The accuracy of the HCG blood test after 4 days of delayed menstruation depends on how long it has been since the previous intercourse, and the blood test after 10 days from the previous intercourse is more accurate. If your period is delayed for 4 days, but the time from the previous intercourse is relatively short, the blood test for HCG will not be accurate. If your period is delayed for 4 days, but the time from the previous intercourse is 10 days, the blood test for HCG will be more accurate. Delayed menstruation for 4 days may be too much stress, long-term malnutrition, polycystic ovary syndrome and other reasons, such as menstruation 2 days before the onset of menstruation, menstruation due to the impact of the above factors delayed for 4 days, 6 days of coitus to take a blood test for pregnancy, the results of the temporary test can not be pregnant. At this time, the fertilized egg may not have implanted, and even if the implantation is successful, the trophoblast cells have not yet secreted HCG, so the pregnancy test is not available for the time being. If your period is delayed for 4 days, more than 10 days after the previous intercourse, the delayed period may be caused by pregnancy, and it would be more accurate to go to the hospital to take a blood test to check HCG for pregnancy. It is recommended that those who have delayed menstruation should have their blood drawn under the guidance of a doctor.