One thing that researchers sometimes do is to keep studies smaller in scope to minimize the scope and size. This is often a budgetary thing. So a study might be limited to first time parents with no complications. That’s normal and appropriate, but as a reader, we need to remember that those limited samples don’t necessarily apply to the general population. You client might be having her third baby and dealing with gestational diabetes. The data from this study doesn’t really apply.
Sometimes it’s not quite so obvious. One example is large scale home birth studies coming out of the Netherlands. You might think that humans are humans and it still applies, but the reality is that the *medical system* could be very different. Transfers for complications are going to be much more seamless in an integrated health system like the Netherlands than in (for example) a state in the US where out of hospital midwifery is illegal.
In the past, most medical research was done with adult white men. But women, children, and people from a variety of cultures and backgrounds need to be represented in the research as well.
Most studies have detailed information on the sample they studied as far as age, parity, socieconomic status, etc etc etc. The closer the population studied is to your clients, the more it applies.