Positively Pregnant tool helps patients time home pregnancy testing

Positively Pregnant, a free online tool, gives users evidence-based estimates of the probability of a positive home pregnancy test based on cycle length and timing of testing, according to Alexandra C. Sundermann, MD, PhD, an epidemiologist in obstetrics and gynecology.

Sundermann worked with the Pregnancy Study Online (PRESTO), a prospective preconception cohort, and her team analyzed data from nearly 20,000 pregnancy tests among 6000 participants to characterize real-world pregnancy test performance.

The findings were published in Fertility and Sterility earlier in the summer and were subsequently translated into the online tool.¹ The goal was to provide day-specific estimates of test performance so patients could make informed decisions about when to start testing and how to interpret a negative result, Sundermann said.

Users accesse the tool at positively-pregnant.com and enter information about their cycle and the timing of their intended test. The tool returns an evidence-based estimate of the probability of a positive test on that day if the user was pregnant, along with a forecast of how that probability changed over the following 3 days, allowing users to see how the chance of detection increased as they approached or moved past their expected period.

Sundermann said the 3-day forecast was built to help patients make an informed testing decision rather than simply naming a single "best" day to test.

"Test sensitivity changes substantially from day to day, particularly when you're testing before your expected period," Sundermann said. As an example, she noted that a 52% probability of a positive test on a given day might rise to 88% just 2 days later—information that could change a patient's decision about when to test.

Sundermann noted that patients approached testing with different priorities. Some preferred to wait until they could trust a result, avoiding the uncertainty that can accompany early testing, whereas others valued the chance of detecting a pregnancy even a day earlier, even at the risk of a false negative that might turn positive within days.

"We wanted people to be able to see the trajectory of test performance, so they could make an informed decision that best aligned with their priorities," Sundermann said.

Sundermann is an epidemiologist in obstetrics and gynecology who worked with the Pregnancy Study Online (PRESTO), a prospective preconception cohort.

REFERENCE

1. Sundermann AC, Jasper EA, Rothman KJ, Wise LA. Home pregnancy test sensitivity by test timing and cycle length with real-world use among pregnancy planners who conceived. Fertil and Steril. 2026 Jun 16:S0015-0282(26)00514-5. doi: 10.1016/j.fertnstert.2026.06.022