What is the earliest stage of prenatal development at which a rudimentary heart beat can be identified?

From the fifth week of pregnancy, its possible to identify the heart beat.

Explanation:

In the third week of pregnancy the heart of the fetus begins to form. For this process, two tubes come together and give rise to five structures: two bulbs, a ventricle, an atrium, and a venous sinus. All this gives rise to the heart tube that, in the fifth week, divides into cavities. This is about the eighth week when the heart is ready.

The heartbeat is noticeable by ultrasound. From the fifth week, on transvaginal ultrasound, depending on the sensitivity of the device, it is possible to hear the heart of the baby. In most such exams you can only hear the baby's heart from 6 and a half weeks.