Pregnancy tests that are OTC (Over the Counter) involve testing a first morning (the most concentrated because you haven't been drinking overnite) urine specimen and test for HCG (human chorionic gonadotropin). HCG is only produced by a pregnancy once the embryo implants (in the uterus in a normal pregnancy, and elsewhere in an abnormal place as in an ectopic pregnancy).
Many women who are trying to become pregnant are so excited at the prospect of pregnancy and can't wait to find out if they are successful. They track ovulation and then want to know when they can find out. The sperm meets the egg in the fallopian tube and forms an embryo, and then the embryo takes the journey over about a week down the tube and into the uterus where it implants.
Once implantation happens, then HCG is secreted and the woman can be tested for pregnancy. HCG is secreted 8-10 days after ovulation, and at this time the level is very low. Blood HCG testing can detect tiny amounts of HCG as low as 5IU/L (anything greater than 5 is considered positive).
In urine, however, HCG cannot be detected until it reaches a level of 20-25 IU/L in most OTC tests. You would expect a level of about 50 IU/L at the time of the missed menstrual period. Therefore, the day or two before, or the day of the missed menstrual cycle one would expect to see a positive OTC test.
Women with histories of ectopic pregnancies, or who have risk factors for an ectopic should be tested ASAP after a period is missed by blood in the doctor's office, and should not rely on OTC testing. Women who are not at high risk should wait for the missed period to test OTC (testing too early can lead to false negatives, where you really are pregnant but the test tells you that you're not) unless otherwise specified by their physician.
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