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Date of expiry doesn't mean that the drug will become ineffective suddenly. The potency of drug starts decreasing from the date it is manufactured & date of expiry is decided on the basis of lab test. It is expected that before date of expiry it will contain the drug in sufficient quantity but after that it might not be effective. Thus you are definitely at the risk of pregnancy
. In a regular cycle of 28 days, ovulation is expected to occur on the 14th day. If you had unprotected sex near by date of ovulation, the chances further increase.
Wish you good health,