What is Naegele's Rule?

Naegele's Rule, published by Franz Karl Naegele in 1830, states that the estimated due date equals the first day of the last menstrual period plus 280 days (40 weeks). In arithmetic terms: add one year, subtract three months, and add seven days. This rule assumes a regular 28-day cycle with ovulation on day 14. For longer or shorter cycles, add the difference from 28 to correct the EDD — for example, a 31-day cycle shifts the EDD three days later.

How accurate is an LMP-based due date?

Only about 5% of babies arrive exactly on the EDD. The standard error of a clinically assigned due date is roughly plus or minus 2 weeks. ACOG and NICE recommend that a first-trimester ultrasound crown-rump length measurement be used to confirm or revise the LMP-based EDD. If the ultrasound date differs from the LMP date by more than 5 days in the first trimester (or 10 days in the second), the ultrasound date takes precedence.

How is an IVF due date calculated differently?

In IVF, the precise embryo age at transfer is known. A Day-3 embryo is 3 days old at transfer, so the conception equivalent is transfer minus 3 days and the EDD is transfer plus 263 days. A Day-5 blastocyst is 5 days old, giving EDD = transfer plus 261 days. Both derive from the 266-day post-conception window (38 weeks), which is the same biological basis as Naegele's 280-day rule (266 + 14 days for the two-week wait).

What does gestational age mean?

Gestational age is the number of weeks and days elapsed since the first day of the last menstrual period — not since conception. A pregnancy is considered full term at 39 to 40 weeks gestational age. Early-term is 37 to 38 weeks, late-term is 41 weeks, and post-term is 42 weeks or beyond. Clinicians use gestational age (not fetal age from conception) because the LMP date is easier to establish than the exact day of ovulation.

What are the trimester boundaries?

The three trimesters divide the 40-week pregnancy. The first trimester covers weeks 1 through 13 (conception through roughly 91 days gestational age), ending at the close of the organogenesis period. The second trimester spans weeks 14 through 27, during which the fetus grows rapidly and most anatomy scans are performed. The third trimester runs from week 28 to birth, covering lung maturation, fat deposition, and preparation for delivery.

What is viability at 24 weeks?

Fetal viability is the gestational age at which a fetus has a reasonable chance of surviving outside the womb with intensive neonatal support. This threshold is widely set at 24 weeks gestational age (168 days), though survival rates are significantly higher from 28 weeks onward. Many UK and US hospitals offer active resuscitation from 23 weeks. This calculator marks the 24-week viability date on your milestone table as a reference point only.

What is the Pregnancy Due Date Calculator?

Free pregnancy due date calculator. Enter your last menstrual period, conception date, IVF transfer date, or ultrasound gestational age to get your EDD, current gestational age, trimester, and a full milestone timeline. Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Pregnancy Due Date Calculator

Name: Pregnancy Due Date Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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A pregnancy due date calculator that computes your estimated due date (EDD) from four different inputs — last menstrual period, conception date, IVF embryo transfer, or ultrasound gestational age — and shows your current gestational age, trimester, a visual progress bar, and a complete milestone timeline with real calendar dates. Everything runs in your browser; no data is uploaded.

Educational tool — not medical advice. Due dates are statistical estimates. Confirm all clinical dating with your midwife or obstetrician.

How it works

The calculator implements four well-established clinical dating methods.

Method 1 — Naegele’s Rule (LMP): The oldest and most commonly used method. The EDD is simply LMP + 280 days. For cycles other than 28 days, the ACOG cycle-length correction adds (cycleLength - 28) extra days to account for late or early ovulation.

Method 2 — Conception date: Conception (fertilisation) normally occurs around day 14 of a 28-day cycle. Clinically, the window from fertilisation to birth is 266 days (38 weeks). So EDD = conception + 266. The calculator also derives the LMP-equivalent date by subtracting 14 days, keeping all gestational-age outputs on the same scale.

Method 3 — IVF transfer: Because embryo age at transfer is precisely known, the formula back-calculates from the 266-day conception-to-birth window:

Day-3 transfer: EDD = transfer date + 263 days
Day-5 blastocyst: EDD = transfer date + 261 days

Method 4 — Ultrasound: If you know your gestational age from a scan report (for example, 10 weeks and 4 days on a given date), the EDD is that scan date plus the remaining days to 280. This is the most accurate method once a crown-rump length measurement has been taken, and ACOG recommends using it to revise an LMP-based date when there is a discrepancy of more than 5 days.

Worked example

Suppose your LMP was 1 September 2025 on a standard 28-day cycle.

Naegele’s Rule: add 280 days to 1 September 2025.

Using arithmetic: LMP (1 Sep 2025) + 280 days = 8 June 2026.

If your cycle is 31 days, add 3 more days: EDD becomes 11 June 2026.

On 1 December 2025 you would be at:

Days elapsed since LMP: 91 days
Gestational age: 13 weeks 0 days (end of first trimester)
Days remaining to EDD: 189 days

If an ultrasound on that same date (1 December 2025) reported 12 weeks 4 days instead of 13w 0d, the ultrasound-derived EDD would be:

Gestational days at scan: (12 x 7) + 4 = 88 days
Remaining days: 280 - 88 = 192 days
Ultrasound EDD: 1 Dec 2025 + 192 days = 11 June 2026

The two methods agree to within 3 days, which is within the ACOG 5-day first-trimester tolerance, so no re-dating would be triggered.

Method	Input	EDD
LMP (28-day cycle)	1 Sep 2025	8 Jun 2026
LMP (31-day cycle)	1 Sep 2025	11 Jun 2026
Conception	15 Sep 2025	9 Jun 2026
IVF Day-5 transfer	20 Sep 2025	9 Jun 2026
Ultrasound 12w 4d	1 Dec 2025	11 Jun 2026

All calculations are local — nothing leaves your device.