We evaluated whether early-life protein restriction alters structural parameters that affect β-cell mass on the 15th day and 20th day of gestation in control pregnant (CP), control non-pregnant (CNP), low-protein pregnant (LPP) and low-protein non-pregnant (LPNP) rats from the fetal to the adult life stage as well as in protein-restricted rats that recovered after weaning (recovered pregnant (RP) and recovered non-pregnant). On the 15th day of gestation, the CNP group had a higher proportion of smaller islets, whereas the CP group exhibited a higher proportion of islets larger than the median. The β-cell mass was lower in the low-protein group than that in the recovered and control groups. Gestation increased the β-cell mass, β-cell proliferation frequency and neogenesis frequency independently of the nutritional status. The apoptosis frequency was increased in the recovered groups compared with that in the other groups. On the 20th day of gestation, a higher proportion of islets smaller than the median was observed in the non-pregnant groups, whereas a higher proportion of islets larger than the median was observed in the RP, LPP and CP groups. β-Cell mass was lower in the low-protein group than that in the recovered and control groups, regardless of the physiological status. The β-cell proliferation frequency was lower, whereas the apoptosis rate was higher in recovered rats compared with those in the low-protein and control rats. Thus, protein malnutrition early in life did not alter the mass of β-cells, especially in the first two-thirds of gestation, despite the increase in apoptosis.