There are no real guidelines for the way in which Catholics and Orthodox fast in the bible. It’s only mentioned that we are expected to: “When you fast, do not look somber as the hypocrites do…” (Matthew 6:16). It’s mentioned in the Didache that we are to keep Wednesdays and Fridays as fasting days but honestly the specifics vary from time to time and place to place. For an Orthodox Christian the mark to aim for in fasting is the same rules the monastics follow. That is not always possible due to health, age, things like pregnancy, maturity, etc. Fasting isn’t an end in and of itself. For Catholics the penitential nature of fasting is stressed more strongly and the “rules” more concretely laid out. The biggest difference is that Orthodox Christians do it more often, fish isn’t allowed and the “rules” may differ a bit more on a case by case basis. Catholics who disregard the fast they are expected to keep will find themselves in a state of mortal sin if done knowingly and with full consent of the will. Orthodoxy doesn’t have it spelled out quite so plainly(or even a concept of “mortal” sins, although some sins are obviously more grevious than others). To knowingly disregard the fast with no good reason(there are some) as worked out between you and your confessor/spiritual father is definitely sinful. Catholics don’t expect pregnant women, the sick and persons in other special circumstances to fast exactly like everyone else is expected to, so I’m not totally sure where the confusion is coming from.