Pregnancy is often thought of as a closed chapter once it begins — one egg, one sperm, one baby (not forgetting the superheroes that are parents of multiples!). But there’s a fascinating and rare phenomenon that challenges this idea: getting pregnant while already pregnant. It sounds like something out of a medical drama, but it’s real, though incredibly uncommon.
It’s called superfetation, and it happens when a second egg is released and fertilized after someone is already pregnant. So yes, technically, it’s possible — but the chances are about as rare as winning the lottery twice. Let’s break down what this means, why it happens, and what doctors know about it.
What are the odds of getting pregnant while pregnant?
In simple terms: extremely rare. Superfetation has only been documented in a handful of medical case reports worldwide — fewer than 10 confirmed cases in medical literature. To put that into perspective, there are around 132 million births globally each year — meaning superfetation is essentially a once-in-a-lifetime medical event, with chances around 0.000008%.
Why is it so unlikely? Pregnancy itself usually shuts down the body’s ability to conceive again. Once implantation happens, hormones like progesterone rise sharply, stopping ovulation and thickening the cervix to block sperm from reaching the uterus. These changes are designed to protect the growing pregnancy and make “double pregnancies” nearly impossible.
Still, biology doesn’t always follow the rulebook (don’t we know it). In superfetation, the body somehow continues to ovulate after conception, leaving the door open for another egg to be fertilized. We still don’t fully understand why this happens, but it’s thought to involve rare hormonal or reproductive system quirks.
Can I get pregnant while being already pregnant?
Here’s the reassuring part: for almost everyone, the answer is no. Once a pregnancy begins, the hormonal changes essentially switch off the possibility of another conception. The odds of ovulating, having sperm reach that egg, and then successfully implanting while already pregnant are so small that doctors classify it as a medical rarity.
That being said, the stories of superfetation cases do pop up in the media because they’re so unusual — and fascinating. They don’t mean it’s something the average pregnant person needs to worry about.
Can you give birth and still be pregnant with another baby?
This is where it gets interesting. If superfetation happens, the second embryo implants days or even weeks after the first one. That means the two babies are technically not twins in the usual sense — they were conceived at different times.
When both pregnancies continue, the babies are usually delivered together, since it’s not practical (or safe) to deliver one baby and leave the other inside. Most reported cases of superfetation result in what looks like twins or triplets, with the younger baby being slightly smaller or less developed at birth.
For example, in 2021, doctors described a case of superfetation where the babies were born with a noticeable difference in size but were otherwise healthy.
What is superfetation?
Superfetation is the medical term for conceiving while already pregnant. It’s different from having fraternal twins, where two eggs are released and fertilized during the same cycle. With superfetation, conception happens in two different cycles.
Here’s how it would have to work:
Questions Women Are Asking
- The body releases another egg after pregnancy has already started.
- Sperm fertilizes this new egg.
- The embryo implants in the uterus alongside the first pregnancy.
For all of those steps to align, the hormonal changes that usually stop ovulation and sperm entry would need to “fail” in rare ways. That’s why it’s considered almost impossible.
What causes superfetation?
To understand how unlikely superfetation is, it helps to know that the body normally has three locks that keep it from happening again once you’re pregnant. The first is hormonal: when an egg is fertilized, rising levels of estrogen and progesterone tell your ovaries to stop releasing new eggs. The second is physical — the cervix creates a thick mucus plug that blocks sperm from getting through. And the third is environmental — the lining of the uterus changes to support the existing embryo, making it extremely difficult for another to implant.
For superfetation to occur, all three of these “locks” would have to fail at once, allowing ovulation, fertilization, and implantation to happen despite the ongoing pregnancy. It’s like trying to sneak into a house where the alarm is set, the doors are bolted, and the guard dog’s awake — the odds are not in your favor. This can only happen when hormonal signaling or uterine responses are disrupted in just the right (or wrong) way, which explains why confirmed cases are vanishingly rare in humans.
How is superfetation diagnosed?
Most of the time, it’s discovered accidentally during pregnancy scans. Doctors may notice one baby is significantly smaller than the other, but still healthy. This size difference can also be caused by growth restriction, so additional scans are usually done to confirm whether superfetation is the likely explanation.
Genetic testing can also help distinguish superfetation from other conditions by showing that both babies are developing normally but are at slightly different gestational ages.
When doctors suspect superfetation, they look closely at ultrasound markers that reveal differences in development — things like the size of each gestational sac, the crown-rump length of each fetus, and the stage of organ formation. These subtle discrepancies tend to become noticeable during the second trimester, when growth milestones are more distinct.
Before confirming superfetation, doctors will rule out more common explanations, such as twin-to-twin transfusion syndrome or placental insufficiency. In confirmed cases, each baby shows a normal growth pattern for its own gestational age — just offset by days or weeks — suggesting two separate conception events rather than one pregnancy with uneven growth.
Could superfetation be risky?
Like any multiple pregnancy, carrying more than one baby comes with higher risks, including preterm birth and complications for both the birthing parent and babies. In cases of superfetation, the second baby may be at a slight disadvantage because it had less time to develop before delivery.
That said, many reported cases have resulted in healthy outcomes, especially with modern prenatal care and close monitoring.
The main challenge is the developmental age gap. If one baby is several weeks behind, doctors must balance the needs of both — waiting long enough for the younger baby to grow while ensuring the older one doesn’t face distress or outgrow the womb. This often means more frequent ultrasounds, fetal monitoring, and sometimes hospital observation in the later stages of pregnancy.
Delivery decisions depend on how both babies are doing. In most cases, the pregnancy is timed to protect the smaller baby’s development while minimizing risks for the older one. C-sections are more common, since labor could pose extra stress when the babies differ in size and maturity.
Superfetation miscarriage
Because the second embryo implants into a uterus that’s already changing to support an existing pregnancy, there’s a higher chance it may not attach or grow properly. The risk of miscarriage or growth restriction is therefore slightly increased for the younger baby. However, with early detection and careful monitoring, many superfetation pregnancies can progress safely — even when the babies start at different points in development.
How does this differ from superfecundation?
It’s easy to mix up superfetation and superfecundation — the words sound similar, but they describe two very different situations.
Superfetation happens when a second pregnancy begins days or weeks after an initial pregnancy has already implanted. It requires ovulation, fertilization, and implantation to occur while a pregnancy is in progress — something human biology is designed to prevent, which is why it’s so rare.
Superfecundation, on the other hand, happens when two eggs released during the same ovulation cycle are fertilized by sperm from separate acts of sex. In extremely rare cases, this can even involve sperm from different partners, meaning the resulting fraternal twins can have different biological fathers.
The key difference: superfetation means two pregnancies starting at different times, while superfecundation means two fertilizations happening close together in the same cycle. Both remind us how complex — and sometimes surprising — human reproduction can be.
Superfecundation is a little more common — researchers estimate it occurs in around 2.4% of fraternal twins, compared to only a handful of confirmed superfetation cases in all of medical literature. Many superfecundation cases come to light through paternity testing, when twins share the same gestational age but have different DNA matches for the father. Understanding this distinction helps scientists and doctors better explain variations in twin development, paternity, and the incredible adaptability of human reproduction.
Why the confusion exists
Because superfetation sounds like something straight out of a medical mystery, it’s easy to see why people might think it’s happening to them — especially when early ultrasounds show one baby measuring smaller than the other. In reality, that size difference is often due to normal variation between twins or conditions like intrauterine growth restriction (IUGR) or vanishing twin syndrome, where one embryo stops developing early on.
True superfetation is vanishingly rare, but uneven growth in the womb isn’t. If your provider notices a size or developmental gap between babies, they’ll monitor both closely with follow-up scans to rule out complications. Most of the time, these differences even out or are explained by factors like placental positioning or blood flow — not a second conception. If anything feels unclear, it’s always worth asking your doctor to walk you through the findings and next steps.
Superfetation: More medical curiosity than reality
While it’s technically possible to get pregnant while already pregnant, it’s so rare that doctors still write about it as a medical curiosity. For almost everyone, pregnancy itself protects against another conception. The few cases of superfetation that do exist remind us of just how complex — and sometimes unpredictable — human reproduction can be.
Tassia O'Callaghan is an experienced women's health content writer and SEO content strategist, having written for brands like Peanut App Ltd, Scary Mommy, Inspire the Mind, Fertility Mapper, Tally Workspace, Fyxer, and Office Christmas. She is an advocate for realistic sustainable living, supporting small businesses (author of A-Z of Marketing for Small Businesses), and equity across all walks of life. Follow her on LinkedIn or TikTok, or see more of her work on Authory or her website.