Due Date Calculator (LMP)
Calculate your estimated due date from the first day of your last menstrual period. Adjusts for cycle length using Naegele's rule.
Due Date Calculator (Conception Date)
Calculate your estimated due date from the date of conception. Simply add 266 days to your known or estimated conception date.
Planning notes, formulas, and examples
About the Due Date Calculator (Conception Date)
If you know when conception likely happened, you can estimate your due date more directly than with a last-period method. This calculator adds 266 days (38 weeks) to the conception date, which is the standard average length of pregnancy from fertilization to birth.
That makes this approach especially useful when you tracked ovulation carefully, conceived through fertility treatment, or have irregular cycles that make LMP dating less dependable. Instead of assuming ovulation happened on day 14, the calculation starts from the date that matters most for this method: conception itself.
Use it as a planning estimate for appointments, screenings, and trimester milestones, then confirm the date with your prenatal provider and first-trimester ultrasound.
When This Page Helps
When the conception date is known or narrowly estimated, this method removes a lot of the guesswork that comes with cycle-based dating. It is most helpful for irregular cycles, ovulation-tracked cycles, breastfeeding cycles, and fertility-treatment pregnancies where timing is clearer than a standard LMP estimate.
How to Use the Inputs
- Enter the date you believe conception occurred.
- If unsure of the exact day, enter the ovulation date — conception typically occurs within 24 hours of ovulation.
- Review your estimated due date, which is 266 days from conception.
- Note the trimester milestones displayed for planning purposes.
- Confirm the date with your healthcare provider at your first prenatal visit.
Formula used
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 equivalentExample Calculation
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.
Tips & Best Practices
- Ovulation predictor kits (OPKs) detect the LH surge 24-36 hours before ovulation — conception likely occurs the day of or after the positive test.
- Sperm can survive up to 5 days in the reproductive tract, but the egg only survives 12-24 hours after ovulation.
- If you tracked your basal body temperature, the thermal shift day is your best estimate of ovulation day.
- Conception date and ovulation date are usually the same day or within 24 hours of each other.
- Your provider may still use the LMP-equivalent date for medical records and gestational age calculations.
- A first-trimester ultrasound remains the gold standard for dating — share your conception date with your provider.
Understanding Conception Dating
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.
Conception vs. LMP Dating
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.
Confirming Your Date
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.
Sources & Methodology
Last updated:
Frequently Asked Questions
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.
More in this topic
IVF Due Date Calculator
Calculate your estimated due date after IVF embryo transfer. Supports Day-3 and Day-5 blastocyst transfers with precise dating.
Ovulation Calculator
Calculate your ovulation day and fertile window based on your last menstrual period and cycle length. Identify up to 6 fertile days each month for conception planning.