The Press-Schechter scheme for mass functions invokesBirkhoff's theorem in the spherical collapse that yields non-linear collapse time. However, the theorem is only proven for asymptotic flatness(vacuum background). We investigate spherically symmetric solutions with pressure and discuss the existence of a dividing shell separating expanding and collapsing regions, for which we obtain gauge invariant conditions relating not only the intrinsic spatial curvatureof the shells to the mass, but also a function of the pressure which we introduce and that generalises the Tolman-Oppenheimer-Volkov equilibrium condition.