In a small number of cases, pregnancy can be detected within a week after intercourse, but in most cases it takes a little longer. The main reason is that if you have intercourse before ovulation, i.e. on day 13-14 of the menstrual cycle, the sperm will enter the fallopian tube first and wait for the egg, and once ovulation occurs, about 10 minutes after ovulation, the egg will enter the fallopian tube under the action of the umbrella end of the fallopian tube and unite with the sperm to form a fertilized egg. On the 7th day of intercourse, which is 5-6 days after the union of the sperm and egg, the fertilized egg has not yet entered the uterine cavity and is therefore less likely to be tested positive. If you have intercourse on the day of ovulation, the sperm and egg will usually unite 1.5-6 hours after intercourse to form a fertilized egg. It takes about 9 days for the fertilized egg to enter the uterine cavity. It takes 2-3 days for the egg to be free and does not settle immediately after entering the uterine cavity, so pregnancy cannot be detected on the 7th day of intercourse. Therefore, it is advisable to take a blood test for chorionic gonadotropin half a month after intercourse, which will often show a positive result if you are pregnant. If urine is used for early pregnancy testing, it is recommended that the test be done 20 days after intercourse and that morning urine be used for the test. Do not drink too much water before taking the urine specimen, as this may lead to dilution of the urine and a false negative result. If you are pregnant, the positive rate will be higher.