You may think you have all the signs of pregnancy — tender breasts, food aversions or cravings, fatigue, nausea — but the pregnancy test comes back negative. You visit your doctor’s office and still, nothing. But you know your body, and the symptoms become consistent. You insist your doctor give you another ultrasound and boom, you’re pregnant. How can this be possible? What can cause a false negative pregnancy test?
While this scenario is uncommon, it can happen. There are many explanations for this outcome, however, the most unheard-of reason — the hook effect.
What Is the Hook Effect?
This concept baffles health professionals and mothers alike because of its effect on lab results. The hook effect is a scientific term for a rare lab test glitch that causes a faulty result. And it doesn’t discriminate. It can alter any kind of medical lab test, such as blood, urine and saliva. It will give you a false negative when you should have gotten a positive result. One reason many women get a false-negative as a result of the hook effect — high levels of human chorionic gonadotrophin (hCG).
During pregnancy, women produce hCG, a hormone created by the placenta after implantation. These hCG molecules bond with a ‘signal’ antibody and a ‘capture’ antibody, which is what a pregnancy test detects. Having too much hCG in urine or blood will ‘swamp’ these antibodies so they can’t be detected. And just like that, your test comes back negative.
Professor Mark Umstad AM, Clinical Director of Maternity Services at the Royal Women’s Hospital Melbourne, told Kidspot that although the probability of this happening is rare, it does happen. In fact, Umstad explained it affects women carrying multiple babies because they produce the highest levels of hCG. Moreover, it’s seen in molar pregnancies, where a non-viable embryo implants in the uterus and forms into a cluster of abnormal cells.
Diluted Levels of hCG
While having too much hCG can produce a false-negative, having low levels can render the same results. If you’re planning on taking a drugstore pregnancy test, it’s best to take it in the morning before drinking water or other fluids.
Diluting the urine can spread out the presence of hCG which can go undetected in a pregnancy test.
Incorrect Use or Early Use of a Pregnancy Test
Taking a pregnancy test at home seems more convenient, but can leave you dissatisfied for a number of reasons. The incorrect use of a pregnancy test or early use of a pregnancy test can produce a false-negative. While you may be feeling all of the pregnancy symptoms, give your body a few more days. The hCG levels in your body aren’t high enough to be detected until six days after conception.
Dr. Anita Sadaty, a board-certified obstetrician-gynecologist, told Verywell Family the earliest you can expect to “feel pregnant” after sex would be around seven days, but it takes two to four weeks before pregnancy symptoms are noticeable.
Miscalculated Menstrual Cycle
The biggest pregnancy-scares and let downs often result from a miscalculated menstrual cycle.
According to the FDA, “10 to 20 pregnant women out of every 100 will not detect their pregnancy on the first day of their missed period.”
Similarly, Dr. Amy Autry, clinical professor of obstetrics-gynecology and reproductive science at UC San Fransisco, told Everyday Health at least 30 percent of women have irregular periods during their childbearing years. Which makes it that much harder to detect a false-negative.