Due Date Calculator: Last Period, Conception, IVF or Scan

How Does the Due Date Calculator Work?

A due date (EDD, estimated date of delivery) marks 40 weeks of gestation. The calculator finds that day from whichever date you know, using the counts in the American College of Obstetricians and Gynecologists' Committee Opinion 700, Methods for Estimating the Due Date. Every method ends in the same place: once the due date is set, the tool dates everything else back from it, so “how far along” and the week-by-week calendar are consistent whichever way you started.

Five Ways to Calculate a Due Date

You know	Calculation	Notes
First day of last period (LMP)	LMP + 280 days, shifted by (cycle length minus 28)	The standard method; assumes ovulation on day 14 of a 28-day cycle
Conception or ovulation date	Conception + 266 days (38 weeks)	Same as LMP + 280 with ovulation on day 14
IVF transfer date	Transfer + 261 days (day-5 embryo) or + 263 days (day-3 embryo)	ACOG: assign the date from embryo age and transfer date, not from a period
Ultrasound with gestational age	Scan date + (280 minus the age on the scan, in days)	First-trimester crown-rump length dating is accurate to about 5 to 7 days
A due date from your provider	Everything is dated back from it	Use this to see your current week and the week-by-week dates

You knowFirst day of last period (LMP)

CalculationLMP + 280 days, shifted by (cycle length minus 28)

NotesThe standard method; assumes ovulation on day 14 of a 28-day cycle

You knowConception or ovulation date

CalculationConception + 266 days (38 weeks)

NotesSame as LMP + 280 with ovulation on day 14

You knowIVF transfer date

CalculationTransfer + 261 days (day-5 embryo) or + 263 days (day-3 embryo)

NotesACOG: assign the date from embryo age and transfer date, not from a period

You knowUltrasound with gestational age

CalculationScan date + (280 minus the age on the scan, in days)

NotesFirst-trimester crown-rump length dating is accurate to about 5 to 7 days

You knowA due date from your provider

CalculationEverything is dated back from it

NotesUse this to see your current week and the week-by-week dates

How Is My Due Date Calculated From My Last Period?

The calculation is a two-step estimate:

Estimated due date: first day of your last period + 280 days, then adjusted for your cycle length.
Gestational age: how far along you are today, measured back from your due date and expressed as completed weeks + days (for example, “12 weeks 3 days”), the same convention your doctor and ultrasound reports use.

Worked Example

Suppose your last period started on March 21, 2026 and you have a typical 28-day cycle:

Due date = March 21 + 280 days = December 26, 2026
Estimated conception = March 21 + 14 days = April 4
On June 13 you are 12 weeks 0 days pregnant (about 30% of the way through), in your first trimester; the second trimester starts on June 27 (14 weeks) and the third on October 3 (28 weeks).

With a longer 35-day cycle, ovulation happens about a week later, so the same last-period date pushes the due date to January 2, 2027, seven days later. If instead you had a day-5 IVF transfer on April 9, the due date would be April 9 + 261 days = December 26 as well: the methods agree when they describe the same pregnancy.

Pregnancy Weeks, Trimesters, and Key Milestones

Stage	Weeks (gestational)	What it marks
First trimester	0 to 13 weeks 6 days	Dating scan and first-trimester screening
Second trimester	14 to 27 weeks 6 days	Anatomy scan (18 to 22 weeks); movement is often first felt
Third trimester	28 weeks onward	More frequent visits; glucose screening at 24 to 28 weeks
Early term	37 weeks 0 days to 38 weeks 6 days	No longer preterm
Full term	39 weeks 0 days to 40 weeks 6 days	Most babies arrive between 39 and 41 weeks

StageFirst trimester

Weeks (gestational)0 to 13 weeks 6 days

What it marksDating scan and first-trimester screening

StageSecond trimester

Weeks (gestational)14 to 27 weeks 6 days

What it marksAnatomy scan (18 to 22 weeks); movement is often first felt

StageThird trimester

Weeks (gestational)28 weeks onward

What it marksMore frequent visits; glucose screening at 24 to 28 weeks

StageEarly term

Weeks (gestational)37 weeks 0 days to 38 weeks 6 days

What it marksNo longer preterm

StageFull term

Weeks (gestational)39 weeks 0 days to 40 weeks 6 days

What it marksMost babies arrive between 39 and 41 weeks

Understanding Your Results

Due Date: the estimated day your baby will arrive. Only about 1 in 20 (4 to 5%) babies are born on their exact due date.
How far along: completed weeks and days of gestation, counted from the start of your last period or the equivalent date for the other methods.
Week-by-week dates: week 1 begins on the first day of your last period, so at “12 weeks 3 days” you are in week 13. Each row shows the calendar days that week covers.

What Can Change Your Due Date?

The length and regularity of your menstrual cycle. If your cycle is longer or shorter than 28 days, or varies month to month, our free menstrual cycle tracker learns your real pattern to predict your next period and fertile window
Ultrasound measurements: an early scan (up to 13 weeks 6 days) is the most accurate way to date a pregnancy, and ACOG advises keeping that date rather than changing it after a later scan
Individual variation in how long pregnancies last
Conception through IVF, where the transfer date and embryo age set the timing

Frequently Asked Questions

How is my due date calculated from my last period?

The calculator adds 280 days (40 weeks) to the first day of your last menstrual period, the convention recommended by ACOG. If your cycle is not 28 days, it shifts the result by the difference: a 35-day cycle moves the due date 7 days later, a 25-day cycle moves it 3 days earlier.

How far along am I?

Gestational age is counted from the first day of your last period (or the equivalent date for the other methods) and written as completed weeks plus days, for example 12 weeks 3 days. If your last period started 84 days ago, you are 12 weeks 0 days pregnant. The calculator shows this along with your trimester and the dates of every week.

How do I calculate my due date after IVF?

Count from the embryo transfer date, not from a period. ACOG assigns 261 days after a day-5 (blastocyst) transfer and 263 days after a day-3 transfer. Choose the IVF option, enter the transfer date and the embryo age, and the calculator applies the matching count.

Can I calculate my due date from an ultrasound?

Yes. Enter the scan date and the gestational age printed on the report, for example 8 weeks 3 days. The calculator adds the remaining days up to 40 weeks. A first-trimester ultrasound is the most accurate way to date a pregnancy, so if it differs from your period date by more than a few days your provider will usually go with the scan.

How accurate is a due date calculator?

It is an estimate of a typical 40-week pregnancy. Only about 4 to 5 percent of babies are born on the exact due date; most arrive between 39 and 41 weeks. Your provider may adjust the date after an early ultrasound.

What is Naegele's Rule?

Naegele's Rule is the classic way to estimate a due date: take the first day of your last period, add one year, subtract three months and add seven days. That works out to about 280 days and assumes a 28-day cycle with ovulation on day 14, which is why a cycle-length adjustment makes the estimate fit better.

Does cycle length change my due date?

Yes. With a longer cycle you ovulate later, so the due date moves later by the same number of days; a shorter cycle moves it earlier. A 35-day cycle pushes the due date about a week past the 28-day estimate.

Can my due date change during pregnancy?

Yes. It can be revised after an early ultrasound, especially in the first trimester, because the scan measures the baby directly. Once a first-trimester scan has set the date, ACOG advises against changing it based on a later scan.

Methodology and Sources

Day counts follow the American College of Obstetricians and Gynecologists, Committee Opinion 700, Methods for Estimating the Due Date (with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine): 280 days from the first day of the last menstrual period; 261 days from a day-5 and 263 days from a day-3 embryo transfer; first-trimester ultrasound as the most accurate dating method. The cycle-length adjustment and the 266-day conception count are the same convention restated from ovulation instead of the period. Trimester and term boundaries follow ACOG's definitions of term pregnancy. The calculator works on calendar days and never uses your device clock to move a date.

Important Considerations

This calculator provides an estimate based on standard formulas and average pregnancy length. Every pregnancy is unique, and your due date may be revised as your pregnancy progresses.

Always consult your healthcare provider for the most accurate due date and for any questions about your pregnancy. An early ultrasound gives the most reliable estimate.

Due Date Calculator