Hmm, boleh je, tapi generally third trimester onwards memang akan jadi uncomfortable, perut dah besar, so lying flat on your stomach akan pressure your belly and baby. A few of my mum friends cakap first trimester okay lagi, tapi as pregnancy progresses, better to sleep on your side (left side is best) or semi-reclined position untuk safety dan comfort. But since this is pregnancy-specific, best to confirm dengan your doctor, okay? They can give you clearer guidance based on your stage of pregnancy!