A little bit out-of-the box, but an expression that can be used is
Let me congratulate you guys on your good news!
You could even simply use news, which would make it truly a one-word expression.
Good news, or great news can mean many things for a couple (getting a job, getting a house, getting married) but when they have recently found out they are expecting a baby, it is not uncommon that nothing else is as good news as the news about that pregnancy.
So, yes, the applicability of good news, (or great or fantastic, or even just plain news) depend very much on context, but that is normally not a problem - unless you have a habit of going around congratulating random people on their going to have a baby.
In the latter case, do not congratulate random people on occasion of the great news!
As an afterthought, there may even be a
zero-word option:
Let me congratulate you guys!
Most expectant parents will immediately assume what it is you are congratulating them with - the same way you do not have to specify to newly-weds right after the ceremony that your congratulations relate to their new marital status!