A blood test to check pregnancy test paper can not be detected, it may be a short period of pregnancy, incorrect testing method, poor embryonic development or ectopic pregnancy and so on. 1. Short duration of pregnancy: women will release human chorionic gonadotropin into the blood and urine after pregnancy, and it will start to be released on the 6th day after ovulation when the trophoblast layer of the fertilized egg is formed, and this hormone can be detected in the peripheral blood after 1 day of implantation, and the hormone content in the urine at this time is low, so it is not detected by the test paper. 2. Incorrect detection: when using early pregnancy test strips should be used correctly in accordance with the instructions, if the test is incorrect, or the pregnancy time is short and the use of random urine test, did not choose the morning urine, but also can not be detected. 3. Poor fetal development or ectopic pregnancy: If the embryo is not well developed or ectopic pregnancy, it will lead to lower than normal levels of human chorionic gonadotropin, which can be detected by blood sampling, but may not be detected by test strips. If you can’t detect it with a blood test, you can wait for a few days and test again. If it is a poorly developed embryo or an ectopic pregnancy, you should intervene as soon as possible.