Pregnancy test sensitivity depends on the timing of the test related to the expected menstrual period, but cycle length matters too. (courtesy of Alex Sundermann)
Using more than 19,000 self-reported home pregnancy test results from 5,979 participants in the pre-conception cohort, Pregnancy Study Online (PRESTO), a recently published study in Fertility and Sterility, found that the probability of a positive pregnancy test is closely linked to cycle length and day of testing among individuals who are pregnant.
“The goal of this research was to determine the sensitivity of home pregnancy tests based on timing of test and menstrual cycle length with real-world use,” said Alex Sundermann, MD, PhD, Assistant Professor in the Division of Quantitative and Clinical Sciences of the Department of Obstetrics and Gynecology and the study’s first author.
“People often want to know how early they can take a pregnancy test, but the earlier you test, the more likely you are to get a negative result even when you’re actually pregnant. We wanted to provide evidenced-based information that helps people make informed decisions about when to start testing — and how to interpret results — so they can better understand what an early negative test may mean.”
Using the data from the self-reported pregnancy test results, Sundermann and her team built a free online tool called Positively Pregnant. This tool allows women to enter information about their menstrual cycle characteristics and timing of intended testing to received evidence-based estimates of expected pregnancy test performance.
As expected, the study showed pregnancy test sensitivity depended strongly on timing of test in relation to the expected menstrual period. One surprising finding, however, was that cycle length also mattered. For the same testing day, those with shorter cycles were less likely to get a positive result than people with longer cycles.
“Instead of giving patients a one-size-fits-all answer, the tool takes those findings and turns them into an individualized estimate,” Sundermann said.
Ronald Alvarez, MD, MBA, Chair and Clinical Service Chief of Obstetrics and Gynecology at Vanderbilt Health, and the Betty and Lonnie S. Burnett Professor of Obstetrics and Gynecology, said Positively Pregnant gives patients more information and control.
“Dr. Sundermann’s new tool provides patients with the power to make a more individualized decision about when to do an early pregnancy test,” he said.
The tool not only gives estimates for the requested day, but also for the subsequent three days. That way, users can see how the chance of detecting a pregnancy increases as they get closer to — or move past — the date of their expected period.
Sundermann said they build a test performance “forecast” so the tool can help people make a decision about when to start testing.
“Test sensitivity changes substantially from day to day, particularly when you’re testing before your expected period,” she said. “We know that individuals have different values when it comes to pregnancy testing.
“Some might choose to take a test later when they know they can trust the result because they want to be spared the uncertainty that can come with early testing. For others, the chance of detecting a pregnancy even one day sooner is worth the risk of having a negative test that may later be positive. We wanted people to be able to see the trajectory of test performance so they can make the best decision on when to test based on their priorities.”
