How soon after having sex can a pregnancy test detect pregnancy?

In normal circumstances, if a woman has a regular menstrual cycle and has sexual intercourse during ovulation for more than two weeks, she can take a home pregnancy test. If the test result shows two lines, it indicates pregnancy; one line indicates no pregnancy. If a woman has an irregular menstrual cycle and cannot determine the exact date of ovulation, to find out if she is pregnant, she can go to the hospital for a blood test to measure serum human chorionic gonadotropin (hCG). This method can detect pregnancy as early as one week after sexual intercourse during ovulation. The blood test results indicating a higher-than-normal hCG level suggest possible pregnancy, while a lower-than-normal level suggests no pregnancy.

Besides using a home pregnancy test and blood test, a woman can also undergo a color Doppler ultrasound examination at the hospital to determine pregnancy. This examination is generally conducted three weeks after sexual intercourse during ovulation. That is, starting from the first day of the last menstrual period, a woman can go to the hospital for a color Doppler ultrasound examination after five weeks to determine pregnancy. The above applies only to women with regular menstrual cycles. For women with irregular menstrual cycles, it is difficult to determine and check for pregnancy based on the menstrual cycle due to unpredictable ovulation times. In such cases, they can only rely on color Doppler ultrasound, blood hCG tests, or pregnancy tests conducted at the hospital for diagnosis.