Hi there mommy, this is very normal actually. Around 12 weeks of age, your baby will be very in tune with his body and discovering how his feet and hands work. They will become utterly fascinated with them and will examine his fingers and thumbs closely and it is inevitable that he puts them in his mouth and try to suck on them. Sucking on his fingers is also very soothing to baby, that's why he is doing the same to your forearm. If you're really bothered by it, you can consider getting a pacifier.
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