Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to support someone facing a breakup while pregnant, use this factual base: acknowledge the loss as they describe it and notice the immediate housing, childcare, work, or social impact.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer one practical task or a form of company that does not require them to retell the story. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not attack the ex, push dating, demand details, or share the news before they are ready. For abuse, stalking, housing danger, child risk, self-harm, or immediate danger, connect them with specialist or emergency support. You do not need the other person to agree with a boundary for you to follow it consistently.