Teen girls trapping their boyfriends

Ok, my brother in law and his girlfriend are both 18 years old. She's in community college and he's a senior in high school. (He was held back as a kid). My mil and I have both noticed how the girlfriend will shove pictures of engagement rings, wedding dresses and babies down his throat. (He can be playing a video game with my husband and she will literally put her phone in his face for him to look.) She just turned 18 in october and its like she trying to be 25 or 30. She's talking about having babies, planning what time of the year she wants to concieve and everything while I've been married for a year and have no interest in babies at all. My mil is really conerened the girlfriend will purposely get pregnant. My brother in law has no interest in marriage and kids. He works at a fast food place on the weekends just for gas and some spending money. His biggest concern right now is his grades and graduating. My mil told me the girlfriend has a bad home life and might be hanging on to him as her way out. I'm not getting into their relationship, but it does worry me too because I wouldn't want that to happen to him. And I feel like a mean person for saying this but if she does get pregnant, I'll blame her for trapping him (I wouldnt say it to anyone I would think it though) because shes told my mil and me that she uses depo, and he wears protection as well. Has anyone else seen a relationship where that happened or experienced it themself?