BACKGROUND This study aimed to examine the immunity level against rubella in pregnant women of different birth cohorts. METHODS In total, 512 pregnant women who visited a primary clinic between May 2019 and March 2020 were enrolled. Information in terms of the patients' hemagglutination inhibition (HI) titers, birthdate, obstetrical history, and vaccination history were collected. Participants were divided into three generational groups according to the vaccination policy in Japan. Publicly funded vaccination was administered twice as part of a routine program in group A (n = 11), once as part of a routine program and once in a catch-up program in group B (n = 181), and once in group C (n = 320). RESULTS All groups had some women with negative rubella HI antibody titers (7.6% of all the women, 18.2% of group A, 9.4% of group B, and 6.3% of group C) and those with rubella HI antibody titers of ≤1:16 (45.1% of all women, 90.9% of group A, 56.4% of group B, and 37.2% of group C). Rubella HI antibody titers differed between the groups; group C had higher titers than that in group B. In groups B and C, the proportions of women with rubella HI antibody titers of ≤1:16 were not statistically different between primipara and multipara. CONCLUSIONS Our study showed that an increase in immunity to rubella, a vaccine-preventable disease, is nevertheless required among childbearing women to prevent rubella and congenital rubella syndrome.