Pregnancy Conception Date Calculator
Estimate your conception date from LMP, due date, ultrasound, or ovulation/IUI/IVF. Get fertile window, gestational age, fetal age, and due date with evidence-based formulas.
Calculate
Select a method, provide the requested dates, and hit Calculate. The results panel will show conception, fertile window, gestational/fetal age, and the estimated due date.
How to Use This Calculator
Choose the entry point that matches the information you have: LMP, due date, ultrasound dating, or ovulation/IUI/IVF. Each method surfaces the required fields. Click Calculate to refresh the results or let the inputs auto-run after you finish typing.
Switch between methods any time; the results card keeps the latest computation visible while the form remembers your cycle length defaults.
Methodology
We rely on Naegele's rule (EDD = conception + 266 days) and standard gestational conventions. Cycle length adjusts the offset between LMP and the estimated ovulation day. Ultrasound dates are back-calculated to LMP before applying the same logic, and assisted-reproduction dates drop the fertile window when the conception day is known.
Full original guide (expanded)
Data Source and Methodology
Primary sources:
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017; reaffirmed 2024). American College of Obstetricians and Gynecologists. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
- Naegele's rule and standard obstetric dating conventions (EDD = LMP + 280 days for 28-day cycles; EDD = Conception + 266 days).
- NICE Guideline NG201: Antenatal care (2021, updated). National Institute for Health and Care Excellence. https://www.nice.org.uk/guidance/ng201
All calculations are strictly based on the formulas and data provided by this source.
The Formula Explained
We denote dates as integers in days. Let L be LMP (first day of last period), C be conception date, E be estimated due date (EDD), and CL be average cycle length in days.
Naegele's rule generalized via conception:
$$ E = C + 266 $$
If LMP is known:
$$ \text{Ovulation} \approx L + (CL - 14), \quad C \approx L + (CL - 14) $$
If EDD is known:
$$ C = E - 266 $$
$$ L \approx C - (CL - 14) $$
If an ultrasound on date U measured gestational age GA (weeks w and days d):
$$ GA = 7w + d, \quad L = U - GA, \quad C \approx L + (CL - 14), \quad E = C + 266 $$
For IVF transfer on date T with embryo age A (A = 5 for day-5; A = 3 for day-3):
$$ C = T - A, \quad E = T + (266 - A) $$
$$ L \approx C - (CL - 14) $$
Fertile window (natural/IUI):
$$ \text{Window} \approx [C - 5,\; C + 1] $$
Glossary of Variables
- LMP (Last Menstrual Period): First day of the last menstrual bleeding.
- Cycle length (CL): Average number of days in your menstrual cycle. Default 28.
- Conception date (C): Estimated day of fertilization, usually within 24 hours of ovulation.
- EDD (Estimated Due Date): Expected date of delivery, computed as C + 266 days.
- Gestational age (GA): Age of pregnancy counted from LMP (weeks + days).
- Fetal age: Age of the fetus counted from conception.
- Fertile window: Period with highest chance of conception, approximately ovulation − 5 to +1 days.
How It Works: A Step-by-Step Example
Scenario: LMP = March 1, 2025; cycle length CL = 30 days.
- Ovulation and conception: C ≈ L + (CL − 14) = Mar 1 + 16 = March 17, 2025.
- EDD: E = C + 266 = December 8, 2025.
- Fertile window: [C − 5, C + 1] = March 12-18, 2025.
- On April 28, 2025, gestational age = days since LMP ÷ 7 ≈ 58 days = 8 weeks 2 days; fetal age ≈ 42 days = 6 weeks 0 days.
These outputs align with ACOG's conventions and standard obstetric dating.
Frequently Asked Questions (FAQ)
How precise is an ultrasound-based date?
First-trimester ultrasound (up to 13 weeks 6 days) provides the most accurate dating. If this date and LMP disagree, clinicians usually adopt the ultrasound-based EDD within accepted tolerance ranges.
My periods are irregular. Which method should I use?
Prefer the Ultrasound method if possible. If using LMP, consider that ovulation may not be cycle length − 14 and the fertile window may be wider.
Why use 266 days from conception?
Pregnancy length is approximately 280 days from LMP in a 28-day cycle, which equals 266 days from fertilization (about two weeks later). This convention underpins EDD calculations.
Is the fertile window the same as the conception window?
The fertile window spans ovulation − 5 to ovulation + 1 days. Conception typically happens within 24 hours of ovulation, so the conception window is narrower.
Can I enter an IVF egg retrieval date instead of transfer?
You can, but you must convert to transfer-based or conception-based dating. This tool accepts transfer date with embryo age or a known conception day.
What if my due date is today or has passed?
The calculator still computes the implied conception date and ages. If dates are inconsistent (e.g., negative ages), double-check inputs.