The number they go with is an average of all the measurements they take. Some may be behind, some may be ahead, but the final number is what you want to look at. Our baby boy's head was measuring almost 3 weeks ahead most of my pregnancy, but slowed down closer to the end, and he had a regular sized melon at birth!
Baby #1 PE 11/3/1993 39 weeks 7lbs. 4oz.
Baby #2 Severe PE/HELLP 9/21/1999 31 weeks 4lbs. 2oz
Baby #3 Mild PE 6/1/2007 39 weeks 7lbs. 1oz.
Baby #4 PE 10/31/2012 37 weeks 6lbs. 11 oz.