Embryonic heart rates before the seventh week of pregnancy.

Previous studies of normal human embryonic heart rates showed an increase in the rates between the 7th and 9th gestational weeks; the rates gradually declined thereafter until the 15th week. The present study added new information about embryonic heart rates before the seventh week. The heart rates are slower than those reported to occur after the seventh week. These observations are connected with morphologic and physiologic changes during this period of embryonic life.