A midwife at a hospital in Bulgaria's capital Sofia is now under arrest after inflicting grievous bodily harm on a newborn baby this week.

The measure is imposed for 72 hours (until Sunday), but prosecution is expected to request that she remain in detention.

In the early hours of Monday Emiliya Kovacheva began hitting a baby using her hands and also a glass bottle used to feed children.

It is not yet known what caused the midwife's outburst of aggression. Kovacheva was on a 24-shift at the time.

The incident has left Bulgaria shocked since information about it was made public on Thursday. Sofiamed, the hospital where the development took place, has also described the incident as "a shock", saying the employee had previously had an impeccable professional record.

CCTV footage available across the hospital also adds to the mystery. It was the video recording that showed Kovacheva even tried to strangle the baby, just seconds after having caressed others babies in the same room.

Kovacheva, who owned up to having hit the baby and clutched at its neck, is already charged with attempted murder of a baby.

Bulgaria's Deputy Health Minister Boyko Penkov briefed reporters on Friday morning, and the Bulgarian National Television showed him saying the newborn, which was now at another hospital, was already in a more stable condition.