steph~ I think it is when there is a septum in your uterus...
Let me see what I can find...

"A bicornuate uterus (a womb with two 'horns') is the most common. Instead of the womb being pear-shaped, it is shaped like a heart, with a deep indentation at the top. This means that the baby has less space to grow than in a normally shaped womb."

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