The age pattern of fecundability: an analysis of French Canadian and Hutterite birth histories.

This paper analyzes the age pattern of effective fecundability from populations with no evidence of deliberate fertility control using a new convolution model of fecundability. The analysis is based on a sample of Hutterite birth histories from the mid-20th century, and birth histories of French Canadians from the 17th and 18th centuries. The main findings are as follows: 1) the level of effective fecundability is higher among the French Canadians compared to the Hutterites; 2) effective fecundability peaks at age 20 for the Hutterites, and in the early to mid-20s for the French Canadians; 3) Hutterite effective fecundability declines almost linearly from age 20 to 45, and French Canadian effective fecundability declines slowly from its peak to the early 30s, and more rapidly at older ages; and 4) the duration of postpartum amenorrhea is longer for the French Canadians than for the Hutterites. Because of the shorter periods of postpartum amenorrhea the Hutterites have about the same average number of children as the French Canadians, even though the French Canadians have higher effective fecundability.