Couples hoping for a positive pregnancy test can have their patience tested while waiting for results – even if the wait is as short as one minute for some home tests. And when the result does show, there is always the fear of false positives, false negatives and dull lines on the pregnancy test. It is therefore helpful for couples to understand what home pregnancy tests actually measure and how sophisticated they have become since their first appearance 60 years ago. Because understanding how pregnancy tests work can help couples read them correctly and boost confidence in a positive pregnancy test result.
What a pregnancy test measures
A positive pregnancy test measures the presence of a hormone called human chorionic gonadotropin (hCG) in the urine. The hCG hormone is produced by the placenta and can be detected in small amounts once a fertilized egg has been implanted in the uterine wall. Couples can get a positive pregnancy test result as early as 8 days after ovulation and 8 days before a missed period.
How a pregnancy test works
All stick-style home pregnancy tests do essentially the same thing: Urine is deposited on one side of the test device and deposited through the test substrate via microcapillary material. For home pregnancy tests, the substrate is basically a strip of material with antibodies that respond to the presence of hCG. When the substrate reacts with the urine, it produces a reaction that displays the result.
It has been pretty much more complicated in history quite a bit to find out how pregnant you were. “Back in the late 60’s we still injected the urine of women in rabbits. It sounds rough, but it’s true, ”said Mary Jane Minkin, MD, clinical professor of obstetrics, gynecology and reproductive science at Yale University School of Medicine. The process at the time relied on the presence of estrogen in the urine. “The extra estrogen will cause the rabbit’s uterus to grow.”
If you’ve ever heard of someone using the euphemism “the hare died” for a woman getting pregnant, this is probably where it came from – although there is little evidence to suggest that the rabbits actually died from the process. .
False Positives on Pregnancy Tests
Most home pregnancy tests claim that they are 99% accurate in detecting pregnancy. But the body does produce hormones that can cross-react with antibody tests, according to Minkin. “The luteinizing hormone, made by the pituitary gland, causes ovulation by helping to kick the egg out of the ovary and can cross-react,” she says. This is especially important for couples undergoing fertility treatments. “Sometimes we administer a large dose of LH to cause ovulation. If you have a lump in your system, there may be a remote possibility of cross-reactivity.”
False negatives on pregnancy tests
Because tests rely on the detection of hCG, there must be a concentration of the hormone before it can work. Current tests require at least 8 or 9 international units of hCG per milliliter. If the concentrations are too low, it is unlikely to be a positive result. If a couple has missed a period and a test comes back negative, the best way to take action is to retest or visit an OBGYN for confirmation.
Blurred line on a pregnancy test
Home pregnancy tests distinguish themselves largely by their speed (as fast as a minute), or the way they indicate results (digitally, using pluses, minuses, lines, or colors). But the indicators are not always clear. Sometimes, because nerves run high, user error or manufacturing differences between tests, the results can be faint or ambiguous.
But home pregnancy tests do not give “maybe” as an answer. They either detect adequate hCG levels or not. Very occasionally, in some tests, urine entering the rash area may evaporate and cause an indicator to faint. In cases where the test has been checked within the recommended time, it is unlikely that evaporation-derived indicators will show. Chances are that if a couple used the test correctly and an indicator appears, they are pregnant. That said, there is nothing wrong with overdoing it.
Minkin, who consulted for First Response, notes that the company has an app for this circumstance. Users can point their phone camera to their test and the application will give a clear “pregnant” or “not pregnant” based on the indicator lines.
It’s definitely better than watching the rabbit’s uterus grow.
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