Pregnancy Test

Topic: What to Make of the Dreaded Faint Line on a Pregnancy Test

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’s always the fear of false positives, false negatives, and faint lines on the pregnancy test. So it’s helpful for couples to understand what home pregnancy tests are actually measuring and how sophisticated they’ve 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 minute amounts as soon as a fertilized egg has implanted in the lining of the womb. 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 essentially do the same thing: Urine is deposited on one end of the testing device and wicked via microcapillary material through the test substrate. For home pregnancy tests the substrate is basically a strip of material impregnated with antibodies that react to the presence of hCG. When the substrate reacts with the urine it produces a reaction that displays the result.

Fairly recently in history, finding out how your were pregnant was far more complicated. “Back in the late 60s, we were still injecting the urine of women into rabbits. It sounds gross but it’s true,” says Dr. Mary Jane Minkin, Clinical Professor of Obstetrics, Gynecology and Reproductive Sciences at Yale University School of Medicine. The process back then relied on the presence of estrogen in the urine. “The extra estrogen would cause the rabbit’s uterus to get big.”

If you’ve ever heard someone use the euphemism “the rabbit died” for a woman getting pregnant, this is likely where it came from — though there’s little evidence to suggest the rabbits actually died from the process.

False Positives

Most home pregnancy tests claim to be 99 percent accurate in detecting pregnancy. But the body does produce hormones that can cross-react with antibody tests, according to Minkin. “The luteinizing hormone which is made by the pituitary gland, triggers ovulations by helping to kick the egg out of the ovary and can cross react,” she says. That’s particularly important for couples who are undergoing fertility treatments. “Sometimes we administer a large dose of LH to induce ovulation. If you have a bunch in your system there can be a remote possibility of cross reactivity.”

Topic Discussed: What to Make of the Dreaded Faint Line on a Pregnancy Test

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