True, we are apex predators, and thus apex toxin accumulators.
But heavy metals and other accumulative toxins are not the reason that pregnant women -- or anyone, for that matter -- should not eat people. I haven't run the numbers, but I'm pretty sure that you won't ingest that much cadmium and whatever, even if human flesh is your primary food source.
The real peril is Transmissible spongiform encephalopathy -- "Mad Human Disease" -- which begins with a single strand of broken protein.
Also, social-evolutionary pressures tend to work against cannibalism, in the long run. Maybe. I hope.