How accurate is the due date calculation?

An EDD is a best guess, not a guaranteed birthday. Research commonly cites that only about 4% of babies are born on the exact calculated due date, while roughly 80% arrive within two weeks before or after it. Our default example (LMP Oct 1, 2025, 28-day cycle) gives EDD 2026-07-08 — on reference date 2026-05-24 you would be 33 weeks 4 days pregnant with about 45 days remaining. Ultrasound dating in the first trimester is often more accurate than LMP alone when cycles are irregular.

How is due date calculated from last menstrual period?

Naegele’s rule sets EDD at 280 days (40 weeks) from the first day of the last menstrual period (LMP). Longer cycles add days: EDD = LMP + 280 + (cycle length − 28). Example: LMP 2025-10-01 with a 28-day cycle → EDD 2026-07-08. The same LMP with a 35-day cycle → EDD 2026-07-15 (7 days later). Ovulation is assumed about 14 days after LMP on a 28-day cycle, which is why conception-based math uses 266 days instead of 280.

Can I calculate due date from conception date?

Yes: add 266 days to the known conception date (280 gestational days − ~14 days before ovulation in the average cycle). Conception Oct 15, 2025 → EDD 2026-07-08, matching the Oct 1 LMP example above. Gestational age by LMP convention is about 2 weeks ahead of fetal age counted from conception.

How does IVF affect due date calculation?

IVF dating uses the embryo transfer date and embryo age: EDD = transfer date + 266 − embryo age in days. Day-5 blastocyst transfer on Jan 10, 2026 → EDD 2026-09-28 (266 − 5 = 261 days after transfer). Day-3 embryos add fewer days back. Clinics may assign a final EDD after early ultrasound — use their chart date if it differs slightly.

Which method is most accurate for calculating due date?

First-trimester ultrasound (before ~13+6 weeks) is generally the most reliable when measured by crown–rump length. LMP is most convenient when you have regular cycles and a known period start. IVF transfer dating is precise for fertilization timing. Example: scan on Feb 1, 2026 at 12+0 → EDD 2026-08-16, which can differ from LMP-only math if ovulation was early or late.

What if my cycle length is irregular?

Standard LMP + 280 assumes ovulation around day 14 of a 28-day cycle. A 35-day cycle adds 7 days to the EDD (see 2026-07-15 vs 2026-07-08 for the same Oct 1 LMP). If periods are unpredictable, enter an early ultrasound instead of guessing LMP, or log ovulation-positive tests and use conception date mode.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of LMP — so at conception you are already “2 weeks pregnant” by convention. Fetal age is about 14 days less early on. This calculator reports gestational age: default example shows 33 weeks 4 days (235 days since LMP-based pregnancy start) on 2026-05-24.

Why do doctors use the last menstrual period to calculate due date?

Most patients remember when their last period started; exact conception is often unknown. LMP-based EDD gives everyone a shared obstetric calendar for prenatal labs, growth scans, and induction discussions. If early ultrasound differs from LMP by guideline thresholds, clinicians may change the official EDD in your chart while you still track both dates.

What does full-term mean?

Full term is 39 weeks 0 days through 40 weeks 6 days gestation. Early term is 37–38+6; late term is 41+0 to 41+6; post-term is 42+0 and beyond. For the default LMP example, the full-term window here is 2026-07-01 through 2026-07-14. Many providers discuss induction between 41 and 42 weeks if labor has not started.

Can the due date change during pregnancy?

Yes. A first-trimester dating ultrasound can shift EDD by several days when fetal size does not match LMP. Later growth scans refine size estimates but usually do not change the assigned due date unless there was no early dating scan. Re-run this tool if your provider updates your EDD after scan.

How many weeks pregnant am I today?

Count completed weeks and days from LMP (or from the LMP equivalent implied by your method). Default: LMP Oct 1, 2025, reference 2026-05-24 → 33 weeks 4 days (83.9% of the usual 280-day / 40-week model). Third trimester starts at 28+0 weeks — this example is in trimester 3 (Third trimester (28 weeks 0 days onward)).

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Pregnancy · EDD

Due Date Calculator — Estimated Delivery Date (EDD)

Calculate when your baby is due with a free due date calculator. Use last menstrual period (Naegele’s rule), ultrasound measurements, conception date, or IVF transfer — plus gestational age, trimester, and days until due.

Last updated: May 24, 2026

Four methods: LMP, ultrasound, conception, IVF

Example: LMP Oct 1, 2025 → EDD 2026-07-08 (33 weeks 4 days on 2026-05-24)

Cycle-length adjustment for long or short periods

Full pregnancy timeline? Pregnancy calculator

Due Date Calculator

LMP, ultrasound, conception, or IVF transfer

Estimate based on

First day of last menstrual period

Average cycle length

Estimated due date (EDD)

Wednesday, July 8, 2026

2026-07-08

Gestational age today

33 weeks 4 days

Third trimester (28 weeks 0 days onward) · 83.9% of 40 weeks

45 days until due date

LMP 2025-10-01 + 280 days → EDD 2026-07-08.

Only ~4% of babies arrive on the exact EDD; most births occur within two weeks before or after. Confirm dating with your clinician.

What is an estimated due date?

Your EDD (estimated due date) marks 40 weeks of gestation from the first day of your last period in the standard model — about 280 days. It is a planning date for prenatal visits, maternity leave, and birth preferences, not a contract for labor day. Most births land in the two-week window around EDD; only a small fraction land on the exact day.

EDD

2026-07-08

Default LMP example

Gestational age

33 weeks 4 days

As of 2026-05-24

Countdown

45 days

Until EDD in this example

Worked examples (same reference date)

Reference: 2026-05-24

LMP · 28 d

2026-07-08

33 weeks 4 days

LMP · 35 d

2026-07-15

+7 days vs 28-day

US 12+0

2026-08-16

IVF day-5

2026-09-28

Calculation methods & formulas

Last menstrual period (LMP)

EDD = LMP + 280 days + (cycle length − 28)

Ultrasound dating

EDD = ultrasound date + (280 − gestational days at scan)

Conception date

EDD = conception + 266 days

IVF embryo transfer

EDD = transfer date + 266 − embryo age (days)

Default LMP walkthrough

LMP 2025-10-01 + 280 days → EDD 2026-07-08.

EDD comparison table

Method	EDD	Gestational age	Days until due
LMP Oct 1, 2025 · 28-day cycle	2026-07-08	33 weeks 4 days	45
LMP Oct 1, 2025 · 35-day cycle	2026-07-15	33 weeks 4 days	52
Ultrasound 12+0 on Feb 1, 2026	2026-08-16	28 weeks	84
Conception Oct 15, 2025	2026-07-08	33 weeks 4 days	45
IVF transfer Jan 10, 2026 · day-5 embryo	2026-09-28	21 weeks 6 days	127

Term definitions (gestational weeks)

Category	Weeks	Notes
Preterm	Before 37 weeks 0 days	May need specialized neonatal care if born early
Early term	37 weeks 0 days – 38 weeks 6 days	Often healthy; some providers prefer waiting
Full term	39 weeks 0 days – 40 weeks 6 days	Ideal window for spontaneous labor
Late term	41 weeks 0 days – 41 weeks 6 days	Monitoring often increases
Post-term	42 weeks 0 days and beyond	Induction may be discussed per guidelines

Limitations

EDD is an estimate — only about 4% of births occur on the exact calculated day; ~80% fall within two weeks before or after
First-trimester ultrasound dating may override LMP when cycles are irregular or LMP is uncertain
Does not replace prenatal care, dating scans, or your clinician’s official EDD in the medical record
Time zones: enter calendar dates as you use them locally; very edge-of-day entries are not modeled to the hour
IVF clinics sometimes assign EDD with clinic-specific offsets — confirm with your embryology team

References

ACOG — Methods for estimating the due dateClinical guidance on LMP vs ultrasound dating
BBC — How accurate are due dates?Why few babies arrive on the exact EDD

Preset scenarios

Standard 28-day cycle

LMP Oct 1, 2025

2026-07-08

33 weeks 4 days · 45 days until EDD

Longer cycles

35-day cycle adds 7 days to EDD

2026-07-15

33 weeks 4 days · 52 days until EDD

IVF day-5 transfer

Transfer Jan 10, 2026

2026-09-28

21 weeks 6 days · 127 days until EDD

Frequently Asked Questions

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Pregnancy · EDD

Due Date Calculator — Estimated Delivery Date (EDD)

Last updated: May 24, 2026

Four methods: LMP, ultrasound, conception, IVF

Example: LMP Oct 1, 2025 → EDD 2026-07-08 (33 weeks 4 days on 2026-05-24)

Cycle-length adjustment for long or short periods

Full pregnancy timeline? Pregnancy calculator

Due Date Calculator

LMP, ultrasound, conception, or IVF transfer

Estimate based on

First day of last menstrual period

Average cycle length

Estimated due date (EDD)

Wednesday, July 8, 2026

2026-07-08

Gestational age today

33 weeks 4 days

Third trimester (28 weeks 0 days onward) · 83.9% of 40 weeks

45 days until due date

LMP 2025-10-01 + 280 days → EDD 2026-07-08.

Only ~4% of babies arrive on the exact EDD; most births occur within two weeks before or after. Confirm dating with your clinician.

What is an estimated due date?

EDD

2026-07-08

Default LMP example

Gestational age

33 weeks 4 days

As of 2026-05-24

Countdown

45 days

Until EDD in this example

Worked examples (same reference date)

Reference: 2026-05-24

LMP · 28 d

2026-07-08

33 weeks 4 days

LMP · 35 d

2026-07-15

+7 days vs 28-day

US 12+0

2026-08-16

IVF day-5

2026-09-28

Calculation methods & formulas

Last menstrual period (LMP)

EDD = LMP + 280 days + (cycle length − 28)

Ultrasound dating

EDD = ultrasound date + (280 − gestational days at scan)

Conception date

EDD = conception + 266 days

IVF embryo transfer

EDD = transfer date + 266 − embryo age (days)

Default LMP walkthrough

LMP 2025-10-01 + 280 days → EDD 2026-07-08.

EDD comparison table

Method	EDD	Gestational age	Days until due
LMP Oct 1, 2025 · 28-day cycle	2026-07-08	33 weeks 4 days	45
LMP Oct 1, 2025 · 35-day cycle	2026-07-15	33 weeks 4 days	52
Ultrasound 12+0 on Feb 1, 2026	2026-08-16	28 weeks	84
Conception Oct 15, 2025	2026-07-08	33 weeks 4 days	45
IVF transfer Jan 10, 2026 · day-5 embryo	2026-09-28	21 weeks 6 days	127

Term definitions (gestational weeks)

Category	Weeks	Notes
Preterm	Before 37 weeks 0 days	May need specialized neonatal care if born early
Early term	37 weeks 0 days – 38 weeks 6 days	Often healthy; some providers prefer waiting
Full term	39 weeks 0 days – 40 weeks 6 days	Ideal window for spontaneous labor
Late term	41 weeks 0 days – 41 weeks 6 days	Monitoring often increases
Post-term	42 weeks 0 days and beyond	Induction may be discussed per guidelines

Limitations

EDD is an estimate — only about 4% of births occur on the exact calculated day; ~80% fall within two weeks before or after
First-trimester ultrasound dating may override LMP when cycles are irregular or LMP is uncertain
Does not replace prenatal care, dating scans, or your clinician’s official EDD in the medical record
Time zones: enter calendar dates as you use them locally; very edge-of-day entries are not modeled to the hour
IVF clinics sometimes assign EDD with clinic-specific offsets — confirm with your embryology team