During what time during the pregnancy is the fetus most at risk to virus' and drug exposure?

First trimester: Or the first 12 weeks is riskiest; but some drugs are extremely dangerous at any time.

Varies : Some exposures are time sensitive, thalidamide produces undeveloped limbs only to those exposed from 6-9 wks. Some are variable, alcohol can alter facial features or heart formation in the 1st trimester but injury to brain function throughout pregnancy.Some infections cause fetal loss early, but organ injury, deafness, blindness etc if it occurs late.