It actually doesn't mean much at all. Fundal height measurement (measuring from the pubic bone to the top of the uterus) is generally accurate with a margin of error of 3 weeks. So if she is measuring 30 cm, then that means she is probably anywhere from 27 to 33 weeks, in a healthy normal pregnancy. If she is measuring very far behind, or very far ahead, then that would indicate a need for further investigation - due date might be off by several weeks, could be twins, something isn't right, etc. But to measure 30.5 weeks at 31 weeks is pretty much measuring exactly right.
Variations can be caused by the baby shifting position, or by two different people doing the measurement. As long as the measurements show steady growth, it doesn't matter if the numbers line up exactly with the number of weeks she is. It's an educated guesstimate, at best. Basically it means that her dates seem to be correct. :-)
at 3:45 PM on May. 20, 2009