You can start from 6th month brain develops in 6th month there is a sloke in ayurveda "shaste budhii"

Garbha sanskar literally means 'education in the womb'. It is traditionally believed that a child's mental and behavioural 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 Garbha sanskara from 7th month onwards.

Hi garbha Sanskar has pre pregnancy and Post pregnancy line of treatment. In pre we do detoxification therapy as well improves sexual health of both . In Post pregnancy we provide medicines, teches yoga & other things.  So it's " never too late " start as soon possible

take proper knowledge about this first