Calculate your estimated due date from the date of conception. Simply add 266 days to your known or estimated conception date.
If you know the date you conceived — whether through ovulation tracking, fertility treatments, or a well-timed encounter — you can estimate your due date with remarkable precision. This calculator adds 266 days (38 weeks) to your conception date to determine your estimated due date, bypassing the assumptions about cycle length that the LMP method requires.
The 266-day figure represents the average human gestation from fertilization to birth. Unlike the LMP method, which adds 280 days and assumes ovulation on day 14, this approach starts the clock at the actual moment of conception, making it inherently more accurate when the conception date is known.
This method is especially useful for women who track ovulation using basal body temperature, ovulation predictor kits, or ultrasound monitoring. It is also the preferred method when conception occurred through timed intercourse in a fertility clinic setting. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation.
When you know your conception date, this calculator provides a more direct estimate than the LMP method. It eliminates guesswork about ovulation timing and cycle irregularities. Women with irregular cycles, those who conceived while breastfeeding, or those who had breakthrough bleeding benefit the most from this approach. Having a precise figure at your fingertips empowers better planning and more confident decisions.
EDD = Conception Date + 266 days Where: EDD = Estimated Due Date 266 days = 38 weeks (average gestation from fertilization) Gestational Age = (EDD date − Conception Date) + 14 days equivalent
Result: October 8, 2026
With a conception date of January 15, 2026, the estimated due date is January 15 + 266 days = October 8, 2026. This is equivalent to an LMP of January 1 with a 28-day cycle, showing how the two methods converge.
Conception occurs when a sperm fertilizes an egg, typically in the fallopian tube within hours of ovulation. The fertilized egg then travels to the uterus and implants 6-10 days later. The 266-day calculation measures from fertilization to the expected birth.
The LMP method adds 280 days from the last period, while the conception method adds 266 days from fertilization. For a 28-day cycle, both give the same result. The difference appears with non-standard cycles — the conception method is inherently more accurate because it does not need cycle-length adjustments.
Even with a known conception date, an early ultrasound between 8-12 weeks provides the most reliable dating. Crown-rump length measurements at this stage are accurate to within 3-5 days. If the ultrasound and conception dates agree, you can be very confident in your EDD.
They are usually within 24 hours of each other. The egg is released at ovulation and must be fertilized within 12-24 hours. For practical purposes, most providers treat them as the same day when calculating the due date.
The 280-day figure used in the LMP method includes approximately 14 days before ovulation and conception. Since this calculator starts from conception itself, those 14 days are excluded, giving 280 − 14 = 266 days.
You can estimate your conception date if you tracked ovulation using OPKs, basal body temperature, or fertility monitor. If you had intercourse only once during the fertile window, that date is a strong estimate.
Yes, when the conception date is truly known. The LMP method assumes a standard cycle and ovulation on day 14, which may not apply. A known conception date removes that assumption entirely.
For IVF pregnancies, use our dedicated IVF due date calculator instead. IVF dating uses the embryo transfer date and embryo age (day 3 or day 5) for the most precise calculation.
Conception most likely occurred on the day of ovulation regardless of when intercourse took place. Sperm can wait in the reproductive tract for up to 5 days. Use your estimated ovulation date as the conception date.
Most medical records use the LMP-equivalent date for consistency. Your provider will convert your conception date to an equivalent LMP by subtracting 14 days. Both methods should yield the same due date.
It is accurate to within about 10 days for most pregnancies when the conception date is well-known. Individual variation in implantation and gestation length means the actual birth may still fall within a 2-week window.