Two fetuses, one womb. Twins? Usually — but not if they were conceived at different times. Possible? Yep.
It’s called “superfetation,” and while rare — there are ten known cases of human superfetation — it is not entirely unheard of. In 2009, an Arkansas woman, Julia Grovenberg, went in for a routine three month ultrasound to check on the baby-daughter-to-be she was carrying when doctors noticed a second, smaller baby conceived two to three weeks after the previously known one.
The babies won’t share a date of conception but will share a birthdate, as when the first is delivered, so comes out the second. This is most likely not a problem, medically, as the second baby will be nearly full term anyway. (The babies should have been born by now, but the Google-able information on them is scant.) In 2007, a UK woman, also pregnant with “twins” via superfetation successfully delivered both; even though the two children were conceived three weeks apart, there were no complications from the superfetation.
Bonus fact: As a corrollary, it’s possible to have fraternal twins conceived by two different fathers. This is called “heteropaternal superfecundation” and it’s also incredibly rare, but, again, not entirely unheard of. In 2009, a Texas woman admitted having an affair around the time her twin boys were conceived, and it’s almost certain that they have two different biological fathers.
From the Archives: The World’s Youngest Mother: Another pregnancy oddity.
Related: Twins, the movie pictured above.