A pregnant woman has stunned doctors in the US after an ultrasound revealed she is carrying a second, younger baby that is not a twin.
Julia and Todd Grovenburg are believed to have conceived a boy some two-and-a-half weeks after falling pregnant with a girl, US television networks report.
Doctors believe the dual pregnancy may be a case of an incredibly rare phenomenon called superfoetation — conceiving while pregnant.
There are reportedly only about 10 recorded cases of superfoetation, which can pose serious health risks to the younger foetus if the babies are born too far apart.
If the gap is longer than 24 days, doctors say the second child is likely to suffer lung development problems.
But doctors believe the Grovenburg's male foetus, dubbed 'Hudson' by the parents, has a very good chance of survival because of the short period between conceptions.
The couple is calling their female foetus Jillian.
Mrs Grovenburg's obstetrician Michael Muylaert confirmed there was a "discordant growth pattern, possibly due to superfoetation".
"This is an unusual and rare condition, but the possibility is real," he told KFSM-TV.
"It can only be confirmed after delivery by chromosomal and metabolic studies on the babies."
If the babies arrive when expected, the girl will be born in 2009 while the boy will have a 2010 birthdate.
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