What do they not mean when doctors say "limit your intake of x to y times or y amount per day?

My understanding, (and I read it somewhere too) that it just means that if you do take x, it should not be more than y amount. An example is coffee consumption by pregnant women. Pregnant women are advised to avoid coffee, so it can’t be that it means 1 cup a day, for 365 days in a row is okay.

Here is an article about pregnacy and coffee consumption.
http://tinyurl.com/pwf2852
“CSPI argues that research published in recent weeks points to a higher risk of low birth weight, miscarriage and childhood leukemia among women who consumed as little as 100 milligrams a day.”

“In its criticism, CSPI cites a recent study in the European Journal of Epidemiology that found women who consumed 100 milligrams of caffeine a day had a 14-per-cent higher risk of miscarriage and a 19-per-cent higher risk of stillbirth.”

“Numerous studies have pointed to possible links between even small amounts of caffeine and miscarriage, stillbirth or low birth weight.”

So, considering the list of harm to the fetus, they can’t mean 365 days per year is okay, regardless of the amount per day.

Is there a site out there that refers to this topic, i.e what doctors or others mean when they say x, y, z.