Fitting age-specific fertility rates by a skew-symmetric probability density function

Mixture probability density functions had recently been proposed to describe some fertility patterns characterized by a bi-modal shape. These mixture probability density functions appear to be adequate when the fertility pattern is actually bi-modal but less useful when the shape of age-specific fertility rates is unimodal. A further model is proposed based on skew-symmetric probability density functions. This model is both more parsimonious than mixture distributions and more flexible, showing a good fit with several shapes (bi-modal or unimodal) of fertility patterns.