I totally know what you mean. My brother can get my SIL pregnant if he sneezes when she is ovulating.

Which is fantastic for them (and me, indirectly) as they produce the most amazing kids I have ever encountered.
But I work with a girl who smokes, doesn't ingest anything other than caffeine, nicotine or sugar and she is pregnant with twins after just giving birth last August. AND she continues to smoke, drink nothing but coke and eat like crap while pregnant. It blows my mind that someone who takes such poor care of herself can get pregnant so easily and maintain the pregnancy. It completely sucks.
While it breaks my heart to go through this all the time, I wouldn't wish this on my worst enemy.