This is a bit of mountain biking slang that showed up a long time ago.
It's a bit weird, if you ask me, but it's a term that is used to describe a
section of the trail that has a lot of stones or rocks in it
that are smooth, round, stuck in the dirt and, well... look like baby
Riding over these things can be a blast though! Depending on how fat these babies are, how friendly they are (i.e. how close they
are together) and what they're in (e.g. mud, water, snow) you can have a
great time trying to head over them at rocket speeds or just trying to get
over them at all, no matter what snail is waiting for you to pass.
Baby heads should not be confused with "turtle heads"...