After reading the reference supplied by Chris, it might be worth asking yourself the question, how would kammic forces know who would be a good parent and a bad parent, and how to direct embryos to "the parents they deserve".
Frankly, this seems a little far fetched to me... it seems to rely on some inate sense that there must be some kind of universal justice at play... and that everything must balance out in the end.... but if you look at the references Chris provided, you'll see that this is not what kamma is about.
"Having understood name-and-form, which is a product of prolificity,
And which is the root of all malady within and without,
He is released from bondage to the root of all maladies,
That Such-like-one is truly known as 'the one who has understood'." (Snp 3.6)
"Whether I were to preach in brief, Sāriputta, or whether I were to preach in detail, Sāriputta, or whether I were to preach both in brief or in detail, Sāriputta, rare are those who understand." (A I 333, Sāriputtasutta)