Due Date Calculator: Pregnancy Due Date, Week & Gestational Age

Pregnancy calculators

One engine. Every pregnancy date.

Each calculator runs on the same dating engine as the tool above, so every answer stays consistent. Start with the due date calculator, or jump straight to the question you have.

Nº 01 · Flagship

Due Date Calculator

Estimate your due date from your last period, conception date, or IVF transfer — with gestational age, trimester, and your personal 40-week horizon.

Best forWhen your baby is likely due

InputOne date (+ cycle length)

MethodsLMP · Conception · IVF

EDD = LMP + 280 days Use calculator

Nº 02Conception

Conception Date

Estimate when conception likely occurred.

LMP + 14 days Use calculator Nº 03How far along

Pregnancy Week

How many weeks and days along you are today.

today − start Use calculator Nº 04IVF & FET

IVF Due Date

Date an IVF pregnancy from the transfer.

transfer + 266 − age Use calculator Nº 05Reverse

Reverse Due Date

Trace a known due date back to its dates.

due date − 280 days Use calculator Nº 06Gestational age

Gestational Age

Weeks-and-days age for any date.

(date − LMP) ÷ 7 Use calculator Nº 07Reference

Pregnancy Terms

Plain-language definitions of every term.

10 key concepts Read definitions

Quick answer

How is a pregnancy due date calculated?

A pregnancy due date is usually estimated as 280 days — 40 weeks — from the first day of the last menstrual period. If the conception date is known, add 266 days. For IVF pregnancies, calculate from the embryo transfer date and subtract the embryo’s age at transfer. These are estimates: an early ultrasound is the most accurate way to confirm dating, and your clinician sets the official due date.

Pregnancy is counted in gestational weeks, starting from the last period.
A clinician sets the official due date, usually after a first-trimester ultrasound.

Source: American College of Obstetricians and Gynecologists, Methods for Estimating the Due Date. See the full methodology below.

Compare the methods

Which dating method fits your situation

Every method estimates the same 40-week span — they just start from a different known date. Here is how they line up.

Method	Best for	Input needed	Formula	Notes
Last period	Most pregnancies	First day of last period (+ cycle length)	LMP + 280 days	The standard convention. Assumes a 28-day cycle; adjust for your own.
Conception	Known ovulation or conception	Conception date	conception + 266 days	About two weeks fewer than the last-period count.
IVF transfer	IVF and frozen transfers	Transfer date + embryo age	transfer + 266 − embryo age	Most precise self-calculation, because the embryo age is known.
Ultrasound	Clinical confirmation	Measured by a clinician	set by measurement	The most accurate method overall — only a clinician can set it.

Methodology

How the due date calculation works

Three methods, one consistent idea: find the gestational start, then count forward 40 weeks. Each formula is shown in full so you can see exactly how your estimate is produced.

Method 01

Last menstrual period

The conventional method. It assumes a regular 28-day cycle with ovulation near day 14. A different cycle length shifts the estimate by the difference from 28 days.

EDD = LMP + 280 days

Method 02

Conception date

If the date of conception is known, dating is more direct. Gestational age is still counted from about two weeks earlier, which is why 40 weeks from LMP is roughly 38 weeks from conception.

EDD = conception + 266 days

Method 03

IVF embryo transfer

Assisted-reproduction pregnancies use the transfer date and the embryo’s age, not a last-period estimate. A day-5 transfer, for example, is transfer + 261 days.

EDD = transfer + 266 − embryo age

Why pregnancy is counted as 40 weeks from the last period

The first day of the last menstrual period is easier to know than the exact day of conception, so it became the standard anchor. Counting begins there even though conception happens about two weeks later — which is why the count appears to start before pregnancy itself.

Why that is about 38 weeks from conception

Conception typically occurs around 14 days after the last period in a 28-day cycle. Subtracting those two weeks from 40 gives roughly 38 weeks, or 266 days, of development from conception to the estimated due date.

How cycle length changes the estimate

A longer cycle usually means later ovulation, so the estimate moves later by the same number of days the cycle exceeds 28; a shorter cycle moves it earlier. This refines the last-period estimate but is not as authoritative as ultrasound.

Why an early ultrasound may confirm or revise it

A first-trimester ultrasound measures the embryo directly and is generally the most accurate way to establish gestational age. When ultrasound and last-period dating differ significantly, clinicians usually rely on the ultrasound, and IVF dating uses the known embryo age.

How to use the calculator

Choose a method
Last period, conception date, or IVF transfer — whichever date you have.
Enter your date
Add the required date, plus your average cycle length if you know it.
Read your estimate
See your estimated due date, how far along you are, your trimester, and the timeline.

The 40-week horizon

The full pregnancy timeline

Pregnancy is measured in 40 weeks across three trimesters. This is the instrument your personal result is plotted on — milestone by milestone.

Week 0

Gestational start

Dating begins from the last period.

Week 14

Second trimester

The first trimester ends at 13w 6d.

Week 20

Halfway point

The midpoint of a full-term pregnancy.

Week 28

Third trimester

The final stretch toward term.

Week 37

Early term

Considered early term from here.

Week 40

Estimated due date

The destination on your horizon.

Where you are now

How far along am I?

“How far along” is your gestational age — the completed weeks and days since the gestational start date. A reading of 24 weeks and 3 days means 24 full weeks have passed, plus three days into the 25th.

Your trimester follows from that age. The first trimester runs through 13 weeks and 6 days, the second from 14 weeks through 27 weeks and 6 days, and the third from 28 weeks onward.

Gestational age is different from fetal age, which is counted from conception and is about two weeks less. Prenatal care uses gestational age, so that’s what this calculator reports.

The calculator updates “days remaining” against today’s date. If the estimated date has already passed, it simply counts the days since — most pregnancies continue a little past the estimate.

What an estimate means

How accurate is a due date calculator?

A due date is an estimate, not an appointment. Only about 1 in 20 babies arrive on the estimated date; a healthy full-term birth is anywhere from 37 to 42 weeks. The calculator gives you a reliable center point, not a guarantee.

Last-period dating is most accurate when cycles are regular and close to 28 days. Irregular or unusual cycle lengths widen the margin, which is why clinicians lean on ultrasound when the two disagree.

When and why a due date may change

A first-trimester ultrasound can confirm or revise the estimate. Once a clinical due date is established — usually early in pregnancy — it is generally not changed by later measurements. Any revision should be made by your clinician, not by an online tool.

For IVF pregnancies, the transfer date and embryo age give a precise gestational start, so ART-derived dating is used rather than a last-period estimate.

Pregnancy definitions

The vocabulary of pregnancy timing

Plain-language definitions of every term this site uses — each written to be quoted directly. These are the concepts behind every estimate above.

01 Estimated Due Date

Definition

The estimated due date (EDD) is the calendar day — about 40 weeks (280 days) from the first day of the last menstrual period — when a baby is most likely to be born.

How it works

It is found by adding 280 days to the last period, 266 days to conception, or counting from an IVF transfer. Most births fall within roughly two weeks on either side of it.

Why it matters

It anchors prenatal scheduling: screening windows, growth checks, and term milestones are measured against it. Only about 1 in 20 babies arrive exactly on the date.

Key takeaway

The due date is a center point to plan around, not a deadline.

02 Gestational Age

Definition

Gestational age is the number of completed weeks and days since the first day of the last menstrual period.

How it works

It is written as weeks + days, such as 24w 3d. It begins about two weeks before conception, because the last period is the easiest reliable anchor.

Why it matters

Clinical care, fetal growth, and trimester boundaries are all tracked in gestational age rather than calendar months.

Key takeaway

When someone asks “how many weeks,” they mean gestational age.

03 Pregnancy Week

Definition

A pregnancy week is an individual week within the 40-week gestational timeline, used to track development, milestones, and pregnancy progress.

How it works

Weeks are counted from the gestational start, so “week 24” means 24 completed weeks have passed. Each week maps to specific fetal development and screening.

Why it matters

Most prenatal appointments, tests, and milestones are scheduled by pregnancy week, making it the unit people track day to day.

Key takeaway

The pregnancy week is the working unit of the whole 40-week timeline.

04 Fetal Age

Definition

Fetal age — also called conception age — is the time since fertilization, about two weeks less than gestational age.

How it works

Because conception usually happens near day 14 of a 28-day cycle, subtracting roughly two weeks from gestational age gives fetal age.

Why it matters

It reflects actual embryonic development, but prenatal care still reports gestational age for consistency between patients.

Key takeaway

Fetal age is true development time; gestational age is the dating standard.

05 Last Menstrual Period (LMP)

Definition

The LMP is the first day of bleeding of your most recent menstrual period before pregnancy.

How it works

Dating counts forward from this day. The standard estimate assumes a 28-day cycle with ovulation near day 14; a different cycle length shifts the estimate.

Why it matters

It is the most widely available dating anchor, which is why due dates are conventionally counted from it.

Key takeaway

“First day of your last period” means the first day bleeding started.

06 Conception Date

Definition

The conception date is the day fertilization occurred, usually close to ovulation.

How it works

It falls about 14 days after the last period in a 28-day cycle. Adding 266 days estimates the due date from conception.

Why it matters

When ovulation or conception is known, dating is more direct than a last-period estimate.

Key takeaway

Conception is roughly two weeks after the date pregnancy is counted from.

07 IVF Transfer Date

Definition

The IVF transfer date is the day an embryo is placed in the uterus during assisted reproduction.

How it works

The due date is the transfer date plus 266 days minus the embryo’s age — for a day-5 blastocyst, that is transfer + 261 days.

Why it matters

Because the embryo’s age is known precisely, IVF dating is the most accurate self-calculation available.

Key takeaway

With IVF, the embryo’s known age replaces the guesswork of ovulation timing.

08 Trimester

Definition

A trimester is one of the three roughly three-month stages of pregnancy.

How it works

The first runs through 13w 6d, the second from 14w through 27w 6d, and the third from 28w until birth.

Why it matters

Each trimester carries its own developmental milestones and screening schedule.

Key takeaway

Trimesters are defined by gestational weeks, not calendar months.

09 Full Term Pregnancy

Definition

Full term is the window from 39w 0d through 40w 6d, when a baby is generally most ready for birth.

How it works

Neighboring ranges are early term (37w0d–38w6d), late term (41w0d–41w6d), and post-term (42w0d and beyond).

Why it matters

Where delivery falls within these windows can affect newborn outcomes, so clinicians track them closely.

Key takeaway

“Term” is a range of weeks, not a single day.

10 Early Ultrasound Dating

Definition

Early ultrasound dating estimates gestational age by measuring the embryo during the first trimester.

How it works

A measurement such as crown-rump length sets or confirms the due date, and is generally trusted over last-period dating when the two differ significantly.

Why it matters

It is the most accurate way to establish dating and is what clinicians use to set the official due date.

Key takeaway

When ultrasound and last-period dates disagree, early ultrasound usually wins.

Questions

Pregnancy timing, answered

The questions people ask most about due dates, pregnancy weeks, and dating accuracy — each answered with a definition, how it works, and why it matters.

Due date basics

How is a pregnancy due date calculated?

Definition

A due date is estimated as 280 days (40 weeks) from the first day of the last menstrual period.

How it works

If conception is known, add 266 days; for IVF, count from the embryo transfer date and subtract the embryo’s age at transfer. Every method estimates the same 40-week span from a different known date.

Why it matters

A consistent method lets clinicians compare pregnancies and schedule care on the same scale, even when the exact day of conception is unknown.

From last periodLMP + 280 days

From conceptionconception + 266 days

From IVFtransfer + 266 − embryo age

Key takeaway

All three methods target the same 40-week due date — they just start from a different anchor.

Is a due date 40 weeks from the last period or from conception?

Definition

A due date is 40 weeks from the first day of the last menstrual period, which is about 38 weeks from conception.

How it works

Gestational age is counted from the last period, roughly two weeks before conception actually occurs in a typical cycle.

Why it matters

Mixing the two anchors is the most common dating confusion; using the last period consistently avoids a two-week error.

From LMP40 weeks · 280 days

From conception38 weeks · 266 days

The gap≈ 2 weeks

Key takeaway

Forty weeks is measured from the last period, not from conception.

Why does pregnancy start before conception?

Definition

Pregnancy is counted from the first day of the last period — about two weeks before conception — because that date is easier to know reliably.

How it works

Conception usually happens near day 14 of a 28-day cycle, so the count begins before fertilization for the sake of a consistent, knowable anchor.

Why it matters

It means “weeks pregnant” always includes roughly two weeks that predate the actual embryo, which surprises many people early on.

Week 0First day of last period

≈ Week 2Conception

WhyA knowable anchor

Key takeaway

The clock starts at the last period because it is the most reliable date to count from.

What if I do not know the first day of my last period?

Definition

If your last period date is unknown, you can date the pregnancy from a known conception or IVF transfer date instead.

How it works

When none of those dates are available, an early ultrasound can establish gestational age directly by measuring the embryo.

Why it matters

Accurate early dating matters for scheduling screening and growth checks, so it is worth confirming with a clinician.

Option 1Conception date

Option 2IVF transfer date

Option 3Early ultrasound

Key takeaway

No last-period date is needed — conception, IVF, or an early ultrasound can date the pregnancy.

Pregnancy weeks & trimesters

How many weeks pregnant am I?

Definition

Your weeks pregnant is your gestational age — the completed weeks and days since the gestational start date.

How it works

It is shown as weeks + days, for example 24w 3d, meaning 24 full weeks have passed plus three days into the 25th.

Why it matters

Nearly all prenatal milestones and appointments are scheduled by this number, so it is the figure to track week to week.

FormatWeeks + days

Example24w 3d

Counted fromGestational start

Key takeaway

“How far along” is your gestational age in weeks and days.

When does each trimester begin?

Definition

The first trimester runs through 13w 6d, the second from 14w through 27w 6d, and the third from 28w onward.

How it works

These boundaries are defined in gestational weeks, which is why they do not line up neatly with calendar months.

Why it matters

Each trimester has distinct development and screening, so knowing the boundary helps you anticipate what comes next.

FirstThrough 13w 6d

Second14w – 27w 6d

Third28w to birth

Key takeaway

Trimesters are set by gestational weeks, not by calendar months.

What is the difference between gestational age and fetal age?

Definition

Gestational age is counted from the last menstrual period; fetal age is counted from fertilization and is about two weeks less.

How it works

Subtract roughly two weeks from gestational age to approximate fetal age in a typical cycle.

Why it matters

Prenatal care reports gestational age, so a fetal-age number from another source can look about two weeks behind.

Gestational ageFrom last period

Fetal ageFrom conception

Difference≈ 2 weeks

Key takeaway

Prenatal care uses gestational age; fetal age is the shorter, development-based count.

Conception & IVF dating

How do I calculate a due date after IVF?

Definition

An IVF due date is the embryo transfer date plus 266 days, minus the embryo’s age at transfer.

How it works

A day-5 blastocyst gives transfer + 261 days, a day-3 embryo transfer + 263 days, and a day-6 transfer + 260 days.

Why it matters

Because the embryo’s age is known exactly, IVF dating is more precise than estimating ovulation from a cycle.

Day 3transfer + 263 days

Day 5transfer + 261 days

Day 6transfer + 260 days

Key takeaway

IVF dating subtracts the embryo’s known age, making it the most precise self-calculation.

What if my cycle is longer or shorter than 28 days?

Definition

Enter your average cycle length, and the estimate shifts by the difference from 28 days.

How it works

A longer cycle usually means later ovulation, moving the estimate later; a shorter cycle moves it earlier.

Why it matters

Cycles far from 28 days make last-period dating less reliable, so an early ultrasound may set a different clinical date.

Longer cycleLater estimate

Shorter cycleEarlier estimate

Far from 28Ultrasound advised

Key takeaway

Cycle length nudges the estimate by its difference from 28 days.

Accuracy & ultrasound

How accurate is a due date calculator?

Definition

A due date calculator gives a reasonable estimate, but only about 1 in 20 babies arrive on the exact date.

How it works

It assumes typical timing; real ovulation, implantation, and labor vary from person to person.

Why it matters

Treating the date as a roughly two-week window, rather than a fixed day, sets more realistic expectations.

On the day≈ 1 in 20

Typical window± 2 weeks

Most accurateEarly ultrasound

Key takeaway

It is a strong estimate, not a guarantee — expect a window, not a single day.

Can an ultrasound change my due date?

Definition

Yes — a first-trimester ultrasound can confirm or revise the due date by measuring the embryo directly.

How it works

When ultrasound dating and last-period dating differ significantly, clinicians generally rely on the early ultrasound.

Why it matters

An accurate due date affects the timing of screening and growth assessments, so early confirmation is valuable.

MeasuresThe embryo directly

Most reliableFirst trimester

Sets the dateYour clinician

Key takeaway

Early ultrasound is the most accurate dating and can revise a last-period estimate.

Can my due date change later in pregnancy?

Definition

Early dating is the most reliable, so once a due date is set in the first trimester it is generally not changed by later measurements.

How it works

Later ultrasounds assess growth rather than re-date the pregnancy, because measurement variation increases over time.

Why it matters

A stable due date keeps the whole schedule of prenatal care consistent, which is why revisions are made cautiously.

Most reliableFirst trimester

Later scansTrack growth

RevisionsClinician only

Key takeaway

Once set early, the due date usually stays fixed; only a clinician should revise it.

The next forty weeks · MyCycle

Your due date is one day. Pregnancy is forty weeks.

This calculator gives you the date. MyCycle carries the rest — track pregnancy weeks, appointments, symptoms, milestones, and your cycle history in one calm companion built for iPhone.

Medical sources & editorial standards

Where this information comes from

The formulas and guidance above follow established obstetric dating conventions from primary U.S. health institutions.

ACOG

American College of Obstetricians and Gynecologists — Methods for Estimating the Due Date. Primary source for the 280-day convention and ultrasound dating.

SMFM

Society for Maternal-Fetal Medicine — guidance on accurate pregnancy dating and management of dating discrepancies.

NIH / NICHD

Eunice Kennedy Shriver National Institute of Child Health and Human Development — background on gestational age and pregnancy stages.

CDC & OWH

U.S. Centers for Disease Control and Prevention and the Office on Women’s Health — patient-facing explanations of trimesters and prenatal timing.

Independent medical review is pending. This page explains widely used dating methods for general information and is not a substitute for professional medical advice. It has not yet been certified for clinical use.

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Back to the calculator

Due Date Calculator

One instrument for the whole pregnancy.

One engine. Every pregnancy date.

Due Date Calculator

Conception Date

Pregnancy Week

IVF Due Date

Reverse Due Date

Gestational Age

Pregnancy Terms

How is a pregnancy due date calculated?

Which dating method fits your situation

How the due date calculation works

Last menstrual period

Conception date

IVF embryo transfer

Why pregnancy is counted as 40 weeks from the last period

Why that is about 38 weeks from conception

How cycle length changes the estimate

Why an early ultrasound may confirm or revise it

How to use the calculator

The full pregnancy timeline

How far along am I?

How accurate is a due date calculator?

When and why a due date may change

The vocabulary of pregnancy timing

01 Estimated Due Date

02 Gestational Age

03 Pregnancy Week

04 Fetal Age

05 Last Menstrual Period (LMP)

06 Conception Date

07 IVF Transfer Date

08 Trimester

09 Full Term Pregnancy

10 Early Ultrasound Dating

Pregnancy timing, answered

Due date basics

Pregnancy weeks & trimesters

Conception & IVF dating

Accuracy & ultrasound

Your due date is one day. Pregnancy is forty weeks.

Where this information comes from

See your estimated due date