In 2021, Deonna Fletcher, who goes by “Dee,” gave birth to three girls. But during Fletcher's first ultrasound, there was just one baby. A couple of weeks later, a follow-up ultrasound revealed the addition of twins who were conceived six days after their sister.
Another mom, Rebecca Roberts, conceived her twins weeks apart. She was shocked when a second baby was found during the 12-week ultrasound, measuring three weeks younger than the baby doctors knew about. Roberts had to be induced due to problems with the younger baby’s umbilical cord and went on to give birth to son Noah at 33 weeks and daughter Rosalie at 30 weeks.
And after three painful miscarriages, Cara Winhold had two babies that doctors said were aged a week apart due to the second baby’s noticeably smaller size. According to Winhold, the math adds up. She had an ovulation tracking app, so she knew exactly when the first baby — who appeared solo in the womb during the five-week ultrasound — was conceived. "Then [my husband] and I had sex the following week when we assumed my ovulation was over," she told Insider.
The phenomenon of getting pregnant while already pregnant is known as superfetation. But what is it, what causes it, and is it something pregnant mothers should be concerned about? Here's what experts say.
What is superfetation?
“Superfetation is a very rare phenomenon in which a woman becomes pregnant a second time with another fetus while already pregnant with one,” says Dr. Samir Babayev, an infertility and reproductive surgery specialist at Mayo Clinic and co-author of Mayo Clinic Guide to Fertility and Conception, 2nd Edition: Expertise From Leading Fertility Specialists for Maximizing Reproductive Health and Growing Your Family. “In essence, two pregnancies occur in parallel, albeit at different stages of gestational development.”
Basically, superfetation is when a “pregnancy occurs in an already pregnant woman,” says Amy Beckley, a PhD scientist and founder of the fertility diagnostic company Proov. “The babies have different gestational ages because the second baby was conceived a few weeks after the other one.”
As with all pregnancies, superfetation is detected via ultrasound, which can “identify the age differences between the fetuses,” says Babayev, “as well as their different sizes and stages of development.”
How rare is superfetation?
All the experts who spoke to Yahoo Life say superfetation is extremely rare in humans.
“One study estimated that less than 10 cases in humans are described in the literature,” says Dr. Mary T. Jacobson, an ob-gyn and chief medical officer at the telehealth provider Alpha. “The hypothesis is that a small discordance in gestational age between dizygotic twins [or, conceived from different ova and therefore not identical] in humans exists very early in pregnancy.”
Dr. Erkan Buyuk, a reproductive endocrinologist, infertility specialist and ob-gyn who practices at RMA of New York, notes that "there are reports of suspected superfetation occurring in IVF pregnancies.” But while Roberts was taking fertility medication, neither she, Fletcher nor Winhold reported using assisted reproductive technology.
“It’s so rare that there is controversy surrounding whether it can even happen naturally in humans,” says Dr. Alan Gaffney, an intensive care physician and co-founder of the health app Tell. “A second fetus noticed on an ultrasound scan may have been missed during the first scan. Two fetuses of different sizes are more likely to be as a result of a twin pregnancy than two fetuses of different ages.”
What can be associated with superfetation?
According to triplet mom Fletcher, she menstruated two or three times a month. But because superfetation is so rare, “it is practically impossible to demonstrate an association” with other conditions, such as more frequent periods, says Buyuk.
In Fletcher’s case, “the fact that she has two or three periods a month signals that her ovaries are firing off more often than they should,” says Beckley. “This normally would not happen in an average woman.”
Babayev agrees, noting that “hormonal imbalances or irregular menstrual cycles could theoretically make it more likely” to have a superfetation pregnancy.
How does superfetation happen?
To get pregnant while already pregnant, you would need to ovulate again — which “doesn’t happen because the high levels of progesterone hormone during pregnancy prevents ovulation,” says Buyuk. “In multiple pregnancies suspected to be the result of superfetation, it is thought that ovulation occurred in the next cycle following the establishment of pregnancy, and the sperm was able to travel to the [fallopian] tubes past the pregnancy and fertilized the ovulated egg. Then the embryo that formed traveled back to the uterus and implanted, despite the presence of an established pregnancy.”
Jacobson says that the difference in gestational age between fetuses could also be due to a luteal out-of-phase (LOOP) event occurring one to three weeks after the original pregnancy. “The LOOP-like event may allow passive sperm transport from the vagina, through the uterine cavity, to the fallopian tube in proximity to where the ovary extruded an egg for the second fertilized embryo to happen.”
Is there a way to avoid superfetation?
“Once a woman becomes pregnant, the body makes hormones that prevent any further pregnancies,” says Gaffney. So most women don’t need to worry.
But to fully prevent the possibility, you would need to use your usual contraception methods, says Babayev, or simply avoid having intercourse after your pregnancy is confirmed, notes Beckley. She adds, “Then, if a second egg was released, it would not be fertilized.” But, again, this is an extremely rare occurrence.
How does childbirth work if you have superfetation?
When superfetation happens, the “usual guidelines are followed as if it is a regular twin pregnancy,” Buyuk explains.
“Typically, the babies are close enough in age that they can both develop fully and be healthy at birth,” says Beckley. But the mom would be “at a higher risk for a C-section as twin pregnancies are at a higher risk.”
Additionally, “your health care provider may recommend a planned C-section to increase the chances of a healthy delivery for both babies without complications,” says Jacobson. This typically happens because “one fetus matures more rapidly than the other,” and thus, “the younger fetus may be born prematurely.”
How are children affected if they are a result of superfetation?
“Children born as a result of superfetation are usually considered to be at higher risk for complications like preterm birth, given their different gestational ages,” says Babayev. “However, with proper prenatal care, many grow up to be as healthy as their peers.”
There is “no long-term follow-up of this condition” due to how rare it is, Buyuk adds.
However, since superfetation is treated like a typical twin pregnancy, Jacobson notes that “twin pregnancies are associated with higher risks than singleton pregnancies” for conditions such as:
Preterm labor and birth
Congenital birth defects
While there's still debate over reports of superfetation in humans, Babayev reiterates that the condition is incredibly rare. But if it happens, “it typically requires a highly specialized level of care to ensure the health and well-being of both mother and fetuses," he adds.