If you are exposed to unprotected sex, a blood test is necessary in the first week of missed period to know exactly about the pregnancy. Home urine pregnancy test are not sensitive enough to detect the pregnancy hormone in the first week of missed period. Only blood test can exactly tell whether you are pregnant or not.
The other way is to wait for further 5 days and if your period does not come, you may do a home pregnancy test again, which in the second week is able to tell with reasonable certainty as pregnancy hormone is raised in urine, by this time.
Apart from pregnancy, there are other causes of missed or delayed periods also;
1) Ovarian hormone dysfunctions
2) Thyroid hormone abnormalities
3) Stress, anxiety, emotional disorders
4) Sudden weight loss or gain
5) Excessive physical activity
Read the resource under the heading secondary amenorrhea;
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