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An Indonesian woman gave birth to a 19-lb. 2-oz. baby behemoth on Sept. 24, but that was only the second weirdest pregnancy tale of the month. The strangest belongs to Julia Grovenburg, a 31-year-old Arkansas woman who has a double pregnancy. No, not twins — Grovenburg became pregnant twice, two weeks apart. Isn't that supposed to be impossible?

Almost. There have been only 10 recorded cases of the phenomenon, dubbed superfetation. In Grovenburg's case, she became pregnant first with a girl (whom she has decided to name Jillian) and then two weeks later with a boy (Hudson). The babies have separate due dates — Jillian on Dec. 24, Hudson on Jan. 10.

Dr. Robert Atlas, chairman of the obstetrics and gynecology department at Baltimore's Mercy Hospital, says he has never encountered a case of superfetation during practice. He says such pregnancies occur when a woman continues ovulating after becoming pregnant and when that second, fertilized egg is able to implant itself in the lining of the womb — two things that wouldn't happen in a normal pregnancy. Typically, hormonal changes prevent further ovulation and thicken the lining of the uterus to preclude a second embryo from attaching. Why didn't that happen with Grovenburg? No one's really sure.

Despite the rarity of Grovenburg's case, Atlas tells TIME the phenomenon shouldn't be cause for concern. Grovenburg's babies should behave much as twins do; in all likelihood, the second baby will be born slightly premature when Julia first goes into labor. Since the difference between the babies is only two weeks, the second baby will be nearly at full term anyway. Indeed, the last known case of superfetation had a happy ending. In 2007, a British woman gave birth to a boy and girl who were conceived three weeks apart, with no undue complications.

• Dan Fletcher
• There have been only 10 recorded cases of superfetation, the phenomenon of a woman getting pregnant twice