Cranial magnetic resonance (MR) imaging was performed in 38 pregnant and postpartum women and 30 nonpregnant age-matched control subjects to establish standards for pituitary gland size and shape during this period. Gland height and infundibulum width were measured on midline T1-weighted sagittal images. Gland convexity or concavity was graded qualitatively. Throughout pregnancy, gland height increased linearly by approximately 0.08 mm/wk. No gland exceeded 10 mm in height during pregnancy. Increases in gland convexity also correlated with progression of pregnancy. The largest glands were seen in the immediate postpartum period; during this period, five of 12 glands measured 10.0-11.8 mm. Beyond the first week post partum, glands rapidly returned to normal size, apparently regardless of the status of breast-feeding. The mean diameter of the infundibulum was 2.2 mm (range, 0.8-4.0 mm). The pituitary gland enlarges throughout pregnancy but should probably not exceed 10 mm during most of this period. Size of up to 12 mm may be acceptable immediately post partum.