In a pregnancy first trimester, when can we begin to detect the heartbeat of a child?

Fetal heart beat. Usually around 14 weeks of gestation.
Depends on tool. The fetal heart beat is generally visible on a trans vaginal ultrasound at 6-7 wks after the last normal menstrual period. It will not become audible to a stethoscope or doppler audioscope until the baby is further along.