This article is from the General Pregnancy FAQ, by swnymph@FensEnde.com (Sabrina Cuddy) with numerous contributions by others.
From: "Corrine R. Johnson" <JohnsonC@calvertgroup.com>
The book I grabbed "Your Pregnancy Week-by-Week --
by Glade B. Curtis" -- had this to say about:
Many women experience an occasional excruciating pain in their buttocks
and down the back or side of their legs as pregnancy progresses. This
is called sciatic-nerve pain because the sciatic nerve runs behind the
uterus in the pelvis to the legs. Pain is believed to be caused by
pressure on the nerve from the growing and expanding uterus.
The best treatment for the pain is to lie on your opposite side. This
helps relieve the pressure on the nerve.