Conception Date Calculator

What this Conception Date Calculator does

This English-language conception date calculator estimates when conception most likely occurred using one of three reference points: the first day of your last menstrual period (LMP), a known estimated due date, or a gestational age measured at ultrasound. Most tools lock you into a single input and assume a fixed 28-day cycle — this one adjusts ovulation timing for your actual cycle length and works from whichever reference you have. To figure out your conception date, the calculator applies Naegele's rule (due date = LMP + 280 days) and places ovulation at cycle length − 14 days from LMP. Results include the estimated conception date, due date, LMP, current gestational age, and a trimester-calibrated uncertainty range per ACOG guidelines: ±5 days for LMP-based dating, ±3 days for first-trimester CRL ultrasound, widening to ±14 days after 22 weeks. 100% client-side — your data never leaves your browser. No uploads, no tracking, no server logs.

Features

• Three input modes. Start from your LMP, a known due date, or a gestational age reported at ultrasound — whichever you have on hand. The calculator works forward or backward from any of the three to produce a consistent set of dates.
• Cycle-length adjustment. Ovulation in a 35-day cycle happens around day 21, not day 14. Enter your average cycle length and the calculator shifts the estimated conception date accordingly, eliminating the systematic error that plagues tools locked to 28-day cycles.
• ACOG-calibrated uncertainty per trimester. Dating accuracy degrades as pregnancy advances: ±3 days for first-trimester CRL ultrasound (before 14 weeks), ±7 days between 14–22 weeks, ±14 days after 22 weeks, and ±5 days for LMP-based estimates. The tool displays the appropriate range for whichever method you use.
• Full output panel. One calculation returns the estimated conception date, due date, LMP, and today's gestational age in weeks and days — all labeled so you can cross-reference your prenatal records without switching between tabs or tools.
• Gestational age vs. conception age — both explained. Medical providers count gestational age from LMP; conception age is roughly two weeks less. The results panel shows gestational age (the clinical standard) so the number matches your provider's records, with a clear note on the offset.

How to use the Conception Date Calculator

Pick a calculation method, enter one reference date, and read the results — the entire calculation runs locally in your browser in under a second.

1. Choose a calculation method. Select "From LMP", "From due date", or "From ultrasound" depending on which date you have available.
2. Enter the reference date. Type the first day of your last period, your estimated due date, or the date of the ultrasound scan. Inputs follow ISO 8601 order (YYYY-MM-DD) — the Stack Overflow Developer Survey 2024 lists it as the most preferred date format among developers worldwide, and it avoids the MM/DD vs DD/MM ambiguity entirely.
3. For ultrasound: add the gestational age at the scan. Enter the weeks and days your provider reported at the time of the scan. A first-trimester CRL measurement gives the tightest uncertainty window (±3 days) and is the reference ACOG recommends for establishing the EDD.
4. Adjust cycle length if needed. The default is 28 days. If your typical cycle is 32 or 35 days, update the field — the calculator shifts the estimated ovulation and conception date forward by cycle length − 28.
5. Read the results. The output panel shows the estimated conception date with its uncertainty range, plus the due date, LMP, and today's gestational age. LMP-based results will display ± 5 days; first-trimester ultrasound results will show ± 3 days.

Common use cases

• Determining conception date from a due date. If your provider gave you a due date but you want to understand the conception-and-due-date relationship, enter the EDD in "From due date" mode. The calculator traces back to the likely ovulation window — about 266 days before delivery — and returns a calibrated estimate.
• Cross-checking LMP vs. ultrasound dating. Discrepancies between LMP-derived and ultrasound-derived due dates are common, especially with irregular cycles. Run both modes and compare the results; ACOG recommends updating the EDD only when the gap exceeds the trimester-specific tolerance (more than 5 days before 9 weeks, more than 7 days between 9–15 weeks 6 days).
• Adjusting for longer or irregular cycles. Someone with a 35-day cycle using a standard online calculator may get a conception date that's off by a full week. Setting the cycle field to 35 corrects the ovulation shift automatically, giving a more accurate window for the conceived date and due date together.
• Education for first-time parents. Many first-time parents confuse conception age with gestational age, or assume implantation (which occurs 6–12 days after fertilization) is the same event as conception. This tool's result panel labels each term clearly. If you're also tracking health metrics during pregnancy, the [BMI calculator](/en/bmi-calculator/) on this site is a useful companion alongside your prenatal visits.

Frequently asked questions