The aim of the study is to examine how toddlers (33 months) with weak mathematical skills differ from their peers, the extent to which those weak skills persist into preschool age (57 months), and how the group with persisting weak skills differs from the group with temporary weak skills. Participants were 692 children (344 girls, 348 boys). The children were observed during play and daily-life activities over two three-month periods to make an authentic assessment of a wide area of mathematical skills. The toddlers with weak skills had weaker skills than their peers in all parts of mathematics. The largest difference was in the numerical areas. Only 25% had persistent weak skills into preschool age. No difference in mathematical skills was found between the persistently weak group and the temporarily weak group at toddler age. The findings indicate that predicting weak development in toddlers is difficult. (author abstract)