<Li> Week 6 starts at 103 and ends at 126 bpm </Li> <Li> Week 7 starts at 126 and ends at 149 bpm </Li> <Li> Week 8 starts at 149 and ends at 172 bpm </Li> <Li> At week 9 the embryonic heart tends to beat within a range of 155 to 195 bpm . </Li>

Where is the cardiac trigger zone where atrial contraction (heartbeat) is initiated