Pregnancy Tests: When Negative Is Positive

When a woman is trying to conceive, a positive pregnancy test is an incredibly exciting, emotional event. For the 43-year-old who is late for her period and doesn’t know if it means peri-menopause or pregnancy (and is hoping for peri-menopause), it’s the negative test that brings the smile and tears of joy. Before getting out the champagne and celebrating, can you be sure that your negative urine pregnancy test is really negative?


The Home Laboratory (Otherwise Known As “The Bathroom”)

Most of us take the convenience and accuracy of home pregnancy tests for granted. Today’s tests work by detecting a hormone that is produced by the future placenta and secreted in the mother’s urine by 2 weeks after conception has occurred, and can easily be performed even if you flunked high school chemistry. It’s hard for most women today to imagine not running off to the drugstore within moments of a missed period to pick up a $20 home kit.


But it wasn’t very long ago that simply peeing on a stick wasn’t an option. Prior to the 1950s, women generally just waited until the physical signs of pregnancy confirmed their suspicions. In the 1930s, the “rabbit test” was performed by injecting a rabbit (or frog, or mouse) with urine from a pregnant woman. The rabbit was then “sacrificed”  (the politically correct scientific way of saying “murdered”) in order to check for changes that occur when urine with high hormone levels are put into rabbit ovaries.


Thankfully, by the 1960s, rabbits no longer had to lose their life in the name of confirming pregnancy, but a woman still had to wait until she had missed 2 or 3 periods before traveling to her doctor’s office to give a urine specimen. Then, she’d wait days for a result, which was frequently wrong.


When the first home test finally hit the market in 1978, it took 2 hours to perform and was falsely negative 80% of the time. Despite the drawbacks, for the first time, women could discover this most private of facts in their own bathroom, forever changing the way in which we learn about our bodies.


Today’s pregnancy tests have come a long way, and that negative blue line is generally assurance that no conception has occurred. But there are times when a negative test is not actually negative. For example, some women have very long cycles, so they ovulate later than others. If ovulation and conception occur around day 24 or 25, (a common occurrence in women in their 40s who are no longer having regular cycles) a pregnancy test on day 30 will be a false negative.


Also, if urine is too dilute, the hormone concentration can be too low to be detected on a home kit performed very early in a pregnancy. Therefore a pregnancy test should always be done with first morning urine, when it will be at maximum concentration.


Even a positive pregnancy test does not always translate to a viable (destined to be a baby) pregnancy, especially if you are older, which is good news for the woman who is not hoping to be pregnant. For the same reason, even if you are excited and happy to be pregnant, refrain from putting the news on your Facebook page the minute the faintest of positive lines is discernable.


As great as the home laboratory is, sometimes you need to have the accuracy of a blood test. So don’t wait too long and don’t invest a week’s salary in repeat home pregnancy tests before you see your doctor to find out the truth.

Added to Women's Health, Pregnancy on Tue 09/28/2010