Does your baby scratch alot? Tink used to leave balls of fur all over the house. By 3 months, the hair on the back of her legs was completely gone.
We never did figure out why her hair came out but she is healthy and has no skin allergies. She is 19 months and every once in a while I find a couple little balls of fur laying on the carpet.
One thing I've learned about Tink is that if she has a diaper on, I have to wash her legsm not just her tulip and tushee when I take the diaper off in the morning or she licks herself silly and the hair on her tummy and the back of her legs disappears!
(just to explain, Tink has seizures at night and in order to let her sleep with me, I put a diaper on her at night so we don't have accidents.)
If this continues much longer, I'd have her looked at by your vet as Deborah says.