Welcome to HCM.
If you are definite that you do not want to be pregnant and in case you are pregnant you will abort it, then there is no harm in taking regestrone
tab. It will not abort the fetus if you are already pregnant, in which case you will not have withdrawal bleeding. If you are not pregnant you will get withdrawal bleeding. Negative urine pregnancy
test doesn't guarantee that you are not pregnant, it only indicates that the pregnancy is not of that duration which can be detected by the test. Only sure way of confirming you are not pregnant is when you get your normal periods.