  This paper sets out a forecasting method that employs a mixture of parametric
functions to capture the pattern of fertility with respect to age. The overall
level of cohort fertility is decomposed over the range of fertile ages using a
mixture of parametric density functions. The level of fertility and the
parameters describing the shape of the fertility curve are projected foward
using time series methods. The model is estimated within a Bayesian framework,
allowing predictive distributions of future fertility rates to be produced that
naturally incorporate both time series and parametric uncertainty. A number of
choices are possible for the precise form of the functions used in the
two-component mixtures. The performance of several model variants is tested on
data from four countries; England and Wales, the USA, Sweden and France. The
former two countries exhibit multi-modality in their fertility rate curves as a
function of age, while the latter two are largely uni-modal. The models are
estimated using Hamiltonian Monte Carlo and the `stan` software package on data
covering the period up to 2006, with the period 2007-2016 held back for
assessment purposes. Forecasting performance is found to be comparable to other
models identified as producing accurate fertility forecasts in the literature.
