To assess early embryonic growth and development, 361 pregnancies were studied from 34 to 56 days from last menstrual period. All pregnancies had a subsequent successful outcome. Transvaginal ultrasonography was performed using an Acuson 128 x P/10 with a 5-7.5 MHz probe. Gestational sac diameter, embryonic pole length and embryonic heart rates were measured. Embryonic heart rates were determined by M-mode. Gestational sac diameter, embryonic pole length and embryonic heart rate increased linearly relative to gestational age and to each other. Regression equations comparing gestational sac diameter and embryonic pole length as well as comparing embryonic heart rate with gestational sac diameter and embryonic pole length were constructed. To be normal, gestations that have (i) sac diameter of 20 mm and 30 mm should contain at least a 2 mm and 5 mm embryo with embryonic heart rates of at least 75 and 100 beats per min, respectively, and (ii) embryos measuring 2 mm, 5 mm, 10 mm and 15 mm should display embryonic heart rates of at least 75, 100, 120 and 130 beats per minute respectively.