It will start working no earlier than the fetal egg attaches to the wall of the uterus, and the production of hCG will increase. It takes 6-8 days from fertilization of the egg to implantation of the embryo. A few more days are required for the concentration of hCG to become sufficient to be able to “color” the second strip of the test.
Most tests show pregnancy 14 days after conception, that is, starting from the first day of a missed period. Some highly sensitive systems react to hCG in the urine earlier and give a response as early as 1-3 days before the expected period. But the probability of error in such a short period of time is very high. Therefore, it is recommended to conduct a pregnancy test no earlier than the first day of the expected menstruation delay or approximately two weeks from the expected day of conception.
Many women are wondering what day pregnancy occurs, and is it possible to do a test in the first phase of the cycle? It's useless. Even if intimacy happened, for example, on the 7-8th day of the cycle, pregnancy will not occur immediately, but only at the time of ovulation - when the egg is released from the ovary. As a rule, this happens in the middle of the cycle - on the 12-14th day. Spermatozoa can live in the fallopian tubes for up to 7 days. They wait for an egg to fertilize it after ovulation. So it turns out that even if sexual intercourse was on the 7th-8th day of the cycle, pregnancy actually occurs only on the 12th-14th day, and it will be possible to determine hCG in the urine using a test only at standard times - on the day the expected menstruation is delayed or a little earlier.