。
She was carrying the baby on her back.
Literal
She [topic-は] baby [object-を] was-carrying-on-back.
背負う (せおう) means 'to carry on one's back' — from 背 (back) + 負う (to bear/carry). The ~ていた form describes an ongoing past scene. Carrying children on the back is traditional in Japan, often using an おんぶ紐 (baby carrier strap). The practice remains common and distinct from front-carrying (抱っこ).