Early date for detection of fetal heart:

“Embryonic investigations suggest that cardiac contractions begin in fetus at 36 days gestational age.” “For practical purposes, many sonologists consider the identification of cardiac activity in an embryo with a CRL of less than 5 mm as 6 weeks GA. Cardiac activity should be detected routinely when the embryo attains a length of 4 to 5 mm. This corresponds to a GA of 6.0 to 6.5 weeks, at which time the MSD is 13 to 18 mm. Using a transabdominal approach, cardiac activity should be evident by 8 weeks GA, when the MSD is 25 mm.” Callen 4/e, p 120 Transvaginal USG At 5-6 weeks of gestational age corresponding to 13-18 mm of MSD. Transabdominal USG At 8 weeks of gestational age corresponding to 25 mm of MSD. MSD: Mean sac diameter. To obtain uniformity gestational sac size is determined by calculating mean sac diameter (MSD).