  We develop a semi-analytical model to describe bound states in the continuum
(BICs) in photonic crystal slabs. We model leaky modes supported by photonic
crystal slabs as a transverse Fabry-Perot resonance composed of a few
propagative Bloch waves bouncing back and forth vertically inside the slab.
This multimode Fabry-Perot model accurately predicts the existence of BICs and
their positions in the parameter space. We show that, regardless of the slab
thickness, BICs cannot exist below a cut-off frequency, which is related to the
existence of the second-order Bloch wave in the photonic crystal. Thanks to the
semi-analyticity of the model, we investigate the dynamics of BICs with the
slab thickness in symmetric and asymmetric photonic crystal slabs. We evidence
that the symmetry-protected BICs that exist in symmetric structures at the
{\Gamma}-point of the dispersion diagram can still exist when the horizontal
mirror symmetry is broken, but only for particular values of the slab
thickness.
