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In a regular menstrual cycle of 28 days, ovulation generally occurs on the 14th day of menstrual period.
One can become pregnant due to intercourse, on or around the day of ovulation.
Thus you could be pregnant, if you had unprotected sex around expected date of ovulation.
Urine test for pregnancy is not very sensitive. Ideally, you should get blood test
for pregnancy done.
In a women of child bearing age, the most common cause of abnormal uterine bleeding
is pregnancy and its complications like ectopic pregnancy and miscarriage
Other causes are hormonal imbalance, trauma, anovulatory cycle
, vaginitis), uterine polyp/fibroid, P.I.D., cancer (of vagina, cervix, uterus, ovaries), certain drugs and bleeding disorder
Detailed history, examination and investigations are required.
Thus you should consult a Gynecologist to establish a diagnosis and treatment.
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