Vestigial twin

So, when a baby is growing inside a mommy's tummy, sometimes there can be two babies in there. But in some cases, one of the babies doesn't grow like it's supposed to and it stays really small. It doesn't turn into a full grown baby, but it stays attached to the bigger, healthier baby. This small, underdeveloped baby is called the vestigial twin. It's like a tiny body part that isn't useful or necessary, kind of like your appendix. But unlike your appendix, a vestigial twin can't be removed easily because it's attached to the bigger baby. Sometimes, people don't even know they have a vestigial twin until they have an x-ray or another test.