Hi!
I Am very surprised with this question. Or did you mean 6-12 months, not weeks? I fairly hope so...
I will try to answer: I n earlier years of education recommended siting for the babies was not before 6 month,with supporting diagnosing treatment of baby's hips, but now new research was done and it is proven that is too much of vertical pressure to baby's spine in sitting ( or worse standing) position, witch can result in latter baby's life time with degenerative changing of the spine, so recommendation is about 8 month( for sitting), and about 12 m for standing. Anyway,main recommendation is to not to forcing baby in next stage of motor development, yet to let her go to develop in her own speed( cause every human being is different), within normal schedule of normal motor development. Thats mean, no supporting in sitting with pillows, no supporting in standing, let your baby's develop in their own way... save their bones and joints they will be needed them whole life time....