Pregnancy tests measure the amount of a hormone called human chorionic gonadotropin (HCG). This hormone is initially produced during very early pregnancy and allows progesterone levels to rise in order to prepare the uterus lining. HCG levels usually begin to rise to a detectable level 7-10 days after successful implantation of a fertilized egg.

During these early weeks, HCG levels usually double every 48-72 hours. Slow-rising HCG levels (i.e. a doubling time that is longer than 72 hours) can sometimes be indicative of a failing pregnancy, although it’s not always the case. Fast-rising HCG levels (i.e. a doubling time that is shorter than 48 hours) can sometimes be indicative of an ectopic pregnancy (when a fertilized egg implants outside of the uterus, often in the fallopian tubes) although it can also be indicative of a perfectly healthy multiple pregnancy!

Home pregnancy tests measure HCG levels on a qualitative basis – that is, they measure whether HCG is present above a certain level. They don’t give an actual indication of the current HCG count. Your exact HCG number at any given time (known as a quantitative result) can only be determined by an HCG blood test, available from your doctor.

Different brands of home pregnancy test have different levels of sensitivity (that is, they detect higher or lower amounts of HCG), so you may get a positive pregnancy result on one brand and a negative on another brand using the same urine sample.

There are websites available that contain galleries of home pregnancy test pictures, such as Can You See A Line? You’ll see that the intensity of colour in a positive result can vary quite dramatically, sometimes because of the stage of pregnancy and sometimes because of the brand or the concentration of urine.

Many women report seeing faint lines on pregnancy tests but they turn out not to be pregnant. This can be caused by a number of factors:

• Sometimes tests can have false positives. It’s not especially common, but it does happen.
• Many tests develop “evaporation lines” after the test has been left to sit for some time. This is simply caused by a combination of the urine sample and the chemicals in the test drying up and usually appears as a faint grey-ish line.
• There’s also the possibility of a very early pregnancy which doesn’t progress to a normal pregnancy. This is known as a ‘chemical pregnancy’ and can be enough to cause a positive pregnancy test but the pregnancy doesn’t “take” and ends around the time the woman’s period is due. Quite often in these cases, the woman wouldn’t have been aware that she had been pregnant had she not done an early pregnancy test.

As a general rule of thumb, the line on your pregnancy test should appear within the specified result time (see your test instructions, but it’s usually 5-10 minutes after adding your urine to the test) and should be visible easily without squinting!  If you have to tilt your test towards a window and look through squinted eyes, the chances are that it’s not yet a true positive result!  That doesn’t mean it won’t become a positive result though, so try again in a few days.