the unit of analysis is often understood as what is scored in your analysis. In research on welfare state spending of OECD countries, most people would say that the countries are the units of analysis. In your study, I would say the situation is less clear because your case – implementation of a health promotion intervention in a birthing center in a specific university – involves multiple levels. My reading is (which I explain in more detail in chapter 5 of my book Case Studies and Causal Inference) that there is not a single unit in your research (and in empirical research more generally). You could say that the hospital is the unit, or the center, or the intervention, or some other feature of your case. You can simply call the unit of analysis what you find most important about your case.
Thank you for your clarification. Indeed, I read in Yin’s book (Case studies) that I could have one case and multiple units of analysis. I will read the reference you gave to have a better insight on this and I will come back to you, if you don’t mind. But is the book accessible somewhere online or do I have to buy it? Thank you,