To make things even more complicated, we might add that it's not even primarily about the things a mother passes on to her child while she's pregnant. Although this connection seems the strongest and most logical. Well, what is logical in our lives anyway?
One can be influenced by many other members of the family and the entire kin. Even people he didn't know, never met, might not have heard of. Big family secrets lend themselves to a forgotten relative suddenly coming to light through a child starting to live his/her life anew. Well, one can even experience the life of one's grandfather's jilted fiancee and whatnot. Rejected people, murders, crimes, secrets and not only - as you yourself write, experiences of wars and any other traumatic, external circumstances that are not related to the family, all this affects a person in his next life without him knowing it.
But you should not be troubled by thoughts like: then I could have lived in a different place, in a different way, I could have done something differently. You see the world is a playground where everything seems to be planned and set long before a person is born. And the trauma is constantly fueled and created again and again. Why don't wars stop, for example? One would say, we are "rational" beings, why should we go to war, kill each other? Even if a person is very careful in their actions, and they should be, and passes on a wonderful legacy to their children and grandchildren, there must be other circumstances to perpetuate the trauma.
Well, I've gone off topic a bit, but that's because just one comment can't cover everything on this complex subject.