Some teratogens are harmless until they reach a certain level of ...

[humphriesbr@me.com]

Some teratogens are harmless until they reach a certain level of exposure; they then become dangerous, thus demonstrating the
 
  critical period effect.
  threshold effect.
  fetal teratogen effect.
  interaction effect.

Question 2

Jan took several prescription drugs early in her pregnancy before she had missed her menstrual period. What advice will the doctor likely give to Jan?
 
  Have an abortion because the embryo will certainly have severe birth
  defects.
  Since the placenta had begun to develop, it is not likely that the drugs caused
  any harm.
  Many prescription drugs are harmless in low doses, but the doctor would
  want to know specific information about the drugs.
  It is highly unusual for prescription drugs to have any effect this early in the
  pregnancy.

[jxjsniuniu]

Answer to Question 1

threshold effect.

Answer to Question 2

Many prescription drugs are harmless in low doses, but the doctor would
want to know specific information about the drugs.

[humphriesbr@me.com]

Excellent

[dyrone]

Thanks for the timely response, appreciate it