Do all babies grow at the same rate in early pregnancy?

Pretty much: Fetal growth does not pick up until it reaches 25 weeks.

As with post-natal : growth, there is a range of normal for each measurement of head & body size at each week of gestation. Likewise, the velocity of growth is important, as each percentile's growth curve represents a normal trajectory. Growth <10th %ile is called Intrauterine Growth Restriction. See http://www.babycenter.com/fetal-development-week-by-week or the Baby Center app, but ask your OB if concerned.