A pregnant woman in Arkansas surprised doctors twice over when she went in for a routine ultrasound this June.
Doctors successfully located Todd and Julia Grovenburg's growing baby girl Jillian, but then discovered another smaller baby -- what could be Jillian's younger brother -- growing beside her.
The Grovenburgs may have conceived their son Hudson a full two-and-a-half weeks after Jillian, according to statements given to KFSM-TV in Ft. Smith-Fayetteville, Ark.
Different from identical twins or fraternal twins, the Grovenburg babies would have separate due dates and are considered to be a rare medical occurrence.
"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, even though babies may differ widely in size at birth.
"It does really sound like this is a true case of different conception times for these children," said Boyle.
Reports of superfetation, or conceiving while pregnant, are 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.
Yet, while superfetation is rarely documented, Dr. Donnica Moore said it may be impossible to tell if this happens more often -- but with a shorter time period between conceptions.
"For these sorts of cases, I have to say, most of our bodies don't read the textbook," said Moore, an obstetrician and president of New Jersey's Sapphire Women's Health Group.
"I would say [it's] extremely rare, but then again, we would never know how often this happens. If the children are born at the same time -- it might be that they were conceived at different times."
Depending on the time between the two conceptions, Boyle said superfetation could be dangerous for the younger baby, who could be born prematurely.
"It [the second conception] can happen up to 24 days later than the first conception, and then you're putting the second baby at risk for lung development problems," said Boyle.
However, in the Grovenburg's case, Boyle said the difference of two weeks would not put the younger baby at much of a risk for health problems.
"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.