Wilma's pregnancy was due to in-vitro fertilization (IVF), in which her husband's sperm was combined with her ova in a petri dish. In an unforgivable breach of proper medical procedure, however, the pipette used to transfer material had apparently been reused after a visit from a previous sperm donor. Wilma Stuart's ova were fertilized by both men, and two of the re-implanted embryos matured into healthy young boys.
Strictly speaking the boys were only half-brothers, even though they were delivered as twins. They entered the medical literature as yet another documented case of heteropaternal superfecundation, a scientific term meaning "different fathers, multiple babies." Most such cases, however, are not the result of IVF, but rather more traditional conception methods.
Though fraternal twins seem less remarkable than their identical counterparts, they are somewhat rarer. About 1 in 150 births results in identical twins, but fewer than 1 in 200 produce fraternal twins. This discrepancy arises from the biology of twinning in each case. Identical twins result from one fertilized ova splitting into two separate embryos. Fraternal twins, on the other hand, are the result of the fertilization of multiple ova. Normally only one ovum is present, but in some individuals on very rare occasions, the ovaries will release more than one. Some fertility treatments also promote the release of multiple ova.
Heteropaternal twins are, in turn, much less likely than even traditional fraternal twins. Sperm are capable of waiting in the fallopian tubes for between three and five days, while the ovum may only survive a matter of hours if fertilization does not occur. So the fertilization time window is relatively narrow, particularly if sperm from two different males must arrive. Given the single-pair reproduction strategy employed by most people, these criteria are very rarely met.
There is, of course, nothing limiting heteropaternity to two children. Triplets, quadruplets, or even nontuplets could all possess different fathers given enough ova and a sufficient variety of male genetic material. But considering the almost inconceivable odds against such a series of events, it is little surprise that no instances of heteropaternal triplets or higher have ever been reported.
Since the mid-1980s, documented cases of heteropaternal twins have been on the rise due in part to better testing methods. Traditional paternity testing relied primarily on blood-type comparison between the mother, child, and alleged father(s). Inconclusive results were unavoidable until modern DNA tests became available. Recent evidence suggests that as many as 2.4% of disputed paternity cases involving fraternal twins are instances of heteropaternity, thought the authors of that particular study caution that the number is likely an overestimate of the general population.
Social factors are also at play in the rising instances of reported heteropaternity. Fraternal twinning is becoming more common as the average age of childbearing rises. As women age it becomes more likely for multiple ova to be present, and many fertility drugs cause the same effect. Also, heteropaternal superfecundation often makes for a salacious story despite its unwieldy name, attracting the "Offbeat News" reporters of countless web news outfits.
Despite better testing and a deeper media penetration in society, most instances continue to go undocumented. After all, not all cases of heteropaternity are black-and-white.