Rubella Affects Only a Small Percent of the Offspring of Mothers

Multiple Choice

Rubella affects only a small percent of the offspring of mothers who contact the virus within 2 weeks of conception, whereas it affects one half of the offspring of mothers infected during the first month following conception. How can these statistics best be explained?

A) The effects of a teratogen depend partly on timing.
B) The physical state of the mother during the first 2 weeks post-conception is much better than it will be at any other point in pregnancy so she is better able to fight off the effects of rubella.
C) Those exposed to rubella within the first month of conception were simultaneously exposed to many other toxic agents.
D) Teratogens differ in how they gain access to the fetus.