Ordinary Differential Equations – Probability of Birth

Say you have a system with a certain number of men $ M (t) $ and a number of women $ F (t) $and we want to model the evolution of populations over time. It is obvious that the respective mortality rates are easy to determine and apply:

$$ frac {dM} {dt} = – d_MM (t) $$
$$ frac {dF} {dt} = – d_FF (t) $$

Now, I want to apply birth rates. My assumptions are that male births are less common than female births, $ b_f> b_m $but $ b_f + b_m = $ 1 and that every woman gives birth to one child each cycle.
Should I just add the terms $ b_iF (t) $ to each respective equation to model births, but this seems false; I feel like he's missing some probability, if we have three moms he's not there $ b_M * $ 3 male children and $ b_F * $ 3 girls, there is a $ b_F $ chance to have a girl …

Thank you.