Is it commom for moms to get pregnant again when their youngest is already a teenager?

I ask this because my brother and I found out that our mom is pregnant (I'm 17 and my brother is 22). Our mom is 45, and she and our dad claim that this pregnancy is unplanned...but they are so happy it happened. So, my question is this: is it really common for older women to get pregnant again, even when the youngest is a teenager/young adult? I'm just a little weirded out that my parents have . lol. Thanks everyone!