A blood test on the 30th day of pregnancy is usually detectable. However, if menstruation is irregular, or if the menstrual cycle is long, it may not be detected. If a woman has regular menstruation with a cycle of about 28 days, ovulation usually occurs around the 14th day of menstruation, and after intercourse, the sperm combines with the egg to form a fertilized egg, and then after 6-7 days, the fertilized egg will implant, and HCG will appear in the blood soon after implantation, and the pregnancy can be detected by blood sampling in 7-10 days after the coitus. If menstruation is regular and the cycle is normal, pregnancy can be detected by blood test in 30 days. If menstruation is irregular, or the menstrual cycle is very long, ovulation may be delayed, and the development time of the embryo will be shorter, so the pregnancy time deduced from menstruation is different from the real pregnancy time, and the development time of the embryo may still be very short when the pregnancy is 30 days old, so there is no way to detect the pregnancy with blood, and you need to review the test after a certain period of time.