Every day thousands of women in the world take a home pregnancy test, the kind that can be bought in pharmacies and that tell us, usually with the appearance of lines, after detecting urine, if we are pregnant or not. But how do pregnancy tests work? How reliable are they?
The operation of the pregnancy test is very simple. Although externally there are differences, inside these sticks, one of whose ends must be impregnated in urine, there is a reactive absorbent strip that detects the presence of chorionic gonadotropin (HGC) in the urine, that is, the pregnancy hormone .
We have to bear in mind that urine pregnancy tests will only be positive when there are sufficient levels of the HGC hormone in the blood, that is why at the beginning of pregnancy a test can be negative and yet we are pregnant. In general, with a HGC level below 25-50 mlU / mL (milliInternational Units per milliliter of urine), the test will be negative.
As the level increases if the urine is more concentrated, many tests recommend that the test be done with the first urine of the morning . If not, it is enough to wait a few days to perform the test (remember that it is recommended to do the test when there is a delay in menstruation for regular cycles) so that the levels of pregnancy hormone have increased and are detected by the test.
How the pregnancy test works
Pharmacy tests have a reliability of between 75 and 97 percent if the result is negative , while if the result is positive the reliability is higher: 99 percent. Remember that a urine test can give us false positives or false negatives.
The process to perform the test is simple. Once the “stick” is unwrapped, the cap that protects the test strip that we must impregnate in urine is removed. It is recommended to wet it directly when urinating (for a few seconds), and then cover it again, without coming into contact with any other substance, and wait a few minutes leaving it on a flat surface.
After three to five minutes we can read the result , which will be indicated in a different way according to each brand marketed. That is why the instructions must be read well in each case. The most common is that in a small window of the stick appear two lines to give a positive . There are also other digital systems that give us a more complete reading, and can even indicate the week of pregnancy if it is positive.
The sensitivity of the pregnancy test
When buying a pregnancy test, you have to look at its sensitivity . In a normal pregnancy, HCG can be detected in urine and blood only seven days after conception. HCG levels continue to increase, exceeding 100 mlU / ml after the first absence and doubling its levels approximately every two days until reaching a range of 11500-300000 mlU / ml in weeks 10-12 of gestation, at the end of the first trimester.
- From 7 to 10 days after conception there is a concentration of 10 mlU / ml of pregnancy hormone (HCG) or higher. This would be equivalent, in regular cycles, to 4-5 days before the lack of menstruation.
- From 10 to 14 days after conception there is a concentration of 25 mlU / ml or higher. This would roughly equal the date your period should arrive. Most tests have this standard sensitivity.
Depending on their sensitivity, pregnancy tests could be classified into:
- Ultrasensitive test: detects a concentration of HCG from 5 mlU / ml
- High sensitivity test: detects an HCG concentration of 10 to 15 mlU / ml
- Sensitive test: 20 to 25 mlU / ml
- Standard test: 25 to 50 mlU / ml
From the first day of the lack of menstruation (always assuming that it is a regular cycle), all pregnancy tests have sufficient sensitivity to detect the presence of the pregnancy hormone.
Therefore, depending on when you perform the test, you should choose the one with the highest possible sensitivity so that the result is reliable. If the result is not clear, it is advisable to repeat the test after a few days.
