On May 4 2019, the Facebook page “Facts that will blow your mind” shared the following meme about the ability of women to “get pregnant while already pregnant”:
Text superimposed on an image of a pregnant woman read:
Not only can a woman get pregnant while already pregnant, but can have the babies of two different men at once.
While the meme included no citation, the general concept described is real and known as “superfetation,” defined as the fertilization and implantation of a second ovum some time after the start of a pregnancy, resulting in two fetuses of different gestations.The accepted definition of the term does not necessarily include different fathers, and while it is not unheard of, it rarely occurs in humans.
A 1999 article [PDF] in an obstetrics and gynecology journal recorded a single case of the phenomenon. That research also observed that the first successful reporting of superfetation in humans likely occurred in 1992:
We report a twin pregnancy in which a 4-week discordance in size between the fetuses was identified in the first trimester. The discordance was maintained throughout the pregnancy, which was otherwise uneventful and resulted in the delivery of a live girl and boy.
Superfetation is the fertilization and implantation of a second ovum some time after the start of a pregnancy, resulting in two fetuses of different gestations. The occurrence is rare, and only a few human cases have been reported. The first reports were by Aristotle and Hippocrates, who speculated that the human uterus had two horns, one becoming pregnant initially and the other at a later date. Conclusive evidence of superfetation came from the case reported by Harris. We report a true case of superfetation, based on the early presentation, documented progress and postnatal findings. To the best of our knowledge this is the second case with early confirmation by ultrasound scanning of a 4-week discordance and discordant but normal growth velocities in both twins.
As of 2017, fewer than ten cases of superfetation had been reported globally. One such case was reported in 2010 in the course of artificial insemination, although only one fetus survived due to the presence of one ectopic embryo. Fertility treatments appear to be a common facet in modern recorded instances of superfetation, such as one reported in 2005:
A case report published in 2005 in the The Journal of Pediatrics, for instance, describe how a 32-year-old woman who had undergone embryo transfer later became pregnant with twins and one additional growth-discordant fetus. Initially, two out of the three embryos transferred to the woman’s uterus implanted themselves and a viable twin pregnancy was pronounced. The third embryo did not develop into a fetus. However, a triple pregnancy was discovered around five months later, when a third smaller fetus was seen by ultrasound. This fetus was deduced to be around three weeks younger than its siblings. This led to the conclusion that there had been a subsequent fertilization and implantation following the successful double implantation from artificial insemination.
In another 2016 case, a woman was being treated for a hormonal disorder and ovulated twice in the same cycle. In another case with a tragic twist, a surrogate mother inadvertently became pregnant with and was forced to surrender her own biological child alongside a baby conceived in the course of her surrogacy. Months later, the woman and her husband were reunited with the child after a fraught legal battle. Another case was reported in 2009, but it wasn’t clear if fertility treatment was involved.
The meme referenced a subset of the already-atypical phenomenon of superfetation, heteropaternal superfecundation. In those cases, twins are determined to have different fathers. A December 2018 Guardian item quoted a researcher familiar with the matter:
There are two ways through intercourse, according to Jason Kasraie, the chair of the Association of Clinical Embryologists. First, a woman can release two eggs at the same time. Since sperm can survive for a few days in the female reproductive tract, loitering in the corner of the womb and the fallopian tube, it would be possible to have sex with one father-to-be in advance of the egg being released, and another just after ovulation.
In the second scenario, the woman releases two eggs a few days apart but in the same reproductive cycle.
Either way, “It’s extremely uncommon,” Carroll says. “It all adds up to many rarities happening in the same cycle.” A sperm’s journey is arduous at the best of times. To have two successful candidates from two different men in a month when two eggs happen to be released…. Well, what are the odds?
Kasraie has found two studies of the incidence of heteropaternal superfecundation, both from the early 1990s. One author claims that one in 400 pairs of fraternal twins (those arising from two eggs) fits the description. The second author puts the figure at one in 13,000 cases of paternity. These sound like best guesses. “They do,” Kasraie agrees. He points out that we cannot know the true number, and research has been scant. Most cases come to light only when paternity is questioned and a DNA test carried out.
One possible case of heteropaternal superfecundation purportedly occurred in England in 2005. A DNA test performed on a set of twins suggested a likelihood of more than 99 percent that twin boys had different fathers — testing performed because of their mother’s involvement with two partners at the time of their conception:
Over a pint the two [possible fathers] cleared the air to ensure both could be part of the boys’ lives. Yet it wasn’t until the twins were one year old that [the mother and both men] were ready to take a DNA test to find out for certain that the boys had different fathers. A few weeks later, the answer arrived in the post. The test proved the twins did indeed have different dads, stating: “The results do not imply that the children Lucas Hilbrandt Nielsen and Marcus Hilbrandt Nielsen have the same biological father. The probability of this preclusion exceeds 99.99 per cent.”
A 2015 claim of heteropaternal superfecundation in New Jersey came to light in the course of a child support matter, and fertility experts weighed in on the possibility that such cases go unnoticed more often than not:
“Normally, when a woman ovulates, she’s only producing one egg, but that’s not always the case,” Dr. Cynthia Austin, a fertility specialist at Cleveland Clinic Women’s Health Institute, told CBS News. “Sometimes there are two.” When two sperm fertilize two eggs, fraternal twins are formed. However, since sperm can remain alive for several days inside the body, it is possible that sperm from two different men could fertilize two eggs during the same menstrual cycle. As a woman is fertile for five to seven days a cycle, “the intercourse with different partners can happen days apart,” Austin said.
DNA expert Dr. Karl-Hanz Wurzinger testified during the New Jersey case that T.M.’s twin daughters were fathered by two different men. He authored a 1997 study that estimated 1 in 13,000 paternity cases involved twins with separate fathers.
“Sometimes it’s very obvious,” Austin said, “if the father is one race and the babies are two different races.” However, there may be instances where both the mother and father are unaware that such an incident has occurred. “I would say the vast majority of times, twins with different fathers, it goes unnoticed,” she said.
Superfecundation is by all accounts an incredibly rare phenomenon, perhaps marginally more common thanks to the advent and popularity rise of fertility treatments. In most cases, fertility treatment has been a factor. Heteropaternal superfecundation, cases in which fetuses carried by the same mother at the same time have different fathers, is believed to be even less common. Once again, fertility treatments (such as that involving a surrogate mother) may have slightly influenced the likelihood of such an anomaly. Some fertility experts posit that naturally occurring cases of heteropaternal superfecundation may go unnoticed because DNA testing for paternity on twins is not routine, but confirmed cases of either phenomenon are few and far between.