Pregnancy Week Calculator: Find Your Week and Due Date

Enter one reliable date into the pregnancy week calculator. The pregnancy week calculator shows completed weeks, extra days, and an estimated due date. Every answer also shows the date source and calculation date. Medical records call pregnancy age gestational age. After medical dating, use the due date in your maternity record. Choose “Calculate as of” to see your age on a different date.

Enter the first day of menstrual bleeding. Add average cycle length only when recent cycles remain consistent.

days

Defaults to today. Choose another date to see your pregnancy age at an appointment.

The pregnancy report shows the calculation date and selected method. The reference date shows when the displayed week count applies. You can print the report or save it as a PDF using your browser. NICHD defines pregnancy age in completed weeks and extra days. The report separates completed weeks from the week in progress.

Which Pregnancy Date Should You Use?

Use the estimated due date already written in your medical record. IVF pregnancies use documented embryo transfer details from official fertility clinic records. Without a recorded due date, choose the earliest reliable record.

Information available Calculator input
Medical record due date Due Date
Fertility clinic transfer record IVF Transfer
Early scan with pregnancy age Ultrasound
Reliable last menstrual period Last Period
Tracked ovulation or insemination Conception or Ovulation
Intercourse date alone Use another date source
Later scan alone Medical record due date, when available

A source label prevents accidental use of intercourse dates or later size estimates. Early ultrasound can support pregnancy dating during medical review. Later measurements mainly answer size and growth questions.

The American College of Obstetricians and Gynecologists places early ultrasound among the strongest dating sources. Its pregnancy dating guidance also covers IVF transfer dates.

The ASRM fertility glossary defines embryo transfer from day one through day seven. Fertility clinic records contain the embryo age needed for IVF calculation.

Why Do Pregnancy Dates Differ?

Pregnancy dates differ when calculations begin from different starting records. Cycle entries, saved app dates, scan timing, and IVF records change results. Compare both sources before interpreting any difference.

Date difference Check first
Different starting dates Input method
Different saved dates Stored due date
Different cycle models Cycle length
Different calculation dates Reference date
One day only Device date and day counting
Different week labels Completed week wording
Later scan date Medical record due date

To compare estimates, calculate each method separately using the same calculation date. Save each report before changing your inputs. This calculator does not choose which medical record is correct.

01

Last Period (LMP) vs Early Ultrasound

Last menstrual period dating uses a recalled date and calendar model. Early ultrasound dating uses the measured crown rump length. Comparing both due dates shows the day difference without diagnosing its cause.

A menstrual estimate of 10 March and scan estimate of 16 March differ six days. The six-day difference only describes two separate dating sources. A maternity team decides which date belongs in the medical record.

ACOG reports first-trimester crown rump length dating is accurate within five to seven days. Menstrual recall and ovulation timing can add extra uncertainty.

02

Pregnancy App vs Medical Record Due Date

Matching due dates and reference dates produce matching pregnancy day counts. First, check saved due dates inside both applications. Then compare cycle entries, device dates, and completed-week wording.

One application may retain an older due date after medical revision. Another application may read the date differently near midnight. Identical calendar inputs produce the same completed pregnancy day count.

03

IVF Transfer Date vs LMP

IVF dating uses embryo age and transfer timing from clinic records. Menstrual dates may reflect bleeding recorded during a treatment cycle. Use the fertility clinic due date in your record.

The IVF method uses transfer date and embryo age. The last-period method uses period date and usual cycle length. Each calculation shows the selected method; use your clinic’s recorded due date for personal care.

04

Later Ultrasound vs Established Due Date

Later scans compare fetal size with the assigned pregnancy age. A measurement-based date cannot replace the medical record automatically. The scan report may display both values without changing the record.

A later scan might print another due date from current measurements. The medical record may retain an earlier, better-supported date. Compare both labels before changing any personal tracker.

Ultrasound dating

Can an Ultrasound Change My Due Date?

Ultrasound findings can support a due date revision during medical review. The day threshold changes across pregnancy stages. Your maternity or fertility team records the official due date.

ACOG reference thresholds
Ultrasound timing Difference that crosses the ACOG reference
Up to 8+6 More than 5 days
9+0 through 13+6 More than 7 days
14+0 through 15+6 More than 7 days
16+0 through 21+6 More than 10 days
22+0 through 27+6 More than 14 days
28+0 onward More than 21 days
01

The phrase more than changes how each threshold works. A five-day difference remains below the more-than-five-day threshold. A six-day difference crosses that same threshold.

02

Consider a scan at 8+2 with a seven-day difference. The comparison crosses the five-day reference with two extra days. This example explains the threshold; the calculator does not apply clinical redating decisions.

03

Third-trimester dating needs extra caution because fetal size varies more widely. The last row applies when the first scan occurs during that trimester. Earlier records and growth concerns remain part of medical review.

ACOG publishes the full ultrasound redating thresholds. The medical team reviews the entire pregnancy record before revising dates.

Understanding your scans

Early Dating Scan vs Later Growth Scan

Early scans support dating through crown rump length. Later scans compare several measurements with the assigned pregnancy age. Serial scans show growth across several separate measurement dates.

Early dating scan Later growth scan
Dates the pregnancy Measures fetal size
Uses crown rump length Uses head, belly, and thigh measurements
Has narrower dating uncertainty Has wider biological size variation
Can establish a due date Tracks size and growth
01

What an Early Dating Scan Measures

Crown rump length measures the embryo from head to bottom. During early pregnancy, that length supports a pregnancy-age estimate. The scan date produces an estimated due date.

Measurement quality depends on the image and marker placement. A medical team compares the scan with menstrual or IVF records. ACOG ranks early crown rump length among the strongest dating methods available.

02

Why a Later Scan Can Measure Ahead or Behind

Later measurements can plot above or below the assigned pregnancy age. The printed measurement age reflects comparison with a reference chart. The medical due date anchors elapsed calendar time.

Example

For example, a scan at 28+0 may show belly size near 29+1. The report compares one measurement with babies at different ages. Eleven extra calendar days have never passed.

03
Size and growth are different

Fetal Size vs Fetal Growth

One scan describes fetal size at one moment. Two scans separated across time can show growth between visits. Doctors also compare percentiles, scan timing, earlier scans, and pregnancy history.

Head width, head circumference, belly circumference, and thigh-bone length measure different areas. NICHD lists them within its fetal measurement definitions. One number alone cannot describe the entire growth pattern.

ISUOG ultrasound guidance distinguishes size at one visit from growth across time. ISUOG also keeps an accurate early due date during later scans.

Reading pregnancy weeks

What Is 18+4 Weeks Pregnant?

At 18+4, eighteen completed weeks and four extra days have passed. The total equals 130 completed pregnancy days. The 19+0 point arrives three calendar days later.

18+4 = 18 completed weeks + 4 days = 130 days

The pregnancy remains within the 18+0 through 18+6 interval. Everyday wording may call 18+4 the nineteenth week in progress. Medical records show 18+4 until 19+0 arrives.

For a Sunday 18+4 result, 19+0 arrives on Wednesday. The calculator shows completed weeks and days for your selected date. NICHD uses completed weeks and days for medical pregnancy-age reporting.

18+0 Week 18 19+0
18+4
18+0 18+1 18+2 18+3 18+4 18+5 18+6 19+0
How pregnancy dating begins

Why Pregnancy Weeks Start Before Conception

Pregnancy age starts from the first day of the last period. The standard model places ovulation near 2+0. Actual ovulation timing varies across cycles and individuals.

0+0
Starting point

First day of the last menstrual period

2+0
Around two weeks

Ovulation and fertilization period

4+0
Around four weeks

Expected next period

40+0
Estimated endpoint

Estimated due date

Pregnancy point Standard menstrual model
0+0 First day of the last menstrual period
Near 2+0 Ovulation and fertilization period
Near 4+0 Expected next period
40+0 Estimated due date
Two different starting points

Pregnancy age and age from fertilization use different starting dates. Pregnancy age starts from the menstrual date. Age from fertilization starts around two weeks later.

01

A last-period date can be hard to recall and may differ from ovulation timing. Early ultrasound or IVF records may provide stronger dating evidence.

02

ACOG uses a standard 280-day menstrual framework for due-date calculation. Early scans or IVF records may offer better evidence. Readers seeking early-month context can review how doctors count the first pregnancy month.

Cycle length adjustment

How Cycle Length Changes Your LMP Estimate

Cycle adjustment changes the standard last-period calendar model. A 32-day cycle moves the estimated due date four days later. A 24-day cycle moves that date four days earlier.

Adjustment formula

Adjusted due date = last menstrual period + 280 days + cycle length - 28 days

24 days Four days earlier
Standard 28 days No adjustment
32 days Four days later
Average cycle length Calendar adjustment
24 days Four days earlier
28 days No adjustment
32 days Four days later
01

Mom News Daily applies that optional model only after cycle entry. The 28-day result remains visible beside the adjusted estimate.

Calendar example

For a last period beginning 1 January 2026, the standard due date becomes 8 October. A 32-day entry moves that date to 12 October. A 24-day entry moves it to 4 October.

02

Cycle adjustment changes calendar assumptions without measuring ovulation. Ovulation timing can change from one menstrual cycle to another. The pregnancy calculator displays both standard and adjusted estimates.

03

Irregular cycles make period-based dating less dependable. Use an early scan or medical due date when available. The pregnancy calculator can still show the standard menstrual calendar estimate.

ACOG describes the conventional 28-day cycle and day-14 ovulation assumptions. The standard assumptions cannot identify one exact ovulation date.

IVF pregnancy dating

How IVF Pregnancy Dating Works

In vitro fertilization dating uses transfer date and documented embryo age. A day-five transfer corresponds to 2+5 on transfer day. Frozen storage time adds zero days to later embryo development.

Due date formula

Estimated due date = transfer date + 266 days - embryo age

Day 3 2+3 Pregnancy age on transfer day 263 days to 40+0
Day 5 2+5 Pregnancy age on transfer day 261 days to 40+0
Day 6 2+6 Pregnancy age on transfer day 260 days to 40+0
Embryo age Pregnancy age on transfer day Days from transfer to 40+0
Day 3 2+3 263
Day 5 2+5 261
Day 6 2+6 260
Same transfer date example

A day-five transfer on 1 January produces 19 September. A day-three transfer performed on 1 January produces 21 September. The day-six result, 18 September, follows the same documented embryo-age formula.

01

ACOG includes day-three and day-five transfer examples. Day-six arithmetic follows the same documented embryo-age calculation.

02

Frozen storage time never increases embryo age. Clinic records combine culture days before freezing and after warming. The ASRM fertility glossary describes fresh and frozen embryo transfers.

03

The word blastocyst cannot identify one exact embryo day. ASRM notes blastocyst development around days five, six, or seven. Always use the documented embryo age written in fertility clinic records.

How We Calculate Pregnancy Weeks

The pregnancy week calculator uses one method for each date source. The form retains your inputs until you reset it, and the report shows the selected calculation date. Each method uses calendar days without clock times.

01 Last menstrual period Add 280 days
02 Cycle adjustment Add 280 days, then add cycle length minus 28
03 Recorded due date Count backward from 40+0
04 Conception or ovulation Add 266 days
05 Ultrasound Add 280 days, then subtract scan age
06 IVF transfer Add 266 days, then subtract embryo age
Date source Calculation
Last menstrual period Add 280 days
Last period with cycle adjustment Add 280 days, then add cycle length minus 28
Recorded due date Count backward from 40+0
Conception or tracked ovulation Add 266 days
Ultrasound Add 280 days, then subtract scan age
IVF transfer Add 266 days, then subtract embryo age
01

A last period of 1 January 2026 produces 8 October 2026. A documented conception date on 15 January produces the same due date. A day-five transfer on 20 January also produces 8 October.

02

Ultrasound calculation uses the scan date and printed pregnancy age. A scan on 1 June at 21+4 produces 8 October exactly. Extra ultrasound days range from zero through six.

03

The recorded-due-date method counts backward from 40+0. A due date of 24 January 2027 produces 18+5 on 28 August 2026. Pregnancy age changes whenever the calculation date changes.

The pregnancy calculator uses named months and date-only values. Time-of-day values never enter any pregnancy formula.

What the Pregnancy Calculator Can and Cannot Do

The pregnancy week calculator performs calendar calculations from entered dates. A correct formula can still begin from an uncertain date source. Medical teams later assign and revise due dates inside their clinical records.

01 Pregnancy calculator can show
  • Pregnancy age from an entered date
  • Estimated due date
  • Difference between two date sources
  • Cycle-adjusted calendar estimate
  • IVF transfer calculation
  • Scan-based date estimate
  • Early, full, late, and postterm dates
02 Medical review covers
  • Official due date assignment
  • Medical date revision
  • Cause of a medical difference
  • Actual ovulation date
  • Clinic record interpretation
  • Scan findings and fetal health
  • Delivery planning
Pregnancy calculator can show Medical review covers
Pregnancy age from an entered date Official due date assignment
Estimated due date Medical date revision
Difference between two date sources Cause of a medical difference
Cycle-adjusted calendar estimate Actual ovulation date
IVF transfer calculation Clinic record interpretation
Scan-based date estimate Scan findings and fetal health
Early, full, late, and postterm dates Delivery planning
01

Menstrual recall, unknown ovulation timing, and intercourse dates can reduce source precision. The pregnancy report identifies the selected calculation method. Readers can then separate calendar accuracy from source uncertainty.

The pregnancy calculator cannot assess pregnancy health, viability, fetal growth, or symptoms. Pregnancy concerns need prompt contact with a maternity or fertility team. Urgent symptoms require local emergency medical services.

Clinical protocols vary across regions and health systems. Use the due date recorded in your own medical record. The medical-record due date remains the best source for personal tracking.

Common Pregnancy Dating Questions

Common questions cover app, scan, appointment, and IVF date problems. Each answer starts with one practical check. Medical records remain the main source for unresolved dates.

01 Why Can Two Apps Show a One Day Difference?

Compare the saved due date and calculation date inside both applications. Matching dates produce the same completed pregnancy day count. A one-day difference may come from device time or hidden date conversion.

One application may still contain an older medical due date. Update both applications after a formal medical revision. Save one private note with old and new dates.

02 Which Weekday Does a New Pregnancy Week Begin?

A new completed pregnancy week begins on the same due-date weekday. Every seven-day interval returns to the same weekday. A Sunday due date places every full-week point on Sunday.

For example, 39+0 and 18+0 both begin Sunday. You can find that weekday from your recorded due date. The weekday result follows the full 280-day pregnancy calendar.

03 Can I Calculate Pregnancy Age for an Appointment Date?

Use the appointment date as the calculation date. Select the Due Date tab and enter your recorded medical due date. The pregnancy week calculator then returns age for that appointment.

Appointment results support personal calendar planning only. Future scan findings or medical decisions remain outside the calculation. Confirm appointment requirements with your local maternity service.

04 Does Frozen Embryo Storage Time Count?

Years in storage never increase the documented embryo age. Fertility records total culture days before freezing and after warming. The transfer date then anchors the IVF calculation.

A five-year storage period adds no pregnancy days. Use the embryo age from your fertility clinic record. The ASRM glossary describes the same frozen-transfer pregnancy dating method.

05 Why Does My Scan Print Another Due Date?

Current fetal measurements can create another printed due date. Your medical record may retain an earlier date from better evidence. Check the medical-record due date before changing any personal tracker.

The printed scan date describes measurement position on a chart. The recorded medical date may come from better early dating evidence. Use the medical-record date after formal assignment.

06 What Should I Update After My Medical Due Date Changes?

Update pregnancy applications, personal calendars, saved receipts, and shared family dates. Save the old date inside a private dated note. Record the clinic or maternity source behind every formal revision.

A dated note prevents old screenshots from causing later uncertainty. The note also shows why archived results differ. Medical records remain the main source for personal tracking.

Medical Sources

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