I don't know about round ligament pain, but my pains have so far been mainly on either side of my groin/hip area. I've been told that it's my hips starting to move to accommodate the ever-expanding uterus... yay!
Copied from ever-useful google:
Many women experience round ligament pain during their pregnancy. Typically, round ligament pain is noticed in the second trimester-but it can occur in early and late stages of the pregnancy as well. The pain is felt in the lower abdomen and groin area. There are two cord-like muscles located at the top of the uterus that extend through the abdominal wall and end in the vulva. The round ligaments are made of the same smooth muscle tissue that the uterus is made of. Throughout pregnancy, the round ligaments will stretch and grow with the growing uterus and this can cause pain for the mother. You might experience the pain on one side or both sides.
Round ligament pain is usually worse when you walk or suddenly change positions in the bed. However, you might also notice the pain for no apparent reason. The pain associated with round ligament syndrome is a sharp, cramping or stretching pain along one of the ligaments. Rather than making abrupt and sudden position changes, try getting up slowly to prevent the stretching pains.