Pregnancy can be detected by a blood test as early as 7 days after ovulation, but there may be individual variations. Fertilization usually occurs within 24 hours of ovulation, and pregnancy is confirmed by a positive blood test for human chorionic gonadotropin (HCG) six to seven days after fertilization when the egg is laid. There are two ways to test for pregnancy by testing for human chorionic gonadotropin, i.e., measuring human chorionic gonadotropin in the urine, measuring human chorionic gonadotropin in the blood, and in addition to the hormone test, ultrasound confirms that a pregnancy sac is visible in the uterus, which can also confirm the diagnosis of pregnancy. Human chorionic gonadotropin in blood can be quantified directly and is more accurate. Urine hormone test can only test whether the hormone is present in the urine, thus not much human chorionic gonadotropin is secreted when the fertilized egg is first laid in the early stages of pregnancy, so there is a possibility of false negatives, and the hormone level will rise in a few days before a positive test can be made. Ultrasound will not confirm pregnancy until the sac is detected in the uterine cavity after the fertilized egg has developed to a certain degree, so it takes longer to detect pregnancy, and the earliest it can be detected is about 3 weeks after the fertilized egg has been deposited.