I lived in Pure Collection's ponchos while I was pregnant. Although they're not officially maternity, they more than accommodated my belly up until my delivery (and let's just say I wasn't one of those women who barely showed). Two of my favorites: the thinner Gassato cashmere poncho (shown far left, $136-$163, originally $274) and the heftier Cable lambswool and cashmere poncho ($189-$223, originally $298). Both styles are available in multiple colors.
Pure Collection Ponchos