Just found out my son’s first mom is pregnant. Should I let her know I know?

We recently found out from another first family member that our son’s first mom is pregnant again by the same man who is the father of our son.  First mom has not told us anything about this (most of our contact is one way – us writing and sending photos to her.)  We can’t decide if we should bring it up with her or if we should act like we don’t know and wait for her to tell us, if she ever does.  Our main concerns are for first mom – to offer her what support we can – and for our son (who is 20 mo old) – to be able to know his sibling eventually – but we don’t want to intrude on first mom’s privacy.

We would appreciate hearing the thoughts of others in open adoption relationships.