How is the estimated due date (EDD) calculated?

The most commonly used method is Naegele's rule, named after German obstetrician Franz Naegele who described it in 1806: EDD = First day of LMP + 280 days (40 weeks) The 280 days assumes: - A 28-day menstrual cycle - Ovulation occurring at cycle day 14 - Fertilisation on the day of ovulation - 266 days (38 weeks) from conception to birth Why 40 weeks from LMP when pregnancy is 38 weeks from conception? Because 40 weeks is measured from the LMP, which is approximately 2 weeks before ovulation. So the first 2 weeks of 'pregnancy' are actually before conception has occurred. Alternative methods: - From conception: EDD = conception date + 266 days - From ultrasound: EDD = scan date + (280 − gestational days at scan) — most accurate in first trimester - IVF Day 3: EDD = transfer date + 263 days - IVF Day 5 blastocyst: EDD = transfer date + 261 days Clinical practice: While the EDD is calculated from LMP, if a first-trimester ultrasound (before 14 weeks) shows a gestational age that differs by more than 7 days from the LMP-based EDD, clinicians typically revise the EDD to match the ultrasound measurement, which is more accurate.

How accurate is the due date?

The EDD is an estimate, not a precise prediction. Statistics on actual birth timing: - Only about 4-5% of babies are born on their exact due date - About 80% of births occur within 2 weeks before or after the EDD - First-time mothers tend to deliver slightly later (average ~8 days after EDD) - Second and subsequent pregnancies tend to be slightly earlier Sources of variation: 1. Cycle irregularity: Naegele's rule assumes a perfect 28-day cycle. If your cycle is 32 days, you likely ovulate at day 18, not day 14 — your EDD may be 4 days later than calculated. 2. Measurement variation: Even ultrasound CRL measurements have a margin of error of ±5-7 days in the first trimester. 3. Biological variation: Actual pregnancy length varies by up to several weeks between individuals. Term definitions (ACOG 2013): - Pre-term: before 37 weeks - Early term: 37w 0d to 38w 6d - Full term: 39w 0d to 40w 6d - Late term: 41w 0d to 41w 6d - Post-term: 42w 0d and beyond First trimester ultrasound (before 14 weeks) is the most accurate dating method — more precise than LMP alone, especially for women with irregular cycles.

What is gestational age and how is it counted?

Gestational age (GA) is the standard way to measure pregnancy duration. It is counted from the first day of the last menstrual period (LMP), not from the date of conception. This creates an apparent paradox: - Week 1 and Week 2 of pregnancy are actually before conception occurred - Conception typically happens around week 2-3 (day 14 of a 28-day cycle) - So 'gestational age' always runs about 2 weeks ahead of 'embryonic age' (time since conception) Why count from LMP? - Clinically practical: the LMP date is reliably known (or can be estimated) - Conception date is rarely known precisely - Ovulation and fertilisation are difficult to pinpoint - The LMP-based system has been used in obstetrics for over 200 years GA format: GA is expressed as 'weeks + days'. For example, 12 weeks 3 days (12w3d) = 87 days from LMP. Ultrasound dating: In the first trimester, ultrasound measures crown-rump length (CRL) of the embryo/foetus. This is converted to GA using standardised charts. CRL-based dating is accurate to ±5-7 days before 14 weeks and is considered more reliable than LMP-based dating for women with irregular cycles.

What is the difference between the three trimesters?

The three trimesters divide pregnancy into roughly equal thirds, each with distinct developmental and clinical characteristics: First Trimester (Weeks 1-12): - Most rapid development — all major organs form - Highest risk period for miscarriage (about 80% of all miscarriages occur in T1) - Morning sickness is most common in weeks 6-12 - Key screening: NT scan at 11-13w, first trimester blood tests - Embryo becomes a foetus at week 10 - By 12 weeks: 6cm, all organs present in rudimentary form Second Trimester (Weeks 13-26): - Often called the 'honeymoon trimester' — morning sickness usually resolves - Most miscarriage risk has passed - Foetus grows rapidly, features become recognisable - Movement felt from ~18-20 weeks (quickening) - Key scan: 20-week anatomy scan (detailed fetal anomaly scan) - Glucose screening typically at 24-28 weeks - At 24 weeks: viability threshold — survival outside womb possible with intensive care Third Trimester (Weeks 27-40+): - Rapid weight gain — baby gains most birth weight - Lungs mature (key development milestone) - GBS (Group B Streptococcus) screening at 36 weeks - Foetal position checked; ideally head down by 36 weeks - Full term at 37 weeks — safe for delivery - Most women deliver between 38-42 weeks

When should I see a doctor after finding out I'm pregnant?

Timeline for first prenatal appointments: Positive test: - Most home tests detect hCG from week 4-5 (4+ weeks after LMP) - A positive test means you should contact your OB/GYN or midwife to schedule first appointment First prenatal appointment: - Typically weeks 8-10 in most countries - Some providers prefer week 6-8 for early ultrasound to confirm heartbeat and dating - Blood tests, physical exam, medical history review - Screening options discussion (NIPT, NT scan, quad screen) Antenatal schedule (typical, varies by country and risk level): - Weeks 4-12: Confirm pregnancy, initial blood work, NT scan (11-13w) - Weeks 13-26: Usually monthly appointments - Weeks 28-36: Every 2 weeks - Weeks 37+: Weekly until delivery Immediate medical attention required if you experience: - Heavy vaginal bleeding - Severe abdominal pain or cramping - Fever above 38°C (100.4°F) - Severe headache, visual changes, or sudden swelling (possible pre-eclampsia after 20 weeks) - Decrease or cessation of foetal movement after 28 weeks - Signs of premature labour before 37 weeks This calculator provides date estimates only — all medical decisions should be made with your healthcare provider.

How does IVF affect due date calculation?

IVF (In Vitro Fertilisation) allows more precise dating than natural conception because the exact age of the embryo at transfer is known: Key concept: the embryo has already been developing in the lab for 3 or 5 days before transfer. So we subtract those days from the standard 266-day conception-to-birth period. Day 3 (cleavage stage) transfer: - Embryo is 3 days old at transfer - EDD = transfer date + (266 - 3) = transfer date + 263 days Day 5 (blastocyst) transfer: - Embryo is 5 days old at transfer - EDD = transfer date + (266 - 5) = transfer date + 261 days Retrival date (egg collection): - Day 0 of embryo development - If you want to count from retrieval: EDD = retrieval date + 266 days (same as conception date method) Frozen embryo transfer (FET): - Same calculation as fresh transfer — the day of transfer is what matters, not when the egg was retrieved - The freeze/thaw process does not affect the embryo's developmental age LMP equivalent for IVF: - For record-keeping, providers often calculate a 'LMP equivalent' = EDD - 280 days - This fictional LMP date allows IVF pregnancies to be tracked using the same gestational age system as natural pregnancies - Your calculator shows this automatically when you use the IVF methods Note: your fertility clinic will provide the official EDD — use this calculator for your own reference.

Pregnancy Due Date Calculator — 5 Methods

The Pregnancy Due Date Calculator estimates your EDD (Estimated Due Date) using 5 methods: Last Menstrual Period (Naegele's rule, LMP + 280 days), Conception/Ovulation Date (+266 days), IVF Day 3 embryo transfer (+263 days), IVF Day 5 blastocyst transfer (+261 days), and Ultrasound gestational age (scan date + remainin..

Medical note: This calculator provides an estimated due date for informational purposes only. Only your healthcare provider can confirm your due date, typically using first-trimester ultrasound. EDD may be adjusted based on ultrasound measurements.

CALCULATE DUE DATE

CALCULATION METHOD

FIRST DAY OF LAST MENSTRUAL PERIOD

Enter the first day of your last period. Assumes a 28-day cycle (Naegele's rule).

Select a method and enter a date to see your due date and milestones.

Created with ❤️ byeaglecalculator.com

HOW TO USE

1
Choose your calculation method from 5 options: Last Menstrual Period (LMP) — the standard clinical method using Naegele's rule (LMP + 280 days); Conception or Ovulation Date — if you tracked ovulation or used an ovulation predictor kit; IVF Day 3 Embryo Transfer — for Day 3 fresh or frozen embryo transfers; IVF Day 5 Blastocyst Transfer — for blastocyst stage transfers; or Ultrasound Gestational Age — if your clinician has already measured a gestational age by crown-rump length (CRL).
2
Enter the relevant date for your chosen method using the date picker. For LMP, enter the first day of your last period — not the last day. For ultrasound, also enter the gestational age shown on the scan report in weeks and days (e.g. 8 weeks 3 days). The date picker is restricted to today or earlier to prevent future dates.
3
Read your results immediately: your estimated due date (EDD) shown in full month-day-year format, your current gestational age in weeks and days (calculated as of today), your current trimester, and the number of days remaining until your due date. Three key dates are also shown: full term at 37 weeks, your EDD at 40 weeks, and the post-term threshold at 42 weeks.
4
Check the pregnancy timeline on the right — a colour-coded progress bar divided into three trimesters (green T1, blue T2, orange T3) with your current gestational week shown as a white-circled marker. The bar spans from week 1 to week 40 with vertical dividers at weeks 13, 27, and 37.
5
Review the milestones list — 18 key pregnancy events from positive test (week 4) to post-term (week 42), each with an emoji icon, date, and a brief clinical note. Milestones are shown in chronological order with past milestones greyed out, upcoming milestones highlighted, and the current week shown in black. This helps you anticipate what's coming in your prenatal care.

WORKED EXAMPLE

Pregnancy Due Date Calculator — worked examples: METHOD 1: LAST MENSTRUAL PERIOD (Naegele's Rule) LMP: January 1, 2025 EDD = Jan 1 + 280 days = October 8, 2025 Gestational age on April 14, 2025: Days since LMP: April 14 - Jan 1 = 103 days GA = 103 ÷ 7 = 14 weeks 5 days Trimester: 2nd (week 13-26) Days until due: Oct 8 - Apr 14 = 177 days METHOD 2: CONCEPTION DATE Conception: January 15, 2025 EDD = Jan 15 + 266 days = October 8, 2025 (Same as Method 1 — conception at day 14 of cycle) METHOD 3: IVF DAY 3 TRANSFER Transfer: January 10, 2025 EDD = Jan 10 + 263 days = September 30, 2025 METHOD 4: IVF DAY 5 BLASTOCYST Transfer: January 12, 2025 EDD = Jan 12 + 261 days = September 30, 2025 METHOD 5: ULTRASOUND Scan date: February 15, 2025 at 8 weeks 0 days (56 days) Gestational days remaining = 280 - 56 = 224 days EDD = Feb 15 + 224 days = September 27, 2025 KEY MILESTONE DATES (from LMP Jan 1, 2025): End of 1st trimester (wk 13): April 2, 2025 20-week anatomy scan (wk 20): May 21, 2025 Viability threshold (wk 24): June 18, 2025 Full term begins (wk 37): September 17, 2025 EDD (wk 40): October 8, 2025 Post-term (wk 42): October 22, 2025

REFERENCE FORMULAS

FORMULA REFERENCE TABLE

Method	Formula	Description
Naegele's Rule (LMP)	EDD = LMP + 280 days (40 weeks)	Standard clinical method. Assumes 28-day cycle with ovulation at day 14. Named after Franz Naegele (1806).
From conception	EDD = conception date + 266 days (38 weeks)	Use if you know the ovulation/conception date. 266 = 280 - 14 (LMP to ovulation interval).
IVF Day 3 transfer	EDD = transfer date + 263 days	Day 3 embryo is 3 days old. So: 266 - 3 = 263 days from transfer.
IVF Day 5 blastocyst	EDD = transfer date + 261 days	Day 5 blastocyst is 5 days old. So: 266 - 5 = 261 days from transfer.
Ultrasound (CRL)	EDD = scan date + (280 - gestational_days)	If scan shows 8w0d (56 days), EDD = scan + 224 days. CRL measurement overrides LMP in 1st trimester.
Gestational age	GA = days since LMP / 7 → weeks + days	Counted from LMP, not conception. Week 0 = first day of LMP. Full term = 37-42 weeks.
Trimester boundaries	T1: wk 1-12 \| T2: wk 13-26 \| T3: wk 27-40	Note: some sources define T1 as 0-13w and T2 as 14-27w. These are approximate, not precise cutoffs.
Full term definition	37 weeks 0 days to 40 weeks 6 days	ACOG 2013 definition. Early term: 37-38w. Full term: 39-40w. Late term: 41w. Post-term: 42w+.

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Last updated: April 25, 2026 · Naegele's rule (1806) · ACOG full-term definitions (2013) · Results are estimates only — confirm with your healthcare provider