Pregnancy raises at least estrogen and progesterone to something like ten times the usual menstrual cycle peak, so I doubt the effect size is comparable. Also, I think pregnancy affects a broad range of hormones, not just the handful in birth control.
Oxytocin levels also increase. That would be the first one I'd test for -- if there was a correlation between oxytocin levels and extent of grey matter loss.
Underrated comment, I would be very interested in a study on this.