This abjectly false and incorrect. Pregnancy is a normal physiological state — there is not increased risk to the mom with just about everything. In the first trimester there is high risk of teratogenesis with exogenous medication administrations. There are some disease states that are deleterious in pregnancy, however, you are painting with far too broad of a brush with your statements.
You're missing the forest for the trees. To the mom, yes. The mom isn't the only player in the game though, which is where everything else that I was trying to point out comes in. No pregnant woman (well, not any that consider themselves pregnant with a child) walks into a doctor's office or hospital thinking "well, so long as I end up ok, that's good enough for me."
Women aren't predictably pregnant according to a clinical trial schedule (given a large enough population, that's really not a big limitation though), they're only pregnant for 42ish weeks maximum, so the duration of their condition is limited, and they are rarely interested in taking part in medical trials, particularly ones early in the process where enough time has passed to give a reasonable approximation of safety. My overall point was that because of these reasons, the sample size is necessarily going to be smaller, so the statistical reliability of any side effect being caused by a medication is less. My second part about an expectant mother's system being stressed is more related to a side effect of an experimental medication being listed as being an effect of the pregnancy or the medication. If the trial administrators are being faithful, if you can't eliminate it as being caused by the drug, every side effect is captured just in case it comes up later.