My suggestion, then, is to make the best of what you can find. There are avenues to find some base layer-type items that will fit. Check the Mothers in Motion Web site (www.mothers-in-motion.com). There you'll find a range of maternity wear for active moms-to-beshorts, pants, tops, that sort of thing. Not really outdoor stuff, but it could be used in conjunction with non-maternity outdoor wear.
Beyond that, consider buying one or two "up-sized" pieces that at least will fit around you, and can always be used again if you decide to give the new baby a sister or brother. Don't splurge herebuy house-brand stuff from REI, L.L. Bean, Lands' End, or Campmor. You can get a nice fleece jacket at Lands' End for $50, for instance. That and an inexpensive rainjacket like Marmot's Precip ($90) will get you halfway there. Alternatively, perhaps the male party responsible for this happy event has some bigger pieces that he will cheerfully loan to you.
Good luck! Send us a picture of the new arrival.