A way of describing the lesbian parent that is not pregnant?

Perhaps something mildly humorous might do the trick. For instance:

"We're pregnant, but I'm still allowed to drink and go bungee-jumping".

"We decided that as I was better at rugby than her, she'd be the one taking a break".

"It turns out I'm having to drink for two".

1. I think my favorite, that is, the most natural and understandable way of communicating this, is to simply say,

   • "my wife is carrying our child"
   • "my wife is expecting our child"

2. Along those same lines, if one were actually looking for a title of some sort that she could to use in cases when:

   • filling out a form... or
   • others need to refer to the non-pregnant partner

   then I think the following two terms could also work equally as well:

   • "non-carrying mother to be"
   • "non-expecting mother to be"

3. I also really like mother-in-waiting, but I don't think it's quite as clear as the two above.

4. A few others that I think can work as well:

   • "Parent to be"
   • "Non-pregnant Mother to be"
   • "Partner-Mother to be"
   • "Wife-Mother to be"

She is a mother to be. Later, she will be a mother. If a male-female couple was waiting to adopt a baby (already being carried by someone else) or using a surrogate, would you need a special word to indicate "will be a mother soon" but "not pregnant"? I don't think so. Use mother-to-be when you want to emphasize the parenting that will be happening, and "pregnant woman" when you want to focus on the physical situation one of them is in. If you need to talk about both of them, "expectant parents" or "expectant mothers" will work.

Now sure, when she tells people she will be a mother soon, and they are confused because she's not visibly pregnant, she needs some sort of reply like "I'm not the one who is pregnant" or "there's more than one way to become a parent, you know" but that doesn't mean you need a word for it.

@DougM's answer of using the gender-inverted ("father-to-be") or gender-neutral ("parent-to-be") terms is a good one for general-purpose, quasi-impersonal descriptions -- such as when you are describing your friend to us. "Mother-to-be", despite the connotations, is still the most technically accurate moniker, however.

When she's asked, or it otherwise comes up for her, she generally has far more control over the structure of her response and thus can take a page from my own friend's book: "My wife* is pregnant." (Substitute "spouse", "SO", "girlfriend", or whatever other moniker she/they prefer.) Since our society is still very heavily hetero-normative, a woman saying she's a "parent-to-be" is still likely to be heard in the exact same light as if she had said she's a "mother-to-be", i.e. it will be too frequently assumed that she's the one who's pregnant. Using "father-to-be" is just likely to sow confusion.

*Note: My friend in this case is not actually married to her SO, as they live in a state that does not (did not? haven't been following progress on that front too closely) recognize homosexual marriage yet, however they think of themselves as being married and thus use those terms (well, term: "wife") in all except legal settings. Which just goes to show that the labels you choose for yourself really are entirely up to you, so to bring this back around to the question at hand: Your friend should use whatever description or moniker she feels comfortable with, regardless of what a hetero-normative society that all too frequently still despises her may think about it.

The non-birthing part of a lesbian relationship having a child is often called the co-mother (last sense—ignore the previous senses, they're very rare in normal settings, at least in my experience).

So your friend would be a co-mother-to-be or (perhaps less likely to make you suffer a hyphen overdose) expecting co-mother.

The "combination" I would use is non-expectant parent. "Non expectant, to demonstrate that her partner is pregnant, and "parent" so say that she is not the biological mother.

To make sure I am answering the question:

How would you convey the message that although somebody is a mother-to-be, they are not the ones that are pregnant?

As you would with a heterosexual couple:

My friend's wife/girlfriend/spouse is pregnant.

If the person to whom you are speaking is aware that your friend in question is female or a woman, then they will probably infer that they are lesbian and that would be the end of that. It is a common enough situation in the United States, anyways.

The more difficult question would be how to easily discuss a pregnant trans man. It would probably require more explaining which could get tiresome.

I think that referring to the wife of the mother as the step-mother is a good convention to use. People unfamiliar with it being used this way may pause when they first encounter the concept, but the meaning should be understood quickly without needing to stop for an explanation.

I think the terminology that is most likely to be adopted will be established using familiar words to which we have extended additional meaning. Inventing completely new words or attempting to establish new and unfamiliar conventions for describing parental roles seems more likely to fail because of human nature, if not active resistance.

The words husband and wife once identified specific gender roles in a marriage relationship which was only defined as being between a man and woman. Describing the spouse of a woman as a "wife" had no meaning. When we later extended the definition of marriage to also include a relationship between a woman and a woman we also extended the meaning of the word "wife" to include both women in the relationship. It was a natural extension. We didn't invent completely new terms for the roles of two women married to each other. Likewise, I think it seems natural to refer to a child's mother and step-mother. If the two women adopt a child then revert back to calling them both "mother", unless you want to be pedantic and insist on calling each an "adoptive mother".

Just asked a doctor. They have to write notes in charts about this occasionally. The term is non-biological parent or non-biological mother.

Doesn't it depend on the relationship? If I (say 'Chris') am partnered with Pat, don't I just say, "Pat is expecting/carrying our child"?

Not like I'd go around telling strangers, wouldn't the conversation be with people I know at least casually? It seems the question itself is phrased to create a way to explain something that one isn't likely to do. Once Pat is really showing, let a stranger ask, and I'd just say, "I'm the other mom."

I speak with a bit of authority here, as the lesbian non-birth parent of a 15-year old. Before the blessed day, I was his "mother in waiting." Brought a smile to frowning lips, and we all got on with things.