Ocean acidification alters the chemical nature of seawater heading towards a more acidic ocean. Several studies have shown the vulnerability of calcifying organisms, especially the sensitivity of their early life stages. Here, we present the results of the effects of increasing pCO2 and associated decreasing pH on the early life stages of the Baltic tellin Macoma balthica. Fertilization success, embryonic and larval development were examined under 3 pH treatments (pHNBS ~ 8.11; pHNBS ~ 7.79; pHNBS ~ 7.56). Fertilization success after 24 hours was only significantly affected under pHNBS 7.66 and was reduced with ~ 15.3%. Embryonic development showed similar hatching rate after 3 days at pHNBS 8.11 and 7.77 however, the hatching rate was significantly reduced by ~ 62.4% under pHNBS 7.54. In contrast to the absence of a significant effect of ocean acidification on both fertilization and embryonic development at pHNBS ~ 7.77, larval development (day 3 – day 19) was significantly altered. Mortality rates were significantly elevated whilst growth rates were significantly reduced. No significant delay in metamorphosis was observed. Remarkably, no more severe impacts were found with a decrease of ~ 0.55 pHNBS unit compared to the decrease of 0.32 pHNBS unit. Despite these reductions, gametes of Macoma balthica are still able to fertilize in acidified seawater and the embryos and larvae are capable to develop even in seawater undersaturated with respect to aragonite. However, the observed increased mortality and reduced growth will augment pre-settlement mortality. Consequently, future Macoma balthica populations are likely to shrink under elevating carbon dioxide pressure and decreasing pH.