We consider the problem of extending the lifetime of wireless sensor networks (WSNs) where sensors report their data to a base station via multi-hop transmission. In-network data aggregation is introduced to reduce the traffic. The lifetime is optimized using a LP (linear programming) framework built on a multicommodity network flow model to which we introduce the concept of flow loss multiplier to express the impact of data aggregation over correlated data. To balance energy consumption across the network, data aggregation is performed only at the first hop (FH) transmission. Heuristics are proposed to obtain significant FH aggregation, accordingly prolonging system lifetime. Simulation results show that FH aggregation possesses characteristics that make it suitable under certain data collection applications/scenarios