Late Period or Pregnant? Probability Calculator

What the bars mean

The bars are an illustrative comparison between pregnancy and usual cycle variation under simplified assumptions. They add to approximately 100%, but are not measured personal odds. Stress, illness, hormonal contraception, breastfeeding, perimenopause and conditions that stop periods are outside this two-part comparison. A high pregnancy bar does not establish pregnancy; a high cycle-variation bar does not identify why a period is late.

Dates and cycle information

Your expected period date is today minus the days-late value you enter. Average cycle length provides the cycle recap and helps identify tests from before this cycle; changing it alone does not change the exposure assumption. The variation slider describes an approximate typical difference from your average, which the calculation treats as a standard deviation. It is not the difference between your shortest and longest cycles. Negative days-late values and future test dates remain available for planning; future results are labelled hypothetical.

Exposure and cycle variation

The comparison uses fixed exposure assumptions and a normal-distribution approximation for cycle variation. It does not account for your age, exact timing of intercourse or specific contraceptive method. A very long delay can make the pregnancy bar approach 100% because other causes of missed periods are not included. The no-exposure choice assumes no sperm exposure or fertility treatment since a normal period; its zero pregnancy estimate applies only to that scenario. A positive test still needs follow-up.

Research describes substantial variation in cycle length (Bull et al., 2019) and variation in the fertile window (Wilcox et al., 2000). The CDC contraception overview explains why protection differs between methods. These are background sources for the factors involved, not a validation of individual percentages from this calculator.

How pregnancy tests affect the comparison

A negative result uses a reference detection rate from Sundermann et al., 2026, Table 2. That study included 5,979 North American pregnancy planners who eventually detected a pregnancy. The reference combines the reported regular- and irregular-cycle estimates in proportion to their group sizes. It is a study average, not the accuracy of your particular test. The outer categories group tests four or more days before, or four or more days after, the expected period; they do not give an exact rate for every day.

Only the latest eligible negative is used in the numerical adjustment. Repeated tests are not multiplied as independent evidence. The calculation updates the pregnancy weight, then normalizes both bars; it assumes a nonpregnant person receives a negative test for this step. The positive-test branch retains a high illustrative value of 99.9%; that number is not a measured chance of pregnancy or a test accuracy claim. Positive and mixed results receive follow-up guidance regardless of the bars.

Test performance depends on timing and use. Gnoth and Johnson, 2014 discuss differences between home tests, while Wilcox et al., 1999 describe implantation timing. The newer study improves the testing reference; it does not establish a reliable personal probability from cycle timing alone.

Testing and follow-up

Follow your test instructions. Testing from the expected period date is useful, but an early negative can miss pregnancy. The FDA advises repeating a negative test after several days when pregnancy is still possible. The calculator provides a two-day reminder as a planning aid, not a guarantee that the next test will be conclusive. Contact your clinician about positive or conflicting results, persistent missed periods, pain, heavy bleeding or other concerning symptoms.

Key sources

• Sundermann AC, et al. Home pregnancy test sensitivity by test timing and cycle length with real-world use among pregnancy planners who conceived. Fertility and Sterility, 2026. Full study.
• FDA. Home pregnancy tests.
• Bull JR, et al. “Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles.” NPJ Digital Medicine, 2019. Link.
• Wilcox AJ, Dunson D, Baird DD. “The timing of the fertile window in the menstrual cycle: day specific estimates from a prospective study.” BMJ, 2000. Link.
• Wilcox AJ, et al. “Time of implantation of the conceptus and loss of pregnancy.” NEJM, 1999. Link.
• Gnoth C, Johnson S. “Strips of Hope: Accuracy of Home Pregnancy Tests and New Developments.” Geburtshilfe und Frauenheilkunde, 2014. Link.
• CDC. “Contraception—Effectiveness of Family Planning Methods.” Link.