Superfetation: Can You Get Pregnant While Pregnant?

Just when you thought your early-pregnancy worry list couldn’t get any longer, enter: superfetation, or the possibility of getting pregnant while already pregnant.

Superfetation is an extraordinarily rare event, but it is technically possible. There have been a few superfetation stories in the news in recent years. In 2021, for example, Rebecca Roberts of Wiltshire, England, gave birth to “super twins” on the same day who were conceived three weeks apart. “It was really, really shocking to be told there were two babies instead of one,” Roberts, who used fertility medication, told Good Morning America. “Then they told me there was a three-week size difference between the two babies that the doctors couldn’t understand.”

So, can you get pregnant while pregnant? As real people have attested, yes—but rest assured that the odds of having “superfetation twins” are extremely slim. Read on to find out all there is to know about the incredibly mind-boggling phenomenon that is superfetation.

Key Takeaways

• Superfetation is the concept of getting pregnant while you’re already pregnant. It’s incredibly rare—but entirely possible.
• Research says that there have only been 14 documented cases of superfetation. However, experts say it’s becoming slightly more common due to the use of assisted reproductive technology.
• Superfetation can occur if a woman continues to ovulate after getting pregnant, or two eggs are released during ovulation—but one of them is fertilized later than the first.
• Some cases of superfetation can result in ectopic pregnancy for one of the embryos. Other risks are babies growing at different rates and preterm birth.

Can You Get Pregnant While Pregnant?

Getting pregnant while already pregnant, also known as superfetation, is incredibly rare, but possible. There are different possible scenarios for how this remarkable phenomenon can occur. Kelli Burroughs, MD, ob-gyn and department chair at Memorial Hermann Sugar Land Hospital in Sugar Land, Texas, describes one: “Typically, after a woman conceives, the body naturally stops ovulating. In very rare cases, ovulation continues after a woman becomes pregnant.” A second scenario is that, in some cases, two eggs can be released during ovulation, which would normally result in a typical fraternal twin pregnancy. However, in the case of superfetation, the second egg essentially hibernates and then implants much later than the first.

What Is Superfetation?

In short, superfetation is an exceedingly rare event that “occurs when an egg is ovulated and fertilized, and the resulting embryo implants during an ongoing pregnancy,” says Sasha Andrews, MD, a board-certified maternal-fetal medicine specialist at Pediatrix Medical Group in Denver. In most cases of superfetation, there’s a two-to-four-week difference in the embryos’ gestational age, per Cleveland Clinic.

Superfetation vs. twins

Superfetation is different from a twin pregnancy because of the timing of fertilization. With a twin pregnancy, both eggs are fertilized at the same time, while in superfetation, there’s a gap between when the first egg and second egg are fertilized. Andrews elaborates, “This is different than a twin pregnancy in that twins are conceived at the same time, either from a fertilized egg that splits (identical twins) or from two eggs that are fertilized at once (fraternal twins).”

Superfetation vs. superfecundation

Superfetation involves two embryos that form during two separate menstrual cycles, says Cleveland Clinic. Superfecundation—an even rarer occurrence—happens when sperm from separate acts of intercourse (and possibly even from separate partners) reach two separate eggs during the same ovulation period.

How Common Is Superfetation?

So, when it boils down to it: Can you get pregnant while pregnant? The answer is yes—with a big caveat. Superfetation is so rare that the chances of it happening to you are practically nonexistent. In a March 2024 case report, researchers listed that there have been only 14 documented cases in recorded history. However, the odds of superfetation are growing ever so slightly higher in modern times. According to Andrews, “it’s becoming more common with assisted reproductive technologies such as in vitro fertilization.” In fact, in a 2010 case report, researchers asserted that most superfetation cases have occurred after assisted reproductive technologies.

Are There Symptoms of Superfetation?

“There are no unique symptoms of a superfetation pregnancy,” says Burroughs. “Patients may experience no symptoms at all or typical symptoms of pregnancy including nausea, breast tenderness and weight gain.”

How Is Superfetation Diagnosed?

Superfetation is challenging to diagnose in early pregnancy because the symptoms can appear the same as any other pregnancy, and it’s so rare that it’s not something doctors routinely check for. Andrews illustrates a scenario in which atypical symptoms would be present: “Some cases of superfetation have resulted in a heterotopic pregnancy, where one embryo implants within the uterus, and the other is an ectopic pregnancy within the fallopian tube. In these cases, symptoms such as abdominal pain or vaginal bleeding can occur.” Ectopic pregnancies are never viable.

Burroughs explains what would be required for a superfetation diagnosis: “Diagnosis of superfetation would include review of the menstrual cycle history and an ultrasound. The most common presentation of a superfetation pregnancy is confirmation of a pregnancy followed by confirmation of an additional pregnancy, on a different date, with significant size difference.”

What Are the Risks of Superfetation?

According to Cleveland Clinic, the odds are good that you’ll have a healthy pregnancy with superfetation. Your provider may recommend a planned C-section to increase the chances of a safe and healthy delivery.

There are some risks to look out for with superfetation, however, including ectopic pregnancy, which can be life-threatening if left untreated. Look out for symptoms such as abdominal pain and swelling, as well as dizziness and shortness of breath.

There are also risks if both fetuses are within the uterus. Burroughs explains that superfetation “can lead to asynchronous growth of the fetuses during pregnancy. This means that while one baby may be fully developed and ready for birth, the other may still be in an earlier stage of development, putting the younger fetus at risk of premature birth.” Additionally, any twin pregnancy is more likely to lead to preterm birth, says Andrews.

Preterm birth can cause a host of issues for baby, including “difficulties with breathing, low birth weight, challenges with movement and coordination, feeding difficulties and serious complications such as brain hemorrhage and neonatal respiratory distress syndrome, which is a breathing disorder stemming from underdeveloped lungs,” says Burroughs.

In Roberts’ case, doctors initially told her the younger twin—a girl named Rosalie—may not survive since she wasn’t growing properly. But after 95 days in the neonatal intensive care unit (NICU)—and a shorter NICU stay for her brother Noah—both babies came home.

“When we lay them down next to each other, it’s like they instantly know and they reach out and touch each other’s faces and it’s just the most beautiful thing,” Roberts told Good Morning America. “Twins have an amazing bond anyways but the story between these two, when they’re old enough to find out, they’ll feel even more special.”

To Sum It Up

Superfetation occurs so rarely that it’s probably not something you really need to have on your radar. However, if you got pregnant with the help of assisted reproductive technologies such as in vitro fertilization (IVF) and your mama senses tell you something’s amiss, it might be worth bringing up superfetation to your healthcare provider. Or, on the very, very off chance that one of your ultrasounds reveals one more fetus than there used to be, you’ll know what to suspect.

Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.