Find out when conception likely occurred based on your last menstrual period or your due date, along with the probable conception window.
Conception usually occurs around ovulation, which takes place approximately 14 days after the first day of the last menstrual period for a standard 28-day cycle. If your cycle length differs, ovulation shifts accordingly — one day earlier or later for each day the cycle deviates from 28.
The conception window spans roughly 6 days ending on ovulation day — the 5 days before ovulation plus ovulation itself — because sperm can survive up to 5 days in the reproductive tract.
Note: Only a DNA test can definitively confirm paternity or exact conception timing. This calculator provides statistical estimates for informational purposes.
Nadia's last period started on December 5, 2026, her cycles run 32 days instead of 28, and she wants to know when she probably conceived and when to expect her baby. Entering those facts here gives a conception date of December 23, 2026, a window from December 20 to 26 and a due date of September 15, 2027, which differs from the textbook 28-day answer. The page accepts either a last period date with a cycle length, or a due date, and adds trimester ends and today's gestational age.
Start with what was typed in: last period December 5, 2026 and a cycle length of 32 days.
Step one slides the start date to its 28-day equivalent: adjusted start = typed date + (cycle length − 28) = December 5 + 4 days = December 9, 2026. This is the cycle-length correction that is commonly paired with Naegele's rule, which assumes ovulation on day 14.
Everything else counts from that adjusted date. In short: conception = adjusted start + 14, window = adjusted start + 11 through + 17, due date = adjusted start + 280, trimester ends = + 91 and + 182 days.
Applied here, conception is December 9 + 14 = December 23, 2026, and the window is December 20 to December 26, a week centered on that date. The due date is December 9 + 280 days = September 15, 2027, with the trimesters ending March 10, 2027 and June 9, 2027.
In due-date mode the same chain runs backward and no cycle length is asked: adjusted start = due date − 280. A due date of September 10, 2027 gives December 4, 2026, conception on December 18 and a window of December 15 to 21. The page's LMP row shows the adjusted date, not necessarily the one you typed. ACOG and other clinical groups use the same 280-day convention, though your provider's dating always takes precedence.
In LMP mode the page first shifts your date by (cycle length − 28) days, so a 32-day cycle moves the start four days later, and then displays that adjusted figure. Your real first day of bleeding has not changed.
The window the page shows is adjusted start + 11 to + 17, centered on the estimate, while the explanatory text below it describes five days before ovulation plus ovulation day. Either way the exact day is uncertain, and a spread of several days is more honest than a single date.
No. A DNA test is the only way to address paternity, and no calendar can fix conception to a single day. The results are statistical estimates for education, not medical advice.
Your midwife or doctor. The page counts from the adjusted start to today's date, whereas clinicians combine LMP with early ultrasound dating, as in ACOG guidance, and may revise a due date. Use their numbers.