There are two accepted forms of making an acronym into a plural form:
As you can see, the first example will not work when you also want a plural acronym in the possessive form. I always prefer the second example for the plural of an acronym.
The possessive form of example 2 is MDs'. Of course, you can never go wrong if you spell out the acronym: Medical Doctors'.
Note: I deliberately described the form MD as an acronym rather than an abbreviation because abbreviations can be so varied (i.e., cont., n.) and clumsy to make plural or possessive. It is always proper to spell out an abbreviation when it is more than a notation.
The plural and possessive of the abbreviation works better if the acronym is used: MD (no periods) or the title spelled out: Medical Doctor.The plural for of the acronym MD is MDs.The plural possessive form is MDs'.The plural form of the compound noun Medical Doctor is Medical Doctors.The plural possessive form is Medical Doctors'.
The possessive form for the plural noun wives is wives'.
The plural possessive noun of molecules is molecules' .
The plural form for the noun buzz is buzzes; the plural possessive form is buzzes'.
The plural possessive noun for "vowel" is "vowels'".
The word sisters is the plural form; the plural possessive is sisters'.
The plural form for the noun tree is trees.The plural possessive form is trees'.
The noun "branch" is a regular plural noun, a noun that forms its plural by adding "s" or "es". The plural form of branch is "branches".The plural possessive form is branches', a regular plural possessive.
pLURAL ; orbits Possessive plural ; Orbits' (NB note the position of the apostrophe).
The plural possessive noun is racers'.
The plural possessive form for the noun senators is senators'.
Singular; Grandchild Singular possessive ; Grandchild's Plural ; Grandchildren Plural possessive ; Grandchildren's