Conflicting evidence exists about whether concentrations of persistent organic chemicals in human milk decrease over the course of lactation. This has implications for the timing of sampling human milk for exposure assessment purposes. We examined this issue by measuring concentrations of polybrominated diphenyl ethers (PBDEs), hexabromocyclododecanes (HBCDs), the HBCD degradation products tetrabromocyclododecenes (TBCDs), and tetrabromobisphenol-A (TBBP-A) in human milk collected in 2010-2011 from 10 first-time mothers from Birmingham, UK. To evaluate whether concentrations varied significantly over the first 12 months postpartum, 12 samples were taken-one per month-from each mother, amounting to 120 samples overall. While concentrations of most of our target contaminants displayed no significant variation (p > 0.1) over the duration of our study, significant increases were detected in concentrations of ∑TBCDs (p = 0.029, average increase 1.4%/month) and BDE-153 (p = 0.058, average increase 4.2%/month). When compared to data obtained from a different set of UK mothers from a related but geographically wider catchment area sampled contemporaneously to this study, the ratio of median concentrations of BDE-153 to BDE-99 was markedly lower in the current study (0.46 compared to 1.32). This may reflect unidentified differences in exposure of the participants in the two studies.