Data on the historical trends for menarcheal age and socio-economic conditions in Norway are used to investigate whether the reproductive system in humans is especially sensitive to environmental stimuli during any particular short age periods. The method which has been used is based on the irregularity in the trend for socio-economic development. These irregularities are reflected in the Gross Domestic Product (GDP), a quantity for which official estimates are available from 1865 onwards. Estimates of menarcheal age for women born in each year from about 1840 have been calculated from information obtained from maternity clinic records. For the period between 1900 and World War II, a close correspondence is found between the GDP in a year, and the menarcheal age of women born in the same or the preceding year. For the years before 1900, the data on menarcheal age is less satisfactory, but the correspondence may be fairly good also for this period. When values of GDP corresponding to higher ages is used, the correlation is less satisfactory. Other information on the susceptibility of the reproductive system towards environmental influences is reviewed. The following conclusion is reached: during the period around or after birth, the processes leading to menarche are clearly more sensitive than at later ages. During adolescence, the maturation process may be influenced somewhat, but probably not much as long as the conditions are not adverse. These observations may be seen as illustrating that phenomena corresponding to the critical or sensitive periods described for animal species, are also observable during the longer-lasting process of human development.