Pregnancy Due Date and Gestational Age Calculator Guide

What This Calculator Does

The pregnancy calculator turns one known dating source into a consistent calendar: an estimated due date (EDD), current gestational weeks and days, trimester, calendar progress, an estimated conception date, pregnancy milestones, and the date ranges for early term, full term, late term, and postterm pregnancy.

It supports five actual modes: last menstrual period (LMP), an already assigned due date, a known conception date, IVF embryo transfer, and gestational age reported on an ultrasound. These modes calculate date relationships. They do not predict the day labor will begin or confirm pregnancy, embryo or fetal viability, fetal growth, or the start of labor. If a clinician or fertility clinic has already assigned an EDD, enter that date directly and use it for care-related decisions.

When to Use It

• You want a preliminary calendar estimate from a certain LMP and a regular cycle.
• A clinician has provided an EDD and you want to see current gestational age, trimester, and upcoming date markers.
• You have a reasonably known ovulation or fertilization date and want to convert it to conventional 40-week obstetric dating.
• You had a day-3 or day-5 embryo transfer and need a timeline based on the transfer date and embryo age.
• An ultrasound report states the scan date and gestational weeks and days, and you want to convert that reported age to an EDD.

If methods disagree, do not treat the website as an automatic redating tool. The best obstetric EDD depends on how the pregnancy was conceived, whether the LMP is reliable, the earliest suitable ultrasound, and the full clinical record.

Inputs Explained

Last Menstrual Period

Enter the first day of the most recent normal menstrual period—not the day bleeding ended or a day of isolated spotting. The page accepts an integer cycle length from 21 to 35 days and starts at 28 days. Cycle length means the interval from the start of one period to the start of the next. Each day above 28 moves the EDD one day later; each day below 28 moves it one day earlier.

The adjustment still assumes ovulation roughly 14 days before the next period. An irregular cycle, uncertain LMP, or cycle outside 21–35 days should not be presented as an equally reliable LMP estimate.

Known Due Date

If an obstetric clinician, midwife, or fertility clinic has already assigned the EDD, enter it directly. This mode does not apply another 280-day interval, cycle correction, or competing dating formula. It uses the entered EDD only to build the current gestational age and timeline.

A clinician-established EDD is the appropriate anchor for pregnancy care, test interpretation, and clinical decisions. A scheduled induction or cesarean date is not the EDD and should not replace it in this field.

Known Conception Date

Enter a reasonably known ovulation, fertilization, or clinically determined trigger date. The page adds 266 days, then uses the conventional obstetric timeline that includes about two weeks before fertilization.

An intercourse date is not a known fertilization date. Intercourse, ovulation, and fertilization can occur on different days, so this mode should not be used to assign conception to a particular sexual encounter or to infer paternity.

IVF Embryo Transfer

Enter the embryo transfer date and select the embryo age recorded by the fertility clinic. The current page supports day 3 and day 5 embryos only. For either fresh or frozen transfer, use the actual transfer date and the clinic-recorded embryo age rather than inventing an LMP.

Reported Ultrasound Age

Enter the ultrasound date and copy the complete weeks-and-days value from the report. The current fields accept 4–42 weeks and 0–6 additional days.

This mode does not accept or interpret crown–rump length (CRL), biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), or femur length (FL). It also does not decide whether the pregnancy should be redated. If the report already provides an EDD, use the Known Due Date mode to avoid recalculating from a rounded gestational-age value.

Date Limits

Except for Known Due Date, the source date cannot be in the future and must fall within the previous 320 days allowed by the page. A known EDD can range from 14 days before today to 294 days after today. The final timeline is likewise limited to no more than 14 days past the EDD and no more than 294 days before it. These are product limits that keep the interaction focused on a current pregnancy; they are not clinical definitions of pregnancy duration.

How the Calculation Works

The page first derives an EDD, then creates one gestational dating anchor at EDD − 280 days.

Mode	Formula used by the page
Last menstrual period	EDD = first day of LMP + 280 days + (cycle length − 28) days
Known due date	EDD = entered due date
Known conception date	EDD = ovulation or fertilization date + 266 days
IVF embryo transfer	EDD = transfer date + (266 − embryo age) days; add 263 days for day 3 or 261 days for day 5
Reported ultrasound age	EDD = scan date + 280 days − (reported weeks × 7 + reported days)

After establishing the EDD, the result uses these relationships:

• Gestational dating anchor = EDD − 280 days.
• Estimated conception date = EDD − 266 days.
• Gestational days as of today = today − dating anchor.
• Completed weeks = floor(gestational days ÷ 7) with a 0–6 day remainder.
• Calendar progress = gestational days ÷ 280 × 100%, displayed only from 0% through 100%.

Inputs are treated as calendar dates without times. The implementation uses UTC calendar arithmetic so a daylight-saving change cannot move a milestone by one day. When cycle-length correction changes an LMP-based EDD, the result's dating anchor may no longer equal the entered LMP exactly. It is a consistent point derived from the final EDD, not a newly observed biological event.

Example

The following inputs deliberately converge on the same EDD so the five methods can be checked against one another.

LMP and Cycle Length

For an LMP of 2026-01-01 and a 28-day cycle:

2026-01-01 + 280 days = 2026-10-08

If the same person has a regular 32-day cycle, add 32 − 28 = 4 more days:

2026-01-01 + 280 days + 4 days = 2026-10-12

Known Conception Date

For a known ovulation or fertilization date of 2026-01-15:

2026-01-15 + 266 days = 2026-10-08

Day-5 IVF Transfer

For a day-5 embryo transferred on 2026-01-20:

2026-01-20 + (266 − 5) days = 2026-01-20 + 261 days = 2026-10-08

Reported Ultrasound Age and Known EDD

If a report says the pregnancy measured 12 weeks, 0 days on 2026-03-26:

2026-03-26 + 280 days − 84 days = 2026-10-08

If the clinician has already selected 2026-10-08 as the EDD, Known Due Date keeps that value unchanged rather than deriving another one.

The agreement in this example demonstrates the formulas, not a promise that LMP, ovulation, embryo transfer, and ultrasound estimates will align perfectly in every pregnancy.

How to Understand the Result

Estimated Due Date and Countdown

An EDD is a reference point for pregnancy dating, not a guaranteed birthday. Australia's government-funded Pregnancy, Birth and Baby service notes that only about 5 in 100 babies are born on the exact due date. “Days until due” and “days past due” are calendar differences, not predictions of labor.

Gestational Age Is Not Fetal Age

Obstetric gestational age conventionally begins at the LMP or an equivalent EDD − 280 days anchor, so it is usually about two weeks greater than age counted from fertilization. A result of 12 weeks, 3 days means 12 completed gestational weeks plus 3 days; it does not mean 12 weeks and 3 days have elapsed since fertilization.

Trimesters

The page uses ACOG's week boundaries:

Stage	Gestational range
First trimester	0 weeks, 0 days through 13 weeks, 6 days
Second trimester	14 weeks, 0 days through 27 weeks, 6 days
Third trimester	Begins at 28 weeks, 0 days

The timeline also marks week 14, the week-20 calendar midpoint, week 28, week 37, week 39, and the week-40 EDD. The week-20 marker is only a calendar midpoint; it does not schedule a test. Appointment and screening windows vary by country, medical history, and care team.

Term-Related Date Ranges

Page label	Gestational range	Date relative to EDD
Preterm	< 37 weeks, 0 days	Before EDD − 21 days
Early term	37+0 through 38+6	EDD − 21 days through EDD − 8 days
Full term	39+0 through 40+6	EDD − 7 days through EDD + 6 days
Late term	41+0 through 41+6	EDD + 7 days through EDD + 13 days
Postterm	≥ 42+0	From EDD + 14 days

These are standardized terms for describing gestational age. They are not recommended delivery dates or a probability interval for birth. Multiple pregnancies use the same dating convention, but care and delivery planning differ; this page does not recommend timing for twins or higher-order multiples.

Dating Hierarchy and Ultrasound Redating Boundary

A practical hierarchy is:

1. Use the best obstetric EDD already documented by the clinician or fertility clinic for ongoing care.
2. For assisted reproductive technology (ART), use the ART-derived age based on embryo age and transfer date.
3. For spontaneous conception, a suitable early ultrasound can confirm or revise an LMP estimate; first-trimester ultrasound is generally the most accurate ultrasound method for establishing gestational age.
4. Use LMP as a preliminary estimate when the date is certain and the cycle is regular, recognizing that cycle and ovulation timing vary.

ACOG gives the following differences between ultrasound and LMP dating as thresholds that can support clinical consideration of redating. The gestational ranges are based on LMP and must be interpreted with the earliest scan, measurement quality, LMP reliability, and the complete clinical picture:

Gestational age at scan	Difference supporting clinical redating consideration
≤ 8 weeks, 6 days	> 5 days
9+0 through 13+6	> 7 days
14+0 through 15+6	> 7 days
16+0 through 21+6	> 10 days
22+0 through 27+6	> 14 days
≥ 28+0	> 21 days, with particular caution for third-trimester-only dating

First-trimester CRL dating has an accuracy of about ±5–7 days. Third-trimester ultrasound dating is the least reliable, with an accuracy of roughly ±21–30 days, and later size differences can reflect fetal growth rather than an incorrect EDD. ACOG advises that once an EDD is established, later changes should be reserved for rare circumstances and discussed and documented by the clinician.

This calculator does not execute the redating table and does not ingest raw fetal biometry. Ultrasound mode only converts a gestational age already stated in a report into calendar dates. A clinician, midwife, or fertility clinic must make any redating decision.

Common Mistakes

• Entering the last day of bleeding instead of the first day of the most recent normal period.
• Treating an intercourse date as a certain fertilization date or using the estimated conception date to infer paternity.
• Applying one average cycle correction to an irregular cycle as though it were a personalized model.
• Replacing an established EDD because one later scan measures larger or smaller.
• Reading the week-20 midpoint, term labels, or countdown as an appointment or recommended delivery schedule.
• Using the same offset for day-3 and day-5 embryo transfers.
• Sharing screenshots or date summaries without considering whether they contain sensitive reproductive-health information.

FAQ

Is the estimated due date the day birth will happen?

No. The EDD anchors a standardized pregnancy timeline; it cannot predict labor. Only about 5% of babies are born on that date. Use the clinician-established EDD for care and scheduling.

What if my cycles are irregular or I do not remember the LMP?

Do not hide uncertainty behind a precise-looking average. This page only accepts regular 21–35 day cycle correction. If the cycle is irregular, the LMP is uncertain, or the value falls outside that range, use the EDD from a clinician or ultrasound report and seek professional guidance.

Why is the ultrasound result a few days different from the LMP result?

LMP recall, ovulation timing, and ultrasound measurement uncertainty can all contribute. Early ultrasound is generally more suitable for dating than a later scan, but the earliest quality examination and the complete clinical context determine whether the EDD should change. The page never makes that decision automatically.

Why does IVF distinguish a day-3 from a day-5 embryo?

The embryo has already developed for a different number of days on transfer day. The formula therefore subtracts embryo age from 266: add 263 days for day 3 or 261 days for day 5, using the fertility clinic's record.

Can this calculator confirm pregnancy, fetal growth, or conception?

No. It has no pregnancy test, hormone, symptom, ultrasound-image, or fetal-growth data. It cannot confirm viability or paternity. Seek urgent local maternity or emergency care for warning signs such as severe or persistent abdominal pain, substantial vaginal bleeding, fainting, chest pain, trouble breathing, fluid leakage, or a marked reduction in fetal movement; do not wait for a website result.

Notes

• This is a calendar estimator, not a pregnancy test, ultrasound device, fetal-growth assessment, labor predictor, antenatal schedule, diagnosis, or medical advice.
• LMP mode accepts only a regular 21–35 day cycle. It does not use multi-cycle history, ovulation tests, basal body temperature, cervical mucus, hormone results, medication, postpartum status, or breastfeeding status.
• IVF mode supports day-3 and day-5 embryos only. For another embryo age, an egg-retrieval date, or a specialized ART protocol, use the EDD supplied by the fertility clinic.
• Ultrasound mode converts reported weeks and days. It does not interpret images, raw measurements, examination quality, or clinical redating criteria.
• Date calculations use calendar days. The 0%–100% progress value is only progress toward the 40-week EDD; it is not fetal-development completion, viability, or birth probability.
• If a known EDD places the gestational dating anchor in the future, the page floors the current display at 0 weeks, 0 days. That technical display is not evidence of pregnancy.
• The calculation runs locally in the browser and does not put entered dates in the URL or persistent storage. It is not a medical record or cloud health-record service. LMP, IVF, and EDD values can be sensitive health information, so review recipients and screenshot contents before sharing.
• Multiple pregnancy, high-risk pregnancy, complications, conflicting dates, and management after the EDD all require individualized clinical care.

Related Calculators

• Menstrual Cycle Calculator: estimate a next period and possible fertile window from a single cycle before pregnancy is confirmed.
• Date Addition Calculator: check the calendar result of adding or subtracting a fixed number of days.
• Date Difference Calculator: calculate the number of calendar days between two dates.

References

• ACOG: Methods for Estimating the Due Date (reaffirmed 2025)
• ACOG: How Your Fetus Grows During Pregnancy
• ACOG: Definition of Term Pregnancy
• CDC/NCHS: Obstetric estimate of gestation at delivery
• South Eastern Sydney Local Health District: Estimating Due Date
• NHS: Pregnancy due date calculator
• Pregnancy, Birth and Baby: Working out your due date
• CDC Hear Her: Urgent Maternal Warning Signs and Symptoms