An Arkansas woman, Julia Grovenburg, surprised doctors when she went in for an ultrasound in June.  Grovenburg had gotten pregnant while already pregnant, but the babies are not twins. Read more and see video below.

Todd and Julia Grovenburg’s doctors saw their baby girl Jillian but then discovered a smaller baby growing beside her, a little boy.

Doctors believe the Grovenburg’s may have conceived their son Hudson two-and-a-half weeks after Jillian, according to statements given to KFSM-TV in Ft. Smith-Fayetteville, Ark.

This is considered to be a rare occurence.

“When the woman had her ultrasound initially, they saw one sack, one baby developing, and that baby had a certain gestational age; then they noticed a second heartbeat in a child that was much, much younger developmentally,” Dr. Karen Boyle of the Greater Baltimore Medical Center, told ABC News’ “Good Morning America Health.”

Boyle said it’s uncommon for fetuses to differ so widely in size and development early in pregnancy, although babies are of varying sizes at birth.

“It does really sound like this is a true case of different conception times for these children,” said Boyle.

Conceiving while you are already pregnant is called superfetation, and is so rare that Boyle said, “There is no prevalence or incidence in the literature. I could only find about 10 reported cases.”

Grovenburg’s obstetrician, Dr. Michel Muylaert, confirmed to KFSM-TV that the Grovenburgs may be dealing with the extraordinarily rare case of superfetation.

“Mrs. Julia Grovenburg is pregnant with twins and there appears to be a discordant growth pattern, possibly due to superfetation,” Dr. Muylaert wrote in a letter to KFSM-TV.

“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,” he continued.

“The interesting thing in this case is if these children were actually born on their due dates, the older child would be born at the end of 2009, and the younger child would be born in the beginning of 2010,” said Boyle.

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