According to NBC's Nancy Snyderman:
Here's how it happens--egg and sperm implant. Of course, that's your first pregnancy. But if you ovulate more than one time a month--and women do--and a sperm happens to meet that egg, and they, too, implant, guess what? You get a second fetus.
This is precisely what doctors think happened to Arkansas couple Todd and Julia Grovenburg. An ultrasound revealed that a male fetus appears to have formed a full 2.5 weeks after the female fetus was formed, according to Arkansas TV station KFSM-TV.
What technically constitutes "twins" may need to be further defined, as twins are thought to be two children produced in the same pregnancy and born during the same birth process. So if Grovenburg has these two children at the same time, by the traditional definition they would be considered twins. (Fraternal twins, the result of two sperm and two eggs, are not technically created at the same instant, unlike identical, or monozygotic twins, when one egg fertilized by one sperm splits.)
In a report on MSNBC, Dr. Snyderman calls superfetation a rare condition, but writing strictly anecdotally, I wonder if it may happen more than we realize.
My own mother had three sets of twins (I'm the younger sister in the first set; my twin brother is 12 minutes older), though her third set resulted in miscarriage. None of us are identical twins, so it is entirely possible, if unlikely, that my mother was ovulating more than once a month as well (not to mention having sex more than once a month; go Mom!).
In fact, my little sisters were born a full 37 minutes apart, which is a long time for twins. The doctor told my mother that the younger girl, Anne, didn't want to come out, but the doctor insisted it was safer to have one labor, not two.
So who knows? Maybe there are not only more twins than we realize, but also more parallel pregnancies of two kids we assume are twins but who are technically not.