What is a Conception Date Calculator?
A conception date calculator estimates the day you most likely conceived a pregnancy. Conception (fertilization) usually happens around the time of ovulation, about 14 days after the first day of your last menstrual period (LMP) in a typical 28-day cycle. This tool works backward or forward from one of four known reference points: a due date, an ultrasound, your last period, or a baby's date of birth. It is a universal clinical date tool based on Naegele's rule and applies to any region.
How to use it
Choose what you want to calculate from, then enter the matching date. If you pick "Ultrasound," also enter the gestational age (weeks and days) that the scan reported. If you pick "Last Period," you can refine the result with your cycle length. The calculator returns your estimated conception date, a fertility window, the trimester and gestational age, and the due date.
The formula explained
Obstetric convention treats full term as 280 days (40 weeks) measured from the first day of the LMP, with conception occurring about 14 days later. That makes conception-to-birth about 266 days (38 weeks). So from a due date, \(\text{Conception} = \text{Due Date} - 266\text{ days}\); from an LMP, conception = LMP + 14 days (or + cycle length - 14 for non-28-day cycles). The fertility window spans conception - 5 days to conception + 1 day, because sperm can survive about 5 days and the egg about 1 day.
$$\text{Conception Date} = \text{Due Date} - 266 \text{ days}$$
Worked example
If your due date is 15 Jun 2026: conception = 15 Jun 2026 - 266 days = Mon, 22 Sep 2025. The fertility window is 17 Sep 2025 to 23 Sep 2025, and at full term the pregnancy is "40 weeks and 0 days, Third Trimester."
$$\text{Conception} = 15\text{ Jun }2026 - 266 \text{ days} = \text{Mon, } 22\text{ Sep }2025$$FAQ
How accurate is the conception date? It is an estimate. Ovulation timing and cycle length vary, so the result can be off by several days, which is why a fertility window and a "plus or minus 2 weeks" due date are shown.
Can I use my baby's birth date? Yes, but it assumes birth happened exactly at 40 weeks, so the conception estimate is only approximate for early or late births.
What if my cycle is not 28 days? In Last Period mode, enter your cycle length. Ovulation is assumed at cycle length minus 14 days, so a longer cycle pushes conception later.
