Imagine Todd and Julia Grovenburg’s surprise when doctors found not one, but two babies on Julia’s ultrasound – and then told the parents the babies weren’t twins, ABC News reported.

Doctors think Julia and Todd conceived baby girl Jillian first, and two-and-a-half weeks later conceived baby boy Hudson, according to statements from KFSM-TV in Ft. Smith-Fayetteville, Ark.

Jillian and Hudson have different due dates, and this rare development is called superfetation – when a mother conceives another child while pregnant.

“It does really sound like this is a true case of different conception times for these children,” said Dr. Karen Boyle of the Greater Baltimore Medical Center. “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.”

Boyle told ABC News she has only heard of 10 reported cases of superfetation.

However, superfetation cannot be absolutely determined until after the babies are born, and doctors are able to conduct chromosomal and metabolic tests on the babies.

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