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 plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural possessive of stepsister is stepsisters'.
The plural form is inches. The plural possessive is inches'.
The plural form is echoes. The plural possessive is echoes'.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The plural possessive is pals'.
The plural possessive of bandit is bandits'.
Studies' is the plural possessive of study.
The plural possessive form of "weeks" is "weeks'."