Reverse Due Date Calculator – Find Conception & LMP Date

A pregnancy due date is conventionally calculated as 280 days (40 weeks) from the first day of the last menstrual period (LMP), based on a standard 28-day menstrual cycle with ovulation on day 14. The reverse due date calculator works backwards from a known due date to reconstruct the pregnancy timeline — giving you the estimated conception date, LMP date, ovulation date, implantation date, and key trimester milestones. The most important input is your average menstrual cycle length, because ovulation timing varies with cycle length. For a standard 28-day cycle, ovulation occurs on day 14, and the LMP is simply the due date minus 280 days. If your cycle is longer — say, 35 days — ovulation occurs on day 21 instead of day 14, meaning the LMP is seven days earlier than the standard formula would suggest. The calculator adjusts all dates accordingly using the formula: LMP = Due Date − (252 + cycle length). Once the LMP is known, all other dates follow from it. The ovulation date is LMP plus (cycle length − 14), placing it in the fertile window around mid-cycle. The conception date is typically the same as or within one to two days of ovulation, since sperm can survive up to five days and the egg is viable for about 24 hours. Implantation follows approximately six to twelve days after conception, with eight days being the most common. Pregnancy milestones in the result panel are based on gestational age measured from the LMP. The first trimester ends at 13 weeks (91 days from LMP). The second trimester ends at 28 weeks (196 days). Fetal viability — the point at which survival outside the womb becomes possible with medical support — is generally reached at 24 weeks (168 days). These dates are estimates; your healthcare provider may use ultrasound measurements to refine gestational age. The optional Current Gestational Age fields allow the calculator to display how many weeks remain until the due date, which is useful for tracking progress mid-pregnancy. If you leave those fields blank, the calculator simply shows the milestone dates without a remaining-weeks figure. All calculations use the Gregorian calendar and assume a singleton pregnancy. Twin and higher-order pregnancies typically deliver earlier than 40 weeks and may have a different expected due date from the outset. Always verify your dates with a qualified healthcare provider or midwife, as individual variation in ovulation timing and cycle regularity means that calendar-based dates are estimates, not certainties.