quick question...

So, if your 10th grader came home and said so&so is pregnant, and he grew up with so&so, and you and so&so's mom are friends... not wonderful friends but you love her and enjoy spending time with her (altho you never do)... and you have always liked so&so a lot because she's friendly, sweet, outgoing, kind... altho you knew that so&so was probably going to get into trouble because of who she picked for friends and how she dresses and holds herself, etc... but you still wish it wasn't so...
do you say anything to so&so's mother?

Because you & the mom are not in close contact, I definitely would not say anything.  

Do you offer support or at least let her know that the whole school is apparently discussing this situation?

If you were in close communication, I'd probably feel differently.  But as thing are now, I wouldn't.

I do not know if this is true, but so&so was discussing it with her friend (how she thinks she is) and said friend then discussed it with HER friend (which happens to be my son). It could all very well be gossip... but, if I were so&so's mother, I'd want to know if that was going around about MY daughter.

Not trying to be snarky Susan, but "why?"  Then what?
Mom's should be supporting their daughters already .... we should always sort of be prepared for bad news, or hurt feelings, or whatever.
If everyone else knows that gossip is spreading, then so & so likely knows it, too.

A few years back when my dd was in high school, a classmate of hers was behaving in a (very) less than respectful manner with the boys.
We went to all the football games because dd was a performer in the Color Guard & I saw first hand the behavior.
Dd told us that what we were seeing was common (for this girl) & she had a reputation.
I felt AWFUL about it, given that her mother was/is pretty good friend of mine.  Well, she had been.
After we changed churches we didn't see each other as much, & in a lot of ways I felt like this mom was too busy for me.
Ultimately, I did not say anything to the mother about her daughter's behavior & what we were witnessing week after week.
She had to know.
Plus, given the character of the boys she was dating, the mom likely knew.
I mean, what is she going to do about it?  Chain her kid to the bedpost & not her her out of the house?
I haven't regretted not saying something.
I figured what I might say could be hurtful to the mom, who likely already knew.
Lots of mom's here helped me to understand that I probably shouldn't say anything.

I don't know... to approach or not to approach? We don't SEE each other so I would be sending her facebook message (I don't have her phone number anymore, nor do I ever see her in person... but we do converse occasionally via facebook... we were closer in the past than we are now... when the kids were in elementary school, etc.).

See, this is the biggest reason of all in why I would not say anything.
Unless you could have a personal heart to heart about it .... I wouldn't go there.
As a mom, I'd probably feel very embarrassed if someone came to me about this.

And you know .... when I used to work ..... the ladies whom I worked with SUSPECTED that I was pregnant before I ever told them ... and we're talking within ONE week of me missing my period (my first child).  They said that I "looked different."  Wow.

Sigh... I'm praying about it, but thought I'd ask what the appropriate thing to do is.

Praying is the best that you can do.   ((hugs))