Everyone here is right on target, i wouldn't worry. Also I remember reading, when my son was a baby, that talking is 'just another skill' they only learn one major skill at a time and really work at it before they move on, my son worked on all things physical.. running, climbing, jumping, walking then he was working on using a fork and a spoon,, finally talking as he got pretty proficient (for his level) he would move on to the next thing.
It does help if they are around people that talk to them alot, it's all soaking into her brain, who knows you may just wake up one day and he decides to talk as if she's been studying forever..or she may work on phrases first, more words, then sentences.. I would just always talk to my son, not baby talk.
Good Luck, but at 18 mos she's still a tad young to worry, i think they say if buy 2.5/3 there aren't plenty of words at least then they explore other options, so you still have some time