Julia Grovenburg told KFSM-TV that she was amazed to discover that she was carrying two babies that were not conceived at the same time.
The expectant mother described her ultrasound as an "absolutely shocking experience" when it showed that one foetus was eight-and-a-half weeks old and the other was 11 weeks old.
Grovenburg said: "It's fun. I mean we feel blessed to have something so rare and as of this point they're perfectly healthy."
Her obstetrician Dr Michel Muylaert said: "There appears to be a discordant growth pattern, possibly due to superfetation. This is an unusual and rare condition, but the possibility is real.
"It can only be confirmed after delivery by chromosomal and metabolic studies on the babies. She was evaluated at UAMS in Little Rock for this condition and they confirmed the suspicion of superfetation."
Although the babies' official due dates are in 2009 and 2010 respectively, it is expected that they will be delivered together in December.