This is related to how many pregnancy massages they have done over their career, but the goal here is to understand how many they do regularly.
The reason this is important is that some therapists used to work on pregnant clients but are no longer proficient in prenatal work and so haven’t seen pregnant clients in some time. Or, they’ve only ever seen prenatal clients intermittently. Their skills may or may not be rusty, but it’s a good idea to check how much prenatal work the therapist is currently doing to have a gauge of their proficiency.
While they may still be happy to book you into their schedule if they don’t regularly see pregnant clients, you might prefer to find a therapist who sees at least a few pregnant clients each week so that you know their skills are fresh.
Something else worth noting: If they bristle at your questions over the phone, it may be a sign that you’re better off with a therapist more open to your fact-finding (and more qualified to work on you during pregnancy).