Overthinking

So I’m not 100% sure if I’m over thinking or just trying to find someone to put the blame on. But part of me thinks my fiancé may be the reason we can’t have kids. All of his 4 step siblings have kids. And my sisters have kids. But neither him or his brother have ever been able to have kids. For awhile I thought it was me but what if it’s really him? We’ve been trying for just over 3 years now and it’s exhausting..