This just came in my weekly "what your baby's doing" email from BabyCenter:
He's shedding most of the downy covering of hair that covered his body as well as the vernix caseosa, the creamy substance that covered and protected his skin during its submersion in amniotic fluid. Your baby swallows both of these substances, along with other secretions, which will stay in his bowels until birth. This blackish mixture, called meconium, will become his first bowel movement.
Okay, I knew that those first poops were nasty, but I went this long without knowing that it was made up of old skin and hair.
Babies are gross.