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Pregnancy Due Date Formulas and Calculation Steps

Review pregnancy due date formulas for LMP, cycle length, conception, ultrasound, IVF, gestational age, pregnancy week, trimester, and term dates.

How do the Due Date Calculator formulas work?

This formula page defines the variables used by the Due Date Calculator, shows how those values are combined, and explains the order of operations behind the result. Use the worked steps to verify an estimate, compare scenarios, and understand which assumptions or rounding rules can affect the displayed answer.

Formula Overview

Each method first produces an estimated due date. The engine then derives estimated LMP, estimated conception, gestational age, pregnancy week, trimester, progress, term dates, and the weekly calendar from that same EDD.

StandardGestation=280daysqquadConceptionToDueDate=266daysStandardGestation=280 daysqquad ConceptionToDueDate=266 days

Variable Definitions

SymbolMeaningUnit
EDDEstimated due datecalendar date
LMPFirst day of last menstrual periodcalendar date
CAverage cycle lengthwhole days
SDUltrasound scan datecalendar date
wCompleted gestational weeks recorded at scanwhole weeks
dAdditional gestational days recorded at scan0 through 6 days
TDIVF transfer datecalendar date
EAEmbryo age at transferwhole days
AODUser-entered as-of datecalendar date

LMP and Cycle-Length Formula

EDD=LMP+280+(C−28)daysEDD=LMP+280+(C-28) days

A 28-day cycle has no adjustment. A longer entered cycle shifts the estimate later by the difference; a shorter cycle shifts it earlier. This is a simple calendar adjustment, not individualized ovulation measurement.

Known Conception Formula

EDD=ConceptionDate+266daysEDD=ConceptionDate+266 days

The 266-day interval corresponds to 38 weeks from the entered conception date.

Ultrasound Dating Formula

MeasuredGestationalDays=7w+dMeasuredGestationalDays=7w+d
EDD=SD+280−(7w+d)daysEDD=SD+280-(7w+d) days

The weeks and days must be the gestational age recorded on the entered scan date.

IVF Transfer Formula

EDD=TD+266−EAdaysEDD=TD+266-EA days

Embryo age is subtracted because those developmental days occurred before transfer.

Estimated LMP and Conception from EDD

EstimatedLMP=EDD−280daysEstimatedLMP=EDD-280 days
EstimatedConception=EDD−266daysEstimatedConception=EDD-266 days

These reconstructed dates are mathematical outputs. They are not independent confirmation of menstrual or conception dates.

Gestational Age on the As-Of Date

ElapsedDays=AOD−EstimatedLMPElapsedDays=AOD-EstimatedLMP
CompletedWeeks=⌊max⁡(0,ElapsedDays)7⌋CompletedWeeks=\left\lfloor\frac{\max(0,ElapsedDays)}{7}\right\rfloor
AdditionalDays=max⁡(0,ElapsedDays) mod 7AdditionalDays=\max(0,ElapsedDays)\bmod7

If the as-of date falls before estimated LMP, the trimester is shown as not started while displayed completed age is held at zero.

Pregnancy Week and Trimester Rules

CurrentWeek=min⁡(42,max⁡(1,CompletedWeeks+1))CurrentWeek=\min(42,\max(1,CompletedWeeks+1))
Completed gestational ageDisplayed classification
Before 0 weeksNot started
0w0d through 13w6dFirst trimester
14w0d through 27w6dSecond trimester
28w0d through 40w0dThird trimester
After 40w0dPast estimated due date

Progress and Remaining-Day Formulas

Progress=round(ElapsedDays280×100)Progress=round\left(\frac{ElapsedDays}{280}\times100\right)
DisplayedProgress=min⁡(100,max⁡(0,Progress))DisplayedProgress=\min(100,\max(0,Progress))
RemainingDays=280−ElapsedDaysRemainingDays=280-ElapsedDays

A negative remaining value indicates days past the estimated due date.

Term Milestone Formulas

EarlyTerm=LMP+259 (37w0d)EarlyTerm=LMP+259\ (37w0d)
FullTerm=LMP+273 (39w0d)FullTerm=LMP+273\ (39w0d)
LateTerm=LMP+287 (41w0d)LateTerm=LMP+287\ (41w0d)
Postterm=LMP+294 (42w0d)Postterm=LMP+294\ (42w0d)

Worked Pregnancy Due Date Calculation

For LMP Feb 22, 2026, cycle length 28, and as-of date Jul 31, 2026:

EDD=Feb 22, 2026+280+(28−28)=Nov 29, 2026EDD=Feb\ 22,\ 2026+280+(28-28)=Nov\ 29,\ 2026
ElapsedDays=159=22×7+5ElapsedDays=159=22\times7+5
Engine EDDNov 29, 2026
Estimated conceptionMar 8, 2026
Gestational age22 weeks, 5 days
Current weekWeek 23
Progress57%
Remaining days121

Date Precision, Assumptions, and Limits

  • Date arithmetic uses whole UTC calendar days to avoid time-zone shifts.
  • Cycle length, ultrasound age, and embryo age must be whole values inside validated ranges.
  • Progress is rounded to the nearest whole percent and limited to 0 through 100.
  • The 42-week schedule contains consecutive seven-day rows from estimated LMP.
  • The engine cannot determine the certainty or clinical priority of an entered source date.

Pregnancy Formula FAQ

Why are 280 and 266 days both used?

Gestational dating begins at estimated LMP, while conception dating begins about 14 days later in the standard model. Their due-date intervals differ by 14 days. Use realistic inputs and review the Due Date Calculator formula assumptions before relying on this result for decisions.

Why is current week one higher than completed weeks?

At 22 completed weeks and 5 days, the 23rd week is in progress. For a useful Due Date Calculator formula result, enter realistic values, compare relevant scenarios, check the calculation details, and review the definitions, assumptions, and limitations before using the estimate for an important decision.

Does changing the as-of date change EDD?

No. It changes progress and gestational-age outputs only. For a useful Due Date Calculator formula result, enter realistic values, compare relevant scenarios, check the calculation details, and review the definitions, assumptions, and limitations before using the estimate for an important decision.

References

Return to the Due Date Calculator or follow the step-by-step pregnancy dating guide.

Use the Due Date Calculator