Most home pregnancy tests screen for the amount of human chorionic gonadotropin, or hCG, in the urine, as this is the major indicator of pregnancy. While the minimum amount of hCG required for any pregnancy is about 5 mIU/ml, hCG levels in early pregnancy vary greatly. A pregnancy that is detected very early, about four weeks after the last menstrual period, should have between 5 and 426 mIU/ml. In most pregnancies, this number should double about every 48 to 72 hours to indicate viability.
The majority of home pregnancy tests can detect a level of about 20 mIU/ml, which means that it may be too early to test positive on the day of the first missed period, as levels may still be under 20 at that point. For this reason, many women do not test positive prior to six weeks gestation, at which point their hCG level should be between 1,080 and 56,500. Around this point, it should be possible to see a heartbeat on an ultrasound, as this usually requires at least 2,000 mIU/ml to be accurate. It should be known that the hCG levels in early pregnancy are often lower in urine than blood, so blood tests are likely to be more accurate, and thus good for detecting pregnancy less than four weeks after the last menstrual period.
It is clear that there is a wide range of acceptable hCG levels in early pregnancy, which is why the real indicator of viability is typically whether the levels double as they should. No matter what the starting number is, it should double every two to three days, though some pregnancies increase levels at a slightly slower or faster rate for no particular reason. To find out whether levels are steadily increasing, a woman usually must go to the doctor to have her blood tested, and then return two to three days later to test it again.
Having decreasing hCG levels in early pregnancy usually indicates a miscarriage. On the other hand, levels that increase at an extremely fast rate could indicate a molar pregnancy, which is when either a placenta forms without a fetus, or a fetus forms incorrectly and does not survive long inside the womb. Women who have hCG levels in early pregnancy that start low and increase very slowly may be experiencing an ectopic pregnancy, which is when the fetus implants outside of the womb, and cannot properly grow. This type of pregnancy usually ends on its own, or needs to be ended medically in order to prevent harm to the mother, such as internal bleeding. Clearly, keeping up with hCG levels, especially prior to six weeks, can tell a lot about how a pregnancy will likely progress.