Find out when conception likely occurred from a due date, plan your fertile window from your last period, or find the best time to conceive for a target due date.
This calculator has three modes to help with conception planning:
Note: These are statistical estimates. Fertility varies significantly between individuals. Consult a reproductive health specialist for personalized guidance.
Gestational age and fetal age are two clocks that run about two weeks apart, and mixing them up is the most common reason a conception date looks wrong. Clinicians count a pregnancy from the first day of the last menstrual period, so at the moment of conception you are already roughly two weeks pregnant by their count. This calculator keeps both clocks in view across three modes: conception date from a due date, fertile window from a last period, and a planning mode for a target due date.
Typical case, due-date mode. A due date of Saturday, August 14, 2027 is 280 days after the first day of the last period (Naegele's rule, the standard LMP-based dating that ACOG describes), so the page puts the last period at Saturday, November 7, 2026. With the default 28-day cycle, conception = last period + (28 − 14) = Saturday, November 21, 2026, which is 266 days before the due date. The window runs from five days before to one day after: November 16 to November 22. The first trimester ends at last period + 91 days (February 6, 2027) and the second at last period + 182 days (May 8, 2027).
LMP mode adjusts for cycle length: conception = last period + (cycle − 14) and due date = last period + 280 + (cycle − 28). With a last period of November 3, 2026, a 28-day cycle gives conception on November 17 and a due date of August 10, 2027. A 35-day cycle gives November 24 and August 17, a full week later on both.
Edge case, a hidden setting. The cycle-length box disappears in due-date mode, but the last value you chose in another mode is still used. If you set 35 days earlier and then switch back, the same August 14 due date returns November 28 instead of November 21, even though conception is about 266 days before the due date whatever your cycle length. Press Clear, which resets the cycle to 28, before using due-date mode, and check the answer against the 266-day rule.
Plan-ahead mode has a related quirk. It starts from the target due date minus 280 days, then steps one cycle length either side to make three options. With a 35-day cycle the middle option's estimated due date is August 21, 2027 rather than the August 14 you typed, so read each option's Est. due line instead of assuming it equals your target.
Not necessarily. Sperm can survive in the reproductive tract for up to about five days and an egg lasts roughly 12 to 24 hours, so intercourse on any of several days can lead to a later conception date. That is why the page prints a range instead of one day.
Enter the due date your clinician settled on. ACOG's guidance on estimating a due date allows an early ultrasound to replace LMP dating when the two disagree by more than a few days, and this page simply works backward from whatever date you give it.
It marks the end of trimester one at last period + 91 days and trimester two at 26 weeks. Sources define the boundaries slightly differently, so a difference of a few days from another site or from your provider is normal.
No. The window spans about a week and no calendar can narrow it further. Only a DNA test can address paternity, and this page is a general-education estimate, not medical advice.
ACOG suggests an evaluation after 12 months of trying if the woman is under 35, and after 6 months if she is 35 or older. A doctor can also look sooner if there are known fertility concerns, so this calculator is no substitute for that conversation.