Gestational Age Calculator — Due Date, Conception & Implantation

Calculate due date, conception date, and implantation window from your LMP using Naegele's Rule. See your current gestational age in weeks and days.

July 8, 2026

Average Cycle Length (days)

Naegele's Rule assumes a 28-day cycle.

First Day of Last Menstrual Period (LMP)

These are estimates, not a diagnosis. ACOG recommends dating a pregnancy by first-trimester ultrasound (crown-rump length) whenever it differs from LMP-based dating by more than 7 days — ultrasound dating is accurate to about ±5–7 days, versus ±7–14 days for LMP-based methods like Naegele's Rule, which also assumes a regular cycle with ovulation on day 14. Every pregnancy varies. Always confirm your due date and gestational age with a healthcare provider.

Due Date = LMP + 280 days (40 weeks), adjusted by the difference between your average cycle length and the standard 28 days. Conception Date = LMP + 14 days, adjusted the same way. Implantation Date = Conception date + 6 to 10 days. Current Gestational Age = days elapsed since LMP ÷ 7, expressed as weeks and remainder days — this is the clinical convention used by every due-date calculation, which is why it runs about two weeks ahead of "fetal age" measured from conception.

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Reference Values

Last verified: Jul 8, 2026

Category	Range	What It Means	Status
Due Date from LMP (Naegele's Rule)	EDD = LMP + 280 days (40 weeks)	The standard due-date formula, assuming a textbook 28-day menstrual cycle with ovulation on day 14. Named for German obstetrician Franz Naegele, who published the rule in the early 1800s.	Good
Cycle-Length Adjustment	±(actual cycle length − 28) days	Naegele's Rule assumes a 28-day cycle. For a longer cycle, ovulation happens later, so add the extra days to the due date; for a shorter cycle, subtract the difference. A 32-day cycle adds 4 days; a 24-day cycle subtracts 4 days.	Good
Reverse Due Date (LMP from EDD)	LMP = EDD − 280 days	Works backward from a known or clinician-given due date to estimate the first day of the last menstrual period, useful when the due date was set by ultrasound and you want an approximate LMP-equivalent date for record-keeping.	Good
Conception Date Estimate	LMP + 14 days (28-day cycle)	Ovulation — and therefore likely conception — typically occurs about 14 days before the next expected period. This is an estimate; actual ovulation timing varies by several days even in people with regular cycles.	Okay
Implantation Date Estimate	Conception date + 6 to 10 days	The fertilized egg typically implants in the uterine lining 6–10 days after conception, per commonly cited clinical and patient-education ranges. This is a range, not a single predictable day, and can occasionally fall outside it.	Okay
Current Gestational Age (from LMP)	(Today − LMP) ÷ 7 = weeks + remainder days	Gestational age is the clinical convention and is always measured from the first day of the LMP, not from the estimated conception date — this is why gestational age runs about 2 weeks ahead of "fetal age" (age since conception).	Good
First-Trimester Ultrasound (CRL) Dating ★	±5–7 days accuracy	Crown-rump length ultrasound performed in the first trimester (up to about 13 6/7 weeks) is the most accurate way to date a pregnancy. ACOG recommends using it to establish or confirm the due date whenever available.	★ Best
LMP-Based Dating Accuracy	±7–14 days accuracy	LMP-based dating (Naegele's Rule) is a wider-margin estimate than ultrasound dating because it depends on accurate recall of the LMP date and assumes a regular, textbook-length cycle with ovulation on day 14 — both of which vary in real cycles.	Okay

Source: Naegele's Rule and cycle-length adjustment per standard obstetric convention (ACOG Committee Opinion on pregnancy dating; Healthline due-date methodology summaries); implantation window per commonly cited clinical/patient-education ranges; ultrasound-vs-LMP dating accuracy per ACOG guidance and PubMed literature reviews on Naegele's rule accuracy. Every pregnancy varies — these are population-level estimates, not individual predictions.

Worked Examples

Due Date from LMP — Standard 28-Day Cycle

Last Menstrual Period: January 1, 2026
Cycle Length: 28 days

Estimated due date: October 8, 2026

EDD = LMP + 280 days = January 1, 2026 + 280 days = October 8, 2026 (Naegele's Rule, no cycle-length adjustment needed since the cycle is the standard 28 days).

Due Date from LMP — Longer 32-Day Cycle

Last Menstrual Period: March 10, 2026
Cycle Length: 32 days

Estimated due date: December 19, 2026

Base EDD (28-day assumption) = March 10, 2026 + 280 days = December 15, 2026. Cycle is 4 days longer than 28, so add 4 days: December 15 + 4 days = December 19, 2026.

Reverse Due Date — LMP from a Known EDD

Estimated Due Date: December 25, 2026
Cycle Length: 28 days

Estimated LMP: March 20, 2026

LMP = EDD − 280 days = December 25, 2026 − 280 days = March 20, 2026.

Conception Date Estimate — 28-Day Cycle

Last Menstrual Period: May 1, 2026
Cycle Length: 28 days

Estimated conception date: May 15, 2026

Conception ≈ LMP + 14 days = May 1, 2026 + 14 days = May 15, 2026, assuming ovulation on day 14 of a 28-day cycle.

Implantation Window from a Conception Date

Conception Date: May 15, 2026

Estimated implantation window: May 21–25, 2026

Implantation typically occurs 6–10 days after conception: May 15 + 6 days = May 21, 2026 through May 15 + 10 days = May 25, 2026.

Current Gestational Age from LMP

Last Menstrual Period: April 1, 2026
Today's Date: July 8, 2026

Gestational age: 14 weeks, 0 days

Days elapsed = April 1, 2026 to July 8, 2026 = 98 days. 98 ÷ 7 = 14 weeks exactly, 0 remainder days. Gestational age is measured from LMP, so this is roughly 2 weeks ahead of fetal (conceptional) age.

How to Use This Calculator

1

Pick the tab for what you know

Due Date from LMP if you know your last period, Reverse Due Date if you already have a due date, Conception Date to estimate ovulation timing, or Implantation Date if you know (or just estimated) your conception date.
2

Enter your average cycle length

Defaults to 28 days. If your cycles run longer or shorter, enter your average — the calculator shifts every date estimate by the difference from 28 days.
3

Enter the relevant date

Your last menstrual period's first day, or your known/estimated due date, or an estimated conception date, depending on the tab.
4

Read the result

Each tab shows the estimated date (or, for implantation, a date range) along with the exact formula used to calculate it.
5

Check your current gestational age

Once an LMP date is entered, a running gestational-age readout appears showing how many weeks and days along the pregnancy is as of today, plus which trimester that falls in.

What Each Value Means

Estimated Due Date (EDD) (calendar date): The predicted delivery date, calculated as LMP + 280 days (40 weeks) under Naegele's Rule, adjusted for average cycle length. About 5% of babies are born on their exact due date; most arrive within about two weeks of it.
Estimated Conception Date (calendar date): The likely date of ovulation and fertilization, estimated as LMP + 14 days for a 28-day cycle and adjusted for actual cycle length. Represents the approximate start of "fetal age" as opposed to gestational age.
Implantation Window (date range): The estimated date range during which the fertilized egg implants in the uterine lining, calculated as 6 to 10 days after the estimated conception date.
Current Gestational Age (weeks and days): The clinical measure of how far along a pregnancy is, counted in complete weeks and days from the first day of the LMP — not from conception. This is the number used on ultrasound reports, prenatal charts, and due-date calculations.

Frequently Asked Questions

How is my due date calculated from my last period?

This calculator uses Naegele's Rule: due date = first day of your last menstrual period (LMP) + 280 days (40 weeks). That assumes a textbook 28-day cycle with ovulation on day 14. If your average cycle is longer or shorter than 28 days, enter it and the calculator shifts the due date by the difference — a 32-day cycle adds 4 days, a 24-day cycle subtracts 4 days. It's a widely used estimate, not a guarantee — only about 1 in 20 babies actually arrive on the exact estimated due date.

How do I find my LMP if I only know my due date?

Use the Reverse Due Date tab. It works Naegele's Rule backward: LMP = due date − 280 days (with the same cycle-length adjustment applied in reverse). This is useful when a clinician gave you a due date from an ultrasound and you want an equivalent LMP-style date for your own records, or when you're trying to reconstruct a timeline after the fact.

How is conception date estimated?

Conception is estimated as LMP + 14 days for a 28-day cycle, since ovulation typically happens about two weeks before the next expected period. The same cycle-length adjustment used for the due date applies here — a longer cycle pushes the estimated ovulation (and conception) date later. This is only an estimate: even people with very regular cycles can ovulate several days earlier or later than the theoretical day 14.

How is implantation date estimated?

Implantation is estimated as 6 to 10 days after the estimated conception date, based on commonly cited clinical and patient-education ranges for how long it takes a fertilized egg to travel to the uterus and implant in the lining. It's shown as a window rather than a single day because the exact timing varies by pregnancy and isn't something that can be pinpointed from dates alone.

What's the difference between gestational age and fetal age?

Gestational age is the clinical standard and is always counted from the first day of your LMP — this is the number your doctor, ultrasound report, and due-date math all use. Fetal age (also called conceptional age) is counted from the actual estimated conception date, roughly two weeks later. So at "10 weeks gestational age," the embryo is actually about 8 weeks past conception. Mixing the two up is a common source of confusion when comparing pregnancy trackers or app estimates.