  We prove a version of Warner's regularity and continuity properties for the
sub-Riemannian exponential map. The regularity property is established by
considering sub-Riemannian Jacobi fields while the continuity property follows
from studying the Maslov index of Jacobi curves. We finally show how this
implies that the exponential map of the three dimensional Heisenberg group is
not injective in any neighbourhood of a conjugate vector.
