Im taking birth control

Because my doctor recommended because i have cysts in my ovaries im trying for 6 months getting pregnant but nothing happened i never used before this its my first time.

My last period was February 23 i started the pill that day

I never missed my period before im late i did afew test in are negative.

I call the doctor office and the nurse said i have to finish all the medication and then my period will come

I don't understand why my period are missing its for the pill ?

This its my first time using the pill so i don't have much idea how really works.