Hello ma'am, thank you for your question!
Usually, the result of a pregnancy test (purchased at a pharmacy) taken on the first expected day of menstruation will be reliable.
A slight delay in menstruation may occur as a result of an episode of severe fatigue, stress, illness (or vaccination), change in schedule, or even without a discernible cause. In the majority of women, especially those with regular cycles, it usually goes away on its own.
In the absence of other symptoms, it is suggested that you wait a few weeks, and if menstruation still does not occur and the last pregnancy test is negative, it is suggested that you seek medical attention.
Hopefully, this has been helpful, so please do not hesitate if you have any further questions!
Tom Samaha, pharmacist