks apart. Your provider can suggest the best time for delivery to maximize the chance of a healthy delivery involving both babies.
There’s a single documented case of a surrogate who got pregnant by IVF only to learn six months later that they were pregnant with their own biological child as well. Your chances of having this happen are close to zero.
Most cases of superfetation involve a two- to four-week difference in the gestational age of the embryos.
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.
A note from Cleveland Clinic
The handful of cases involving superfetation highlight just how effectively pregnant bodies prevent a subsequent pregnancy from happening. Superfetation is nearly impossible in all pregnancies, including those involving assistive reproductive technology. Researchers are still exploring what causes superfetation in the rare event that a back-to-back pregnancy does occur. In the meantime, you shouldn’t worry about getting pregnant a second time if you’re already pregnant.
Last reviewed by a Cleveland Clinic medical professional on 01/06/2023.
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