I periodically re-watch Jaws, and that animatronic fish gets me every. single. time. When the great white shark takes down that kid and there’s all that blood in that water, all I can think is how nothing would be scarier than starting my period mid-swim. It’s a thought that bleeds (hah) directly into one of the wackiest sea myths out there: whether or not you should you be extra worried about the threat of…sharks (and, okay, other potential dangers—like ruining your bikini, or sinking like a rock because your cramps are a mess) when it comes to swimming while on your period.
The confusion conjures that cult-beloved Anchorman line about bears being able to smell menstruation, and expert intel leads me to believe the conceit of this shark question—about period blood attracting sharks—should be regarded with humor in equal measure. Read: Even if the sharks could smell the menstruation, they’re not extra interested in you because of it.
“There is no medical reason a woman should avoid water or swimming while having a period,” says sexual medicine specialist, Dr Serena McKenzie, ND, IF, NCMP and Medical Advisor for Rory. Another related period factoid to keep in mind? Women simply do not produce enough blood while menstruating to attract sharks. I know that sounds crazy on days when your vagina feels like the elevator from The Shining, but it’s true. Research shows that on average, a woman loses only 60 milliliters of blood during her period, which only measures out to about a shot glass and a half.
“There is no medical reason a woman should avoid water or swimming while having a period.” —sexual medicine naturopath Serena McKenzie, ND
Maybe that’s less volume than you would have assumed, but, hey, blood’s blood—and it’s a bloody mess. “Most women want to avoid a mess in their swimwear, of course, and usually a tampon can provide this protection.” Dr. McKenzie adds. This has never personally been an issue for me because I only wear pads and am more than happy to opt out of swimming. But if you, like me, aren’t the monthly mascot for Team Tampon, you could certainly still take a dip, because you don’t technically need to wear anything in the ocean, pond, or lake water to plug things up. Water doesn’t necessarily stop your flow, but the upthrust and gravity change can, for the most part, temporarily stifle things. (But forces like jumping and coughing may lead to some blood coming out onto your suit and into the water.)
The main health concern to consider regarding getting in water during your period actually has nothing to do with blood. “If you are prone to severe cramping, I wouldn’t suggest swimming alone if you could start cramping and be unable to swim,” Mary Jane Minkin, MD, clinical professor of obstetrics and gynecology at Yale University, tells me.
So ultimately, if it’s your heart’s desire to free-bleed in the ocean, go for it, because when it comes to swimming on your period, the only real danger is the status of your new white one-piece.
Leave it to medical dramas to highlight some of the rarest health conditions out there (Grey's Anatomy or ER, anyone?). While some of the issues these shows highlight seem far-fetched, many are based in reality, as in, healthcare experts have seen cases of it before. One such condition? Getting pregnant while you're already pregnant. It's a phenomenon called superfetation1, and while extremely rare—a 2008 study recorded only 10 cases worldwide2—it is totally real.
board-certified OB/GYN, reproductive endocrinologist, and director of education at the Fertility Centers of Illinois.
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Curious as to how this happens? Read on to learn how double pregnancies occur and how to potentially prevent them.
Why superfetation pregnancy happens
Before we get into how superfetation happens, let's cover some of the basics of conception. First, an egg gets released from an ovary (female reproductive organ) during ovulation—a few days during the menstrual cycle when you're most fertile. Then, if you have unprotected sex (i.e., without a condom or birth control method) with a partner with a penis, their sperm can fertilize your dropped egg, and voilá—conception occurs. The combined sperm and egg will form an embryo, which implants into the lining of the uterus and grows into a fetus.
With superfetation, this entire process happens twice, meaning the second embryo joins the first one in the uterus.
For most people, ovulation stops while pregnant, meaning the uterus won't allow another embryo to implant when there's already one in there, says Allison K. Rodgers, MD, a board-certified OB/GYN, reproductive endocrinologist, and director of education at the Fertility Centers of Illinois. In fact, the hormonal changes your body goes through during pregnancy almost ensures this scenario doesn't happen. These changes including the following:
Your hormones stop your ovaries3 from releasing another egg, making ovulation super unlikely.
A mucus plug also forms in your cervix, blocking sperm from reaching your uterus.
When superfetation occurs, a new pregnancy circumvents these natural safeguards. But medical experts are still not exactly sure why or how this ends up happening.
How common is it to get pregnant while pregnant?
“It is exceedingly rare,” Dr. Rodgers says. Only about 10 or so cases of superfetation have ever been recorded in humans, though it is more common in other species like rodents and other small mammals.
“Some even question if it is a real phenomenon in humans,” Dr. Rodgers says. A more likely scenario is that superfetation gets confused with another reproductive issue. For example, if you’re pregnant with twins and one is developing slower, it might seem like the second implanted later, she says. But in reality, they both implanted at the same time—one’s just growing at a different pace.
At the end of the day, you have “less than one in 100 million” chance of experiencing superfetation, Dr. Rodgers says. Put simply, your odds are practically zero. (But it's still fascinating to learn about!)
Are certain people more likely to experience it?
Because only a handful of superfetation has been documented, “it’s hard to determine” whether there’s a pattern of risk factors, Dr. Rodgers says. That said, many instances of superfetation have involved some form of assistive reproductive technology (ART), such as intrauterine insemination5 (IUI) or in vitro fertilization (IVF). In other words, people receiving fertility treatments may be unaware that they are already pregnant, resulting in two (or multiple) embryos at once.
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That said, medical researchers cannot completely confirm the link between superfetation and fertility treatments. Even with ART, your chances of superfetation are extremely low, per Dr. Rodgers. You'll just want to make sure you're checking in with your fertility specialist regularly and taking pregnancy tests to ensure you're not already pregnant before treatments.
Another (unconfirmed) theory suggests that an ectopic pregnancy—when a fertilized egg implants outside of the uterus, usually in the fallopian tube—could slightly raise the risk of superfetation. However, more medical research is needed to confirm this connection. Dr. Rodgers does point to one case study6 of superfetation which reported an ectopic pregnancy at 10 weeks and a "normal" pregnancy at 5 weeks. But again, this is extremely rare and the exact biological processes behind it are largely unknown.
If you're concerned about your risk of superfetation or pregnancy complications with ART, let your healthcare provider know. They can help offer guidance and support.
Does it mean you’re having twins?
Technically, no, superfetation does not mean you're having twins. Even though both embryos grow together in the same uterus and will probably be born at the same time, they weren’t conceived in the same menstrual cycle (if they had been, that's a phenomenon called superfecundation). Since each fetus was conceived during separate menstrual cycles, they are at different stages of pregnancy, with one being a few weeks older than the other.
Are there any potential risks or complications?
According to Dr. Rodgers, there are a few potential complications and risks with superfetation. For one, "when the uterus is in labor," both fetuses would be delivered. But the problem is, if one fetus develops faster than the other, the younger one might be born prematurely, Dr Rodgers says.
When an infant is born early—before week 37 of pregnancy—it doesn't have enough time to fully develop certain organs (like the heart and lungs, for example), which can cause serious health problems and developmental delays. If this is the case, the premature infant would need special care in the neonatal intensive care unit (NICU) at the hospital.
In the cases of superfetation, a healthcare provider may recommend a planned Cesarean section (C-section) birth, rather than vaginal birth, to increase the chances of a smooth delivery for both infants.
How to prevent pregnancy while pregnant
It's not likely that you'll have to worry about superfetation when you're expecting. “This is not a typical thing,” Dr. Rodgers says. “It’s not in books, and we don’t learn about it in medical school,” she adds. In other words, superfetation is so rare that, while interesting to learn about, it won't likely happen to you. (You already have enough going on when you're expecting and prepping to be a new parent!)
That said, if you're feeling uneasy about having sex while pregnant, or are worried about other potential pregnancy complications, reach out to your healthcare provider for advice and support. They can help come up with ways to have more comfortable, and safe, sex while pregnant—such as abstaining from penetration or wearing condoms. Providers can also run tests to ensure you're receiving proper care for any pregnancy side effects you may have.
Tarín, Juan J et al. “Unpredicted ovulations and conceptions during early pregnancy: an explanatory mechanism of human superfetation.” Reproduction, fertility, and development vol. 25,7 (2013): 1012-9. doi:10.1071/RD12238 ↩︎
Pape, O., et al. “Superfœtation : à propos d’un cas et revue de la Littérature.” Journal de Gynécologie Obstétrique et Biologie de La Reproduction, vol. 37, no. 8, Dec. 2008, pp. 791–795, https://doi.org/10.1016/j.jgyn.2008.06.004. ↩︎
Claudia Main, Xinyue Chen, Min Zhao, Lawrence W Chamley, Qi Chen, Understanding How Pregnancy Protects Against Ovarian and Endometrial Cancer Development: Fetal Antigens May Be Involved, Endocrinology, Volume 163, Issue 11, November 2022, bqac141, ↩︎
Roellig, Kathleen et al. “Superconception in mammalian pregnancy can be detected and increases reproductive output per breeding season.” Nature communications vol. 1,6 78. 21 Sep. 2010, doi:10.1038/ncomms1079 ↩︎
Lantieri, Teresa, et al. “Superfetation after ovulation induction and intrauterine insemination performed during an unknown ectopic pregnancy.” Reproductive BioMedicine Online, vol. 20, no. 5, May 2010, pp. 664–666, https://doi.org/10.1016/j.rbmo.2010.01.017. ↩︎
Hassani, Karim Ibn Majdoub et al. “Heterotopic pregnancy: A diagnosis we should suspect more often.” Journal of emergencies, trauma, and shock vol. 3,3 (2010): 304. doi:10.4103/0974-2700.66563 ↩︎
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