An effective field theory is derived that describes the low-frequency spin
dynamics in the low-temperature orthorhombic phase of La2CuO4. Restricted t
o a single CuO2 layer the effective theory is a simple generalization of th
e relativistic nonlinear sigma model to include all spin interactions allow
ed by symmetry. Incorporating a weak interlayer interaction leads to two co
upled nonlinear sigma models which provide an efficient description of the
complete bilayer dynamics. Particular attention is paid to the weak-ferroma
gnetic and spin-flop transitions induced by external magnetic fields. The m
ain features of the observed (covert) weak ferromagnetism are thus accounte
d for in a straightforward manner but some of the finer theoretical predict
ions would require further experimental investigation. The derived framewor
k is also suitable for the study of the structure and dynamics of magnetic
domains in undoped La2CuO4.