Okay kiddo, imagine if you are going to have a party and you invited two friends. But when the day of the party comes, one of your friends couldn't come. It can be sad for you because you were looking forward to playing with both friends, right? Well, that's kind of like what happens with vanishing twin.
When a woman is pregnant with twins or more, sometimes one of the babies disappears during pregnancy. It usually happens in the first trimester (the first three months) when the body is still figuring out how to grow the babies.
The body naturally absorbs the disappearing baby, so it doesn't affect the health of the remaining baby or the mom. In some cases, the disappearing baby can still be seen on the ultrasound, but it will stop growing and eventually fade away.
It can be sad to hear that one of the babies won't make it, but it's important to remember that the surviving baby has a better chance of being healthy since it won't have to share resources and nutrients with another baby. The mom will still need to take good care of her body to make sure the remaining baby grows big and strong.