Relationship between decreasing fertility during the post-war period and maternal age in a Japanese population.

This research was performed in an effort to understand the decrease in fertility that has occurred over the past few decades. The objective of the study was to analyze female fertility according to maternal age; data were based on the number of children born per mother. The records of 18-year-old college students were obtained, and the mothers of the students were categorized into age groups according to the year of their birth (1915 to 1949). The number of children born to each mother was then analyzed. The total sample size was 4078. The results showed that an increase in two-children families led to a reduction in the mean number of children per mother. While the decrease in the maternal age at the time of the birth of the last child in the family was observed, the maternal age at the time of the first birth did not increase. Thus, the reduction in fertility may not be the result of delayed motherhood. The group of mothers, who gave birth to the largest number of children, had their highest fertility rate in the twenties. In addition, their fertility rate in the thirties was almost equal to other groups, who had the same fertility level in their twenties.