Transferring sows and their litters to group lactation (GL) after an initial period of farrowing crate (FC) housing could enhance the viability of GL for commercial production. Group lactation from 7 d postpartum would reduce the time sows spend in confinement, but the effects of early mixing on animal welfare and productivity require examination. Two experiments were conducted on sows and piglets kept in GL from 7, 10, or 14 d postpartum, compared with FC. Experiment 1 utilized 180 sows and 1,887 piglets over five time replicates (n = 60 sow and litter units per treatment) comparing GL from 7 or 14 d postpartum to FC. In experiment 2, 108 sows and 1,179 piglets were studied over three time replicates (n = 36 sow and litter units per treatment) comparing GL from 10 or 14 d postpartum to FC. All sows farrowed in FC. Group lactation sows were transferred to pens (one pen of five sows at 8.4 m2/sow and one pen of seven sows at 8.1 m2/sow, per GL treatment and replicate) with their litters at 7 (GL7), 10 (GL10), or 14 (GL14) d postpartum. Farrowing crate sows and their litters remained in their FC. Data were collected on sow feed intake and reproduction, piglet mortality (from day 6 postpartum), and sow and piglet weight changes, plasma cortisol concentrations, and injuries. Piglet mortality was greater in the GL7 (17 ± 1.8%) and GL10 treatments (12 ± 0.9%) compared with GL14 (8.3 ± 1.8% P ≤ 0.001 and 8.1 ± 0.9% P ≤ 0.001 in experiments 1 and 2), and greater in GL overall compared with FC (2.7 ± 1.5% and 1.8 ± 0.9% in FC in experiments 1 and 2; P ≤ 0.001). Piglets from GL7 were also lighter at weaning than GL14 piglets (P < 0.001), whereas GL10 and GL14 did not differ (P > 0.05). Overall, piglets in GL were lighter at weaning than piglets in FC (P ≤ 0.01). Sows from GL7 were heavier at weaning (P = 0.001), and GL10 sows tended to be heavier at weaning (P = 0.08), than GL14 and FC sows. Post-mixing, sow cortisol (P ≤ 0.01) and sow and piglet injuries (P ≤ 0.02) were greater in GL than FC. Treatment had no effect on sow feed intake or reproductive performance (P > 0.05). Under the conditions of this research, the known benefits of two-stage GL housing were achieved at a cost particularly to piglets in terms of increased piglet mortality and injuries after mixing, and reduced piglet growth. The risk of piglet mortality decreased with older age at mixing.