I point out a simple expression for the 'Hubble' parameter defined by Borde, Guth and Vilenkin in their proof of past incompleteness of inflationary spacetimes. I show that the parameter which an observer O with four-velocity v will associate with a congruence u is equal to the fractional rate of change of the magnitude ξ of the Jacobi field ξ associated with u, measured along the points of intersection of O with u, with its direction determined by v . I then analyse the time dependence of and ξ using the geodesic deviation equation, computing these exactly for some simple spacetimes, and perturbatively for spacetimes close to maximally symmetric ones. The perturbative solutions are used to characterise the rms fluctuations in these quantities arising due to possible fluctuations in the curvature tensor. © 2020 IOP Publishing Ltd.