If you are pregnant, is it possible to become pregnant again? It sounds impossible, but it can and does happen, although it may happen only once out of a few million pregnancies. This condition is called superfetation.

Superfetation is when an additional fetus is conceived days or even weeks after its “elder” sibling. Such babies usually share a birthday, as it is medically necessary in most cases to induce labor or to deliver them by C-section, but gestationally, one baby is “older” than the other.

Superfetation is much more common in the animal kingdom—among mammals it is well documented in badgers, buffalo, mink, and panthers—but it is nearly unheard-of in humans, which leads some to question whether it can reliably be said ever to happen at all.

How Uncommon is Superfetation?

To characterize superfetation in humans as “rare” would be to understate how unusual the phenomenon truly is. Only about ten cases of superfetation are documented in the medical literature, and experts are suspicious even of some of these documented cases.

On the other hand, many experts admit that it cannot be known for certain exactly how often superfetation occurs (if it occurs), as it is unlikely that every occurrence is detected and reported; some cases of superfetation may be mistaken for ordinary twin pregnancies. The discrepancy in apparent gestational age (and size) may in some cases be ascribed to a medical condition such as twin-to-twin transfusion syndrome, a condition in which one of a pair of twins who share a placenta (most twins do not) consumes a larger share of the nourishment provided by the placenta due to abnormal formation of important blood-vessel connections. This results in one twin being larger than the other, although in these cases they are always identical twins, while in cases of superfetation the two babies can never be identical.

This discrepancy in apparent gestational age and size between twins (i.e., “Hey, one of ‘em looks bigger than the other one!”) is known as growth discordance. Growth discordance is not uncommon; as noted above, it can be caused by twin-to-twin transfusion syndrome. Other possible causes of growth discordance include congenital infection and chromosomal abnormalities. With fraternal twins, it can literally just be the “luck of the draw” since genetically some babies are programmed to be bigger or smaller at birth than others.

Is Superfetation Medically Possible?

Some scientists are still skeptical about whether superfetation occurs at all, and believe there must be some other explanation for cases in which it appears to have happened. Ordinarily, physical and hormonal changes occur after conception that make additional conceptions impossible while the first pregnancy is ongoing.

First and most important, once a woman becomes pregnant, ovulation stops. After conception occurs in a normal pregnancy, the corpus luteum (and later the baby’s placenta) releases hormones that stop further ovulation. Also, the lining of the uterus thickens in a way that should prevent a second embryo from attaching itself. Finally, the cervix forms a barrier known as the mucus plug, which is designed to protect the developing fetus from microbes that might make their way into the uterus from the outside world. Just as it prevents microbial infection, the mucus plug is also an effective barrier to sperm.

To sum up all of the above: superfetation is so unlikely because three near-impossible things need to happen in order for it to occur:

Ovulation must occur while a woman is already pregnant. Sperm must somehow make it past the mucus plug. Implantation must occur in a uterus that is no longer prepared for it.

How Well-Documented Are Cases of Superfetation?

While not every story can be considered reliable, superfetation seems to have occurred in a number of well-documented cases:

In 1960, a Baltimore woman named Mary Tress delivered what appeared to be a pair if twins. Oddly, the firstborn twin, Anthony, appeared to be premature. X-rays were immediately taken of the boys’ thigh bones, and the results showed a difference in bone age—the second-born twin, Mark, appeared to have been conceived a full two months before his “older” brother.

In a more recent case, an Arkansas couple, Todd and Julia Grovenburg, learned in 2009 that Julia was carrying two fetuses, rather than just the one that had been detected on her initial ultrasound. Further examination by doctors revealed that their son Hudson had likely been conceived almost three weeks after his sister Jillian. Both children were safely delivered via C-section. Speaking to ABC News, one doctor involved with the case remarked that “The interesting thing [is that] if these children were actually born on their due dates, the older child would be born at the end of 2009, and the younger child … in the beginning of 2010.” (This may just go to show that not everyone has the same perspective on what is “the interesting thing” about a case like this.)

Finally, a 2008 case of superfetation involved Charlotte Mullineux of Essex, England, who had been told she was carrying twins, only to learn that one twin had been miscarried and was being reabsorbed by her body (an uncommon but not rare phenomenon known as vanishing twin syndrome). One of her early ultrasound examinations had revealed a small blob of tissue in addition to the twins, but no importance had been attached to the observation. Ms. Mullineux was told to undergo another ultrasound a couple of weeks later, by which time it was clear that the little blob that had been observed in the previous ultrasound was growing into a baby!

What Causes Superfetation?

Artificial fertility treatments are thought to be responsible for cases in which superfetation is believed to have occurred, although exactly how these treatments cause this phenomenon is not well understood.

One of My Unborn Twins Appears to Be Larger Than the Other; Could This Be a Case of Superfetation?

No, almost certainly not.