Congrats!!! What an exciting time for you and your family! My suggestion depends on whether you plan on having more children? This is why:
We purchased a convertible crib for our first daughter (she is turning 2) and planned to use the stages, too. But it skips a step: the twin bed! What child needs a double bed after a toddler bed? And we don't have the room!
We realized how impractical it would be to use the toddler bed stage of it if we ever decide to a second child--like now. (We are due next month!) Our daughter was almost outgrowing the crib/toddler mattress already and it wouldn't be long that the toddler bed would be uncomfortable. So we'd have to transition her to a twin mattress anyway. So, we decided to head to straight for the "big girl bed" this winter. She helped pick out the bedding (butterflies) and we made it a very exciting transition for her. She's done great. So, now the new baby will use the crib and we didn;t have to spend money on a second crib!
So, instead of us spending money on a crib and then spending money again on a twin bed later, we saved ourselves some money. Seriously, once you get the safety bars for a twin bed, there isn't much difference and now our daughter can use the twin for years and years to come in her room.
Enjoy your pregnancy and good luck with ALL your decisions!! There is sooo many to make! :-> D.