The knowledge of garbhsanskar dates back to ancient scriptures and is included in the Ayurveda. The word garbh in sanskrit refers to foetus in the womb, and sanskar means education of the mind. So, garbhsanskar translates into the process of educating the mind of unborn baby.
It is traditionally believed that a child's mental and behavioral development starts in womb itself. From the seventh month onwards, unborn babies can hear and respond to their parent's voice, different sounds, music... So you can start Garbhasanskara from 7th month onwards