A U.K. woman was surprised to learn her twins were conceived ten days apart.
Susie Chong was told she was having twins when she was seven weeks pregnant, according to the Daily Mail.
“I lay on the scanning table and the sonographer told us that she could see two heartbeats," Chong told the Daily Mail. "I couldn't believe it, that I was actually pregnant with twins."
But at her 12-week scan, she discovered her twins, Leni and Jenson, were conceived in an unusual way when the scan showed Jenson was much larger than his sister.
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“We were so shocked to hear what the sonographer said about the twins,” Chong said.
“[The sonogrpaher] said that she had only ever seen it a handful of times in her career, and that it was incredibly rare,” she added.
“From the sizes of the twins on the scan, she put their two conception dates at about ten days apart,” added Chong.
Such an event is known medically as superfetation, TIME reports. It occurs when the woman continues to ovulate even though she has conceived. A second egg then manages to attach itself to the womb following fertilization.
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There are only 25 recorded cases in which the simultaneous development of two fetuses in the womb have been considered likely, according to the Daily Mail.
“We had never even heard of it before and I never knew it was possible,” Chong added.
Chong gave birth to the twins just ten months after having her son, Hendrix, with her partner Matthew. After Leni and Jenson spent ten days in medical care, Chong was able to bring them home.
While Matthew wanted to have seven children, Chong ruled out having any more.
“I gave birth to three children within ten months, and that's enough for any mum," she told the Daily Mail. "If I was to fall pregnant again, goodness knows how many I would have!”
The phenomenon of superfetation appears to have become easier to detect over recent years. In 2009, there had only been 10 recorded cases, TIME reports.
Julia Grovenburg, an Arkansas woman, was one of them. Her two babies, Jillian and Hudson, were conceived two weeks apart and even had separate due dates.