Astrology, séances, the MMR vaccine causing autism, and superfetation, initiation of a new pregnancy during an ongoing pregnancy in the same mother – all of these scenarios are fiction, except for last one. Superfetation sounds strange, but actually it happens sometimes in nature, though only extremely rarely in humans. Also called superconception, superfetation was first noted in classical Greece, when the ancient physician Hippocrates of Cos (c. 450-380 BCE) described a pregnant woman, Larista, giving birth to twins who were apparently healthy, but “40 days [distant]” in what we would call gestational age. A little bit later, also in classical Greece, the philosopher Aristotle of Stagira (384-322 BCE) described superfetation occurring in a hare.
While it is clear that superfetation is more likely to occur in those mammals, such as hares, that deliver large litters, than in humans or apes, it’s not really possible for us to know whether superfetation was the reason for Hippocrates, Aristotle, describing newborns or fetuses of differing gestational ages. That’s because there are other reasons why this would happen. When human women are found to be carrying two fetuses of vastly differing sizes and maturity levels, those other conditions must be ruled out before an obstetrician can diagnose superfetation and start writing up a case report to publish in a medical journal, and contacting reporters to obtain a television spot to describe the case. In reality, only a handful of such cases have ever been published, notably one case occurring at the end of the last century, in Leicester, UK.
As for those other conditions that can mimic superfetation, one is that during a twin pregnancy one twin can have chromosomal anomaly that causes a problem in growth and development, so that twin will be less mature that her sibling. Another condition that must be ruled out, and which occurs only in monozygotic (identical) twins that share a placenta, is called twin-twin transfusion syndrome (TTTS). Essentially, TTTS is a blood vessel problem that results in one twin hogging much of the blood supply, such that the other twin gets gypped.
While superfetation is extremely rare and probably something that you’ll never witness in a human woman, even if you’re an obstetrician who remains in practice for 60 years, the fact that it is physiologically possible raises another prospect: The two fetuses that the mother is carrying at different stages within the womb can have different fathers. That might make for nice conversation material at a birthday party! More realistically, the fact that superfetation can occur, but hardly ever does, makes it good material for literature.
As for why superfetation would occur, in human women, it is though that it could happen as a result of the woman taking high doses of the ovulation stimulating treatment, clomiphene citrate. In such a case, a woman would have continue to ovulate several weeks into a pregnancy, followed by fertilization of one ovum and implantation into the already pregnant uterus.
So what does one do if there are two fetuses, both healthy but different gestational ages, and conditions other than superfetation have been ruled out after a bunch of tests? The answer recommended by the tiny number of case reports on the matter is to let the pregnancy go as long as possible, to 40 – 42 weeks, if possible before delivering the neonate, although never longer than 42 weeks, since that could be dangerous to both fetuses. Once labor begins, due to the more mature fetus being ready, the other one has to be delivered as well. It cannot remain in their with uterine contractions and other things happening to empty out the uterus.