Is it allowed in Islam when a mother is breastfeeding ,to use preventive like sheaths or in order to safe the baby.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

There is no harm in using such a shield as long as there is no fear of injury to the mother or inconvenience to the child because of it. The presumption is that all things are deemed lawful unless proven otherwise. Allaah The Exalted says (what means): {It is He who created for you all of that which is on the earth.} [Quran 2:29] Allaah said this in the context of reminding people of His favors. So it indicates that what He created for them is permissible.

Allaah Knows best.