Yes, You Can Get Pregnant If You're Already Pregnant—Here's How It Happens

While it's incredibly rare, you can theoretically get pregnant while you're pregnant. Here's what to know about superfetation.

Medically reviewed by Andrea Chisholm, MD FACOG

Have you ever wondered if a person could get pregnant when they're already pregnant? This is theoretically possible, and there's even a technical term called "superfetation." That said, it's so incredibly rare that most people shouldn't worry about it (even if they're not using any form of birth control while expecting).

We reached out to experts to learn why some people can conceive if they're already pregnant, including how common it is and whether you'll notice any symptoms.

Related: Your Pregnancy Symptoms Week by Week

Can You Conceive While Pregnant?

The technical term for getting pregnant while already pregnant is superfetation. And yes, it's something that can happen. But it's so uncommon that most doctors and reproductive specialists have never seen it.

"Superfetation in humans is possible, but it is extremely rare," says Sasha Andrews, MD, board-certified maternal-fetal medicine specialist at Pediatrix Medical Group in Denver, Colorado. "I have never seen it in my practice nor heard of any local cases." 

Kelli V. Burroughs, MD, chief of staff and department chairman of obstetrics and gynecology at Memorial Hermann Sugar Land Hospital, also emphasizes how exceptionally rare superfetation is and likewise has never seen it in her practice.

Katie Sagaser, MS, LCGC, a genetic counselor at Juno Diagnostics, characterizes superfetation as "incredibly rare in humans" but possible. She has witnessed one case, but it was only a possible case. "In the past decade, I've only seen one instance in which one of our fertility clinic patients was suspected to have a superfetation diagnosis in her twin pregnancy," she shared.

How Common Are Double Pregnancies?

Superfetation hasn't been widely studied, but a 2021 case report notes that only 10 cases have been recorded in literature. It's actually more common in other animals besides humans, and for some species may be a type of adaptation and means of reducing the reproductive burden in female animals.

In humans, superfetation is more common when assisted reproduction is used. "The use of assisted reproductive technologies such as in vitro fertilization (IVF) can make superfetation a little more likely, but still very rare," says Sagaser. "In theory, if a person has unprotected intercourse and becomes [naturally] pregnant shortly before their embryo transfer, the possibility of superfetation exists."

However, she notes, this is unlikely for several reasons, including the fact that patients are generally cautioned against having unprotected intercourse while on IVF hormones.

Related: 5 Signs You Might Be Pregnant With Twins

How Is Superfetation Possible?

Usually, after you've conceived, your hormonal make-up changes, making it impossible for you to ovulate and conceive again. This is a natural process, says Dr. Burroughs. But in very rare cases, ovulation continues after a person becomes pregnant, which can lead to superfetation.

"Superfetation occurs when after the start of a pregnancy, a second egg (or ovum) is fertilized and implants, resulting in two babies with different gestational ages," says Dr. Andrews. In most cases of superfetation, there are several weeks between conceptions. As Dr. Andrews notes, there was one reported case of superfetation back in 1999 where there was a four-week difference between the two babies.

Again, it's very rare for superfetation to occur naturally or spontaneously. "Superfetation more commonly happens in the setting of assisted reproduction, or fertility treatment, with medications that cause the release of multiple ova," describes Dr. Andrews.

Are There Signs of a Superfetation Pregnancy?

There's no real way to know that you're experiencing superfetation. "There are no unique symptoms of a superfetation pregnancy," says Dr. Burroughs. "Patients may experience no symptoms at all or typical symptoms of pregnancy including nausea, breast tenderness, and weight gain."

Most of the time, superfetation is diagnosed via an ultrasound during pregnancy, says Sagaser. Usually a disparity in sizes between the two fetuses alerts the provider to the fact that superfetation may be at play. "Based on the handful of superfetation case reports, the growth difference with superfetation can be anywhere from two to four weeks; this is probably due to differences in the frequency of ovulation," she says .

It's possible that a diagnosis of superfetation doesn't happen until the second or third trimester of pregnancy, says Sagaser. "For example, if you have a viability ultrasound at six to eight weeks gestation (counting from the date of your last menstrual period) which confirms a viable intrauterine pregnancy in line with those dates, the smaller (younger) twin may not yet be visible by ultrasound," she describes. "If your next ultrasound doesn't take place until the 20-week anatomy sonogram, or perhaps later, there really would be no way to know about superfetation until that time."

Sometimes the diagnosis of a superfetation pregnancy doesn't happen until after the baby is born—sometimes not for years, says Dr. Andrews. "Interestingly, there was a case of superfetation diagnosed by home genealogy testing, after a set of twins in their 50s discovered they had different fathers," says Dr. Andrews.

What's the Difference Between Superfetation and Twins?

Sometimes superfetation pregnancies are referred to as a type of twin pregnancy. But superfetation isn't the same thing as twins—either fraternal or identical. "Twin pregnancies occur either when one ovum is fertilized and splits to form two embryos [identical twins], or when two eggs are fertilized simultaneously by different sperm cells [fraternal twins]," says Dr. Andrew. "Superfetation occurs when an embryo or fetus is already implanted, and a second ovum is fertilized and implants."

Related: What Are the Costs of a Multiple Pregnancy and Birth?

The Bottom Line

Keep in mind that superfetation is super unusual and it's highly unlikely that you would be experiencing it, even if you're having unfamiliar symptoms or you're concerned about when and how your conception took place. If you have further questions about superfetation, or any question about the progression of your pregnancy, don't hesitate to reach out to a doctor or midwife.

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