Fetal erection?

I have a crazy question that I haven't found an answer to elsewhere on
the web. (Maybe it's in this group's archives and I just haven't found
it.)

Yesterday I went in for amniocentesis, during which an ultrasound was
done and my approximate due date was calculated. (Looks like the fetus
is about 17 1/2 weeks along.) The doctor working on me also went ahead
and tried to make a guess as to the baby's sex.

I noticed that once the doctor zoomed in on the relevant piece of
anatomy, he seemed to be struggling to maintain his professional
demeanor and not snicker. When I peered at the ultrasound display, it
looked as though the baby had an erection. Not knowing as much about
penises as my husband who was also there, I later asked for his
opinion, and he agreed with me. (Also trying hard not to snicker.)

My first reaction was "Kid, what ARE you doing in there?"

My second reaction is to wonder whether that's even possible. Do
little boys get erections when they're small, fairly undeveloped
fetuses? I suppose there are a variety of things that can bring one
on, so it doesn't necessarily have a sexual connotation or require
that all the male hormones be fired up.

Your experiences, please.