Due to the efforts of Norman Ryder, the influence of fertility tempo on the population renewal process is now well-known to the demographers. The fertility tempo as formulated is composed of at least two parts: mean age of childbearing and variation in the age of childbearing. Empirlcally the two parts might be highly correlated in affecting the quantity of birth, theoretically the two parts can have distinct effects on the birth. By manipulating the distribution of age-specific fertilities, this paper simulates and examines the changes in the number of births in Taiwan since 1905. Two humps in birth number after the 2nd World War were indentified in the baseline analysis. The first hump can be attributed to the decline in mortality since 1920 and the decline in fertility since 1951; the growth in population of childbearing age coupled with the decreasing propensity of bearing children have resulted in the growth and decline of birth during the 50’s and 60’s. The second hump was determined a compressed replication of the first one through the renewal process. As a consequence of the continuous fertility compression after the war, the second hump appears to be shorter in length and larger in amplitude than the first hump. Much of the “excess”birth can be eliminated if the variation of age-specific fertilities were held constant to allow for an uninterrupted operation of the "averaging effect".