Thanks for the query.
Regesterone is a progesterone
hormone which help in withdrawal bleeding after use in ammenorhea (no menses). It has no role as a contraceptive ie to avoid pregnancy.\It would definitely regularize your cycles if you use it for last 10 days in your cycle.
But that wont provide you contraception
. You can still conceive if you have unprotected intercourse
. As you say you had unprotected intercourse this cycle and you have missed your periods , it is possible that you are pregnant. So i suggest you do a urine test for pregnancy. Incase its negative, then you can relax. If you have irregular cycles for more than 3 months, you need a proper evaluation which includes ultrasound and hormonal tests to rule out pathological causes of irregular periods
. TO regularize your periods , you can use oral contraceptive pills
which will also provide you contraception instead of regesterone which would just help you in getting periods but doesnot provide contraception.
Hope i have answered your query.