The factor influencing the effect that teratogens have on prenatal development is the timing of exposure.
1. **Timing of Exposure:** When a teratogen (substance that can cause birth defects) interacts with the developing embryo or fetus during a critical period of development, the impact can be more severe. For example, exposure to certain teratogens during the first trimester of pregnancy when major organs are forming can lead to significant birth defects.
Therefore, the correct answer to the question is:
- Timing of exposure
