When, in terms of gestational age, can embryonic cardiac activity be first visualized by transvaginal ultrasound? Transabdominal ultrasound?

Transvaginal - 6 weeks gestational age

Transabdominal - 8 weeks gestational age

The pregnancy is considered abnormal if no fetal heart tones are noted after 10 - 12 weeks.


Source

Cosby, K. and Kendall, J.  Practical Guide to Emergency Ultrasound.