A pair of non-identical twins born in New Jersey have been tested and found to be the products of one womb, one pregnancy, but two fathers. To simplify this perplexing concept: one woman ovulated, had sex with Gentleman Caller 1, then ovulated a second time within a few days (a rare but not unheard of phenomenon) and had sex with Gentleman Caller 2. Each coital act yielded a zygote; each zygote matured into fetus-hood, then live birth, and now personhood.

The two-into-one situation has a fancy medical-sounding term—heteropaternal superfecundation. Though this may sound like a PAC or a new juice cleanser, it is a simple way in medical-speak to denote that a super-fertile (or fecund) woman is carrying two or more babies fathered by two or more distinct fathers. In the New Jersey case, the woman claims she had sex with different men a few days apart, resulting in the heteropaternal offspring.

We have seen this before—in fact in sort-of-history, Hercules, the original Mr. America, shared the womb with a brother, Iphicles. Zeus was Hercules’ father whereas Iphicles—well, he was the dull plodding issue of a mortal father and mortal mother. Zeus’ sperm co-mixed with mortal seed in another womb—that of Leda. She really had a bad night when Zeus “came to her” and fertilized two eggs while her mortal husband on the same night fertilized another two eggs. To further make matters less, um, realistic there is the additional fact that the human, Leda, laid eggs that hatched her glorious daughters: Helen (of Troy) and Clytemnestra.

Much more recently, the phenomenon of two eggs meeting two distinct guy sperm-lines in the same womb at more or less the same time has been studied. In 1992, in the Roman medical journal, Acta geneticae medicae et gemellologiae, authors from Maryland USA wrote an article entitled, How Frequent Is Heteropaternal Superfecundation?

In it the authors used a database of people involved in paternity suits—not exactly the regular U.S. population. In a sub-group of 127 paternity cases where the progeny in question is exclusively a set of fraternal (dizygotic or non-identical) twins, 3 cases of heteropaternity were found. In the authors’ words, a “separate copulatory event” with a separate male partner (see New Jersey case above) accounted for the conception of each child.

A key question in understanding current frequency is this: in medical school it is taught (and therefore must be true, sort of) that women ovulate with a single egg per menstrual cycle. One egg, one shot at fertilization, with ovulation stopping as soon as pregnancy occurs or, alternatively, without fertilization, till the next menstrual cycle.

This rule of thumb works perfectly except when it does not.

The entire world of fraternal or non-identical twins flies in the face of this adage. In such cases, for whatever reason two eggs are loosed from the ovary and begin the precarious descent down the Fallopian tube perhaps to encounter sperm and perhaps to not encounter it. The eggs might be released within minutes of each other or perhaps within a day or two. In such a case, the opportunity for separate coital acts to result in separate offspring exists—witness the number of fraternal twins paraded on the streets on any city. Some were conceived with a single copulation, some with two—most involving the same male partner.

The authors of the 1992 paper are prescient when they point that with increasing use of fertility drugs, there will be much more multiple egg ovulation per menstrual cycle—that’s why fertility drugs are called fertility drugs: they send more eggs downstream and increase the chances that a sperm and egg with fuse and start the entire complex dance of human conception.

They authors also suggest, as many other articles on the topic do, that increased “promiscuity” might also fuel an increase in heteropaternity. Maybe—but it is never a good bet to think that one generation is more or less sexually active than the next. Rates of gonorrhea in the 13th century were sufficiently high that Pope Boniface had to get involved. In fact, 800 years ago the problem was so bad, the Pope made doctors treat the sick as people with, gulp, physical diseases, not spiritual problems. Now, that’s what I would call a pressing issue.