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What is What is the effect of Progesterone-only pills? and explain in detail?
Ref. KDT. Page. 343
Progestogen-only pills have multiple biological effects on the female body.
Multitude effect of POP:

Inhibition of follicular development
Inhibition of ovulation
Suppression of corpus luteum formation
Changes in the nature of the cervical mucus which inhibits the passage of sperms into the uterine cavity – Thickening of cervical mucous secretion
Effects on the endometrium which affect implantation – by making it thin