Compatibility, why so difficult now?

In the days long gone, most especially in Africa, marriages were arranged even right from birth. A girl who's in the womb is already promised to a man who agrees to train her and sponsor her up keep and they finally get married and have children with absolutely no complications, except the usual -pregnant women under 20 are at a greater risk for death and disease including bleeding during pregnancy, toxemia, hemorrhage, prolonged and difficult labor, severe anemia, and disability. Life-long social and economic disadvantages may also be a consequence of teenage birth. Apart from these they're completely free from complications from Rhesus factor, blood group, genotype...etc. But these days, why have complications from things like this become more pronounced. I mean, there was no test or facilities to conduct tests in the past and their marriages end up fine. If evolution is false, then what suddenly let to this incompatibility? Or maybe things like this happened in those days and they never knew the cause of why their infants kept dying...I guess we'll never know.