Human chorionic gonadotropin or hCG is the biomarker used to detect pregnancy, as it is produced exclusively by the trophoblast (which is what gives rise to the placenta). This hormone can be detected in both blood and urine before pregnancy can be detected by ultrasound.
HCG begins to be secreted into the maternal circulation after implantation, which normally occurs 8 to 10 days after ovulation, and its concentration doubles approximately every 48 hours during the first 30 days after implantation.
Normally, urine pregnancy tests will be able to detect hCG at 4 weeks gestation, as they will only be positive when the hCG concentration is at least 20mIUI/ml. It should be borne in mind that pregnancies are counted from the last menstrual period, so that 4 weeks of gestation will actually be 2 weeks from ovulation, which is when fertilization has occurred and the embryo has been created. In other words, the hCG concentration will be easily detected about 8-10 days after implantation.
The determination of hCG in the blood will be much more sensitive and can be positive when the urine test does not yet detect the hormone. According to studies, at 8 days after ovulation it would be possible to detect concentrations of around 10MUI/ml of hCG in the blood.
