Hello. My son was 10 weeks early, and I wasn't sure when to start, either. Have you read How Smart is Your Baby? They outline the things you can do according to the developmental stage of the baby, not the age (actual or adjusted). I found this really helpful - they mark our the skills needed to proceed from very simple outline cards to detailed cards to fast flashing. It might help you to judge whether your baby is physically ready for flashcards (no point flashing if they can't see them!
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I will say that from my own experience, he was a 'newborn' baby for about four months (the looks I got when I started weaning - spoon feeding a 'newborn' when we went out
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BUT his eyesight, hearing and speaking developed much more along the lines of a child his actual age, lending some support to what Doman says - in the hospital he had plenty of noises to listen to, shapes to focus on, time to practice breathing...
I would say, if you feel like your 4-month old premature baby can SEE the cards and HEAR you name them, there is no reason not to start now. Otherwise, work on the seeing and hearing, first!