I just dont get it

Why is it that females who are bad mothers who are out there drinking alcohol like getting drunk and doing drugs and treat their kids bad end up getting pregnant or have abortions in the past end up pregnant again and then the ones whos wish is to get pregnant and have a healthy baby dont get pregnant.?? I just get so sad that i havent been able to give my husband a baby. :( i feel like crying just thinking about it.