The American Academy of Pediatrics suggests avoiding any blankets in the crib until age 12 months. However, I have found babies older than 5 months with core strength and good arm control do very well having a small 6 “ by 6” lovie blanket to help them soothe. Repeatedly stroking different textiles between their fingers is very soothing. Not a must have, but can be a nice to have if it's the right size and your baby is mobile. The perfect lovie blanket has a few different textures on it for your baby to touch.
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