this pattern raises the question of

this pattern raises the question of whether birthrates, even in times of rapid change, show a seasonal cycle To answer this question, we need to compare the columns of the table

there is clearly an annual cycle: birthrates are lowest in April and May, are highest in the summer,and seem to cycle smoothly
month-to-month the cycle is clear in the sequence of medians, in both the low and high hinges, and even in the outliers (mostly values from 1946 and 1947)