A blood test is necessary for the first week of missed period to know exactly about the pregnancy. The home urine pregnancy test is not sensitive enough to detect the pregnancy hormone in the first week of the missed period. The only blood test can exactly tell whether you are pregnant or not.
The other way is to wait for further 3 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;
The laboratory testing done for the absence of period (termed as secondary amenorrhea) should include measurements of serum prolactin, thyrotropin, and FSH to test for hyperprolactinemia, thyroid disease, and ovarian failure.
The screening tests for the patient with amenorrhea include urinary HCG, complete blood count, sedimentation rate, thyroid function, prolactin, FSH and LH, and assessment of estrogen status.
Causes of amenorrhea/delayed period are pregnancy, drugs, and systemic diseases, hypothalamic and pituitary amenorrhea, "postpill" amenorrhea; hyperprolactinemia, androgen resistance.
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