This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax email@example.com with numerous contributions by others.
Should be avoided during the first trimester of pregnancy. People who
have had a reaction (which is very rare) should avoid it.
Vaccine components capable of causing adverse reactions: for
diptheria, thimerosal and toxoid; for tetanus, thimerosal and toxoid
(Travel Medicine Advisor).
(Note: There is currently, as of 1999, a move toward replacing
thimerosal in vaccines, so the reference to thimerosal here may
shortly be out of date.)