Nabothian follicles occur in:
Cervical erosion: Condition where squamous epithelium of ectocervix is replaced by columnar epithelium which is continuous with endocervix. 

It occurs when estrogen levels are high as in pregnancy and use of oral contraceptives (OCP’s). 
As a result of healing of an erosion, the mouth of the cervical gland is blocked. 
The blocked gland becomes distended with secretion and forms small cysts which can be seen with naked eye and so-called Nabothian cyst.
Erosion of cervix
Ca endometrium
Ca cervix
Ca vagina