How accurate is a conception date estimate?

Conception timing is estimated within a window. Natural cycles vary and ovulation can shift several days. Ultrasound in the first trimester provides the most reliable dating when available.

What if my cycle is not 28 days?

The calculator adjusts ovulation as cycle length − 14 days. Longer cycles shift ovulation later; shorter cycles, earlier.

Can I use an IVF transfer date?

Yes. For day-5 transfers, EDD = transfer date + 261 days; for day-3 transfers, EDD = transfer date + 263 days. Conception date ≈ transfer date − embryo age (3 or 5 days).

What is the difference between gestational and fetal age?

Gestational age counts from LMP; fetal age counts from conception (about two weeks later in a 28-day cycle).

Which method should I choose?

Use LMP if cycles are regular and you recall the date. Use Due Date if provided by a clinician. Use Ultrasound if you have scan date and gestational age. Use Ovulation/IUI/IVF if you tracked ovulation or had assisted reproduction.

Is this medical advice?

No. This tool is for education only and does not replace clinical evaluation. Consult your healthcare professional for personalized care.

Pregnancy Conception Date Calculator

Name: Pregnancy Conception Date Calculator
Creator: Ugo Candido

Estimate your conception date and fertile window using Last Menstrual Period (LMP), a known Due Date (EDD), ultrasound measurements, or an ovulation/IUI/IVF date. You’ll also get gestational age, fetal age, and an evidence-based estimated due date.

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.

Strumento sviluppato da Ugo Candido,. Contenuti verificati da,.
Ultima revisione per l'accuratezza in data: .