  We study $T=0$ spin density wave transitions in two dimensional Fermi liquids
in which the ordering wavevector $\bf Q$ is such that the tangents to the Fermi
line at the points connected by $\bf Q$ are parallel (e.g. $Q=2p_F$ in a system
with a circular Fermi line) and the Fermi line is not flat. We show that the
transition is first order if the ordering wave vector $\bf Q$ is not
commensurate with a reciprocal lattice vector, $\bf G$, i.e. ${\bf Q} \neq {\bf
G}/2$. If $\bf Q$ is close to ${\bf G}/2$ the transition is weakly first order
and an intermediate scaling regime exists; in this regime the $2p_F$
susceptibility and observables such as the NMR rates $T_1$ and $T_2$ have
scaling forms which we determine.
