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 prevents someone from getting pregnant while they’re already pregnant?
Once you’re pregnant:
- Your hormones change to prevent your ovaries from releasing another egg. Ovulating during pregnancy is highly unlikely.
- A mucus plug forms in your cervix, making it difficult for sperm to enter your uterus. Your cervix is the opening between your vagin* and uterus. If you were to have intercourse during pregnancy, sperm would have difficulty reaching your uterus and fallopian tubes, where fertilization takes place.
- Your hormones change to prevent an embryo from implanting in your uterine lining. Even if a second embryo were to form somehow, it’s unlikely that it would be able to begin growing in your uterus.
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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.
Example case: in vitro fertilization (IVF)
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.
Example case: artificial insemination
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.
Example case: surrogate
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.
What are the symptoms of superfetation?
There aren’t any pregnancy symptoms unique to superfetation.
