Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
Some abbreviations must take periods, some optionally take them
and some don't take them. Periods are more likely to be used when
an abbreviation is sounded out as letters than when it is an acronym,
but this distinction cannot be taken as a rule. In cases where periods
are optional, the general trend today is to omit them, but this is by
no means universal.
The following are rules and guidelines on when to include periods:
INITIALS OF PEOPLE'S NAMES
Always include periods.
C.S. Lewis
F. Scott Fitzgerald
George C. Scott
Franklin D. Roosevelt
TITLES, HONORIFICS
Follow the convention that is most appropriate for your audience.
In North America, periods are expected.
Mrs. America
Mr. Magoo
Dr. Seuss
Mount St. Helen
Fred Wincourt, jr.
Godfrey Barker, Esq.
Screech Owl Recording Studios, Inc.
Porcupine Quill Duvet Manufactur-
ers, Ltd.
In Britain and some other Commonwealth countries, the period
is omitted for abbreviations that include the final letter of the word:
thus, Mrs, Mr, Dr, etc.
Note that the honorific Ms-or Ms. -is a special case. Many writers
spell it with a period, and indeed some style guides say to do
so. Presumably this is to make it equivalent to its male counterpart,
Mr. However, given that the word isn't actually short for anything,
leaving the period off is legitimate as well.
For academic degrees, the period is optional, or at least varies
according to different style guides.
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R.N. RN
M.D. MD
B.A. BA
M.Sc.
Ph.D.
D.D.S.
MSc
PhD
DDS