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Pregnancy Calculator – Due Date, Current Week & Trimester

Calculate how many weeks pregnant you are online free - no login. Enter the first day of your last period or conception date to find your current pregnancy week and estimated due date.

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Health & Wellness Disclaimer

Calculations and estimates provided by this tool are for informational and educational purposes only. They are not intended as medical advice, diagnosis, or treatment. Always consult a qualified physician or healthcare professional regarding any health condition or medical decision.

About this tool

Accurate gestational dating is foundational for scheduling prenatal screening tests, interpreting fetal growth scans, and planning obstetric care milestones. This calculator applies Naegele's rule (LMP + 280 days) and post-conception dating (conception + 266 days), aligning with baseline dating conventions outlined in clinical guidance from the American College of Obstetricians and Gynecologists (ACOG Committee Opinion No. 700, 'Methods for Estimating the Due Date').

Because cycle regularity and ovulation timing vary significantly across individuals, formula-based calculations serve as standard initial estimates that clinical providers systematically reconcile against early first-trimester ultrasound biometry.

Enter the first day of your last menstrual period (LMP), a known conception date, or an ultrasound-confirmed due date to calculate your estimated due date (EDD), current gestational age in completed weeks, trimester status, and milestone calendar. Calculations use standard obstetric dating conventions (LMP + 280 days or conception + 266 days) supported by the American College of Obstetricians and Gynecologists (ACOG).

How to Use Pregnancy Week Calculator

Enter LMP Date

Input the first day of your last menstrual period.

Calculate

The tool calculates your gestational age and due date.

See Week & Trimester

View your current week, trimester, and days remaining.

View Due Date

See your estimated due date (EDD) clearly displayed.

Common Workflows

Calculate Due Date from Last Menstrual Period (LMP)

Select 'Last Menstrual Period', choose the first calendar day of your last normal menstrual cycle, and click Calculate to receive an estimated due date based on the standard 280-day obstetric timeline.

Calculate from Known Conception Date

If you tracked ovulation through luteinizing hormone (LH) surge testing, basal body temperature, or timed intercourse, select 'Conception Date' and input the ovulation or fertilization date for a 266-day post-fertilization timeline.

Check Current Gestational Age and Trimester

View your real-time gestational status displayed in completed weeks and current trimester classification (First Trimester: under 13 weeks; Second Trimester: 13 to 26 weeks; Third Trimester: 27 weeks and beyond).

Review Prenatal Milestone Dates

Inspect the projected milestone schedule to plan for key clinical benchmarks, including the start of the second trimester (Week 13), mid-pregnancy anatomy ultrasound (Week 20), start of the third trimester (Week 27), and early term status (Week 37).

Reconcile LMP Dating with Ultrasound Confirmation

When you receive your first-trimester crown-rump length (CRL) ultrasound dating scan, enter the clinician's revised due date using 'Due Date' mode to determine the corresponding clinical conception date and updated weekly milestone schedule.

Best For

  • Calculates estimated due date using standard obstetric conventions: LMP + 280 days or conception date + 266 days.
  • Tracks real-time gestational age in completed weeks, active trimester category, and countdown of days remaining.
  • Projects key prenatal milestone dates, including the start of the second trimester (13w), anatomy ultrasound (20w), viability/third trimester milestone (27w), and full term (37w+).

Examples

Standard 280-Day Calculation from Last Menstrual Period

Input Selection

Method: Last Menstrual Period; Date: January 1, 2026

Calculated Timeline

Estimated Due Date: October 8, 2026; Estimated Conception: January 15, 2026; Milestone Schedule: Week 13 Second Trimester (April 2, 2026), Week 20 Anatomy Scan (May 21, 2026), Week 27 Third Trimester (July 9, 2026), Week 37 Early Term (September 17, 2026)

Naegele's rule adds 280 days (40 completed weeks) to the first day of the last menstrual period, assuming a conventional 28-day cycle with ovulation occurring approximately on cycle day 14.

266-Day Post-Conception Calculation

Input Selection

Method: Conception Date; Date: February 14, 2026

Calculated Timeline

Estimated Due Date: November 7, 2026; Equivalent LMP: January 31, 2026; Gestational Age: calculated from equivalent LMP date (+14 days prior to fertilization)

When the conception or ovulation date is known, human gestation is calculated as 266 days (38 completed weeks) post-fertilization, with gestational age standardized by backdating 14 days to simulate the equivalent menstrual start.

Reverse Milestone Tracking from an Established Due Date

Input Selection

Method: Due Date; Date: November 20, 2026

Calculated Timeline

Estimated Conception: February 27, 2026; Equivalent LMP: February 13, 2026; Milestone Schedule: Week 20 Anatomy Scan (July 3, 2026), Week 37 Early Term (October 30, 2026)

Subtracts 266 days from the verified due date to compute estimated conception, establishing a standardized milestone schedule aligned with obstetric charting conventions.

Common Mistakes

Problem

Entering the End of Bleeding Rather Than the First Day of LMP

Solution

Obstetric dating strictly measures from the first calendar day of your last normal menstrual flow, not the day bleeding stopped. Using the last day artificially shortens the calculated timeline by several days.

Problem

Confusing Gestational Age with Fetal Age

Solution

Gestational age includes the approximate two weeks prior to ovulation when follicular development occurs. At conception, gestational age is already considered 2 weeks; by the time a missed period occurs (approx. 4 weeks), embryonic development is roughly 2 weeks post-fertilization.

Problem

Assuming the Due Date is a Guaranteed Delivery Date

Solution

Only 4% to 5% of deliveries occur exactly on the estimated due date. The EDD is a reference point for monitoring fetal growth and clinical safety, with spontaneous delivery normally occurring across the term delivery window.

Problem

Relying on LMP Dating with Irregular or Non-28-Day Cycles

Solution

The standard 280-day formula assumes ovulation occurs exactly 14 days after menstruation begins. If your cycle length is 35 days or irregular, ovulation occurs later, making LMP-only calculations systematically overestimate gestational age until adjusted by early ultrasound.

Problem

Overriding Clinical Ultrasound Dating with an Online Calculator

Solution

Under ACOG Committee Opinion No. 700 guidelines, when an accurate first-trimester crown-rump length (CRL) ultrasound measurement differs from LMP dating by more than 5 to 7 days, the ultrasound date becomes the authoritative clinical due date.

Tips & Best Practices

Record the Exact First Day of Your Last Period Before Your Initial Prenatal Visit

Note whether the start date of your last menstrual period was certain and whether your preceding cycles were regular; this distinction helps your obstetrician determine whether LMP dating is clinically reliable.

Understand ACOG's Definition of Term Pregnancy

Under clinical guidance from ACOG and the Society for Maternal-Fetal Medicine (SMFM) on the 'Definition of Term Pregnancy' (Committee Opinion No. 579), term gestation is divided into four distinct categories: Early Term (37 0/7 weeks through 38 6/7 weeks), Full Term (39 0/7 weeks through 40 6/7 weeks), Late Term (41 0/7 weeks through 41 6/7 weeks), and Postterm (42 0/7 weeks and beyond).

Use Conception Mode for Timed Cycles or Ovulation Tracking

If you tracked your LH surge or basal body temperature shift, entering your conception/ovulation date directly provides a more precise 266-day post-fertilization estimate than an LMP date from a variable cycle.

Update the Calculator with Your Clinician's Due Date

Once your healthcare provider confirms your final estimated due date via first-trimester ultrasound, switch the calculator to 'Due Date' mode to align your weekly milestones with your medical chart.

Limitations

Assumes a Standard 28-Day Cycle with Mid-Cycle Ovulation

The calculator's LMP mode uses Naegele's rule (LMP + 280 days), which relies on the biological assumption of a 28-day menstrual cycle and ovulation on day 14. Variations in follicular phase length or delayed ovulation alter true gestational timing.

Does Not Factor In Ultrasound Crown-Rump Length (CRL) Biometry

First-trimester ultrasound biometry is the most accurate clinical method for establishing gestational age (ACOG Committee Opinion No. 700). Online mathematical formulas cannot assess anatomical measurements or physiological development.

Conception Timing Window Variation

Sperm can survive in the female reproductive tract for up to 5 days prior to ovulation, meaning intercourse date does not always correspond exactly to the date of fertilization.

Generalized Trimester and Milestone Thresholds

While trimester transitions and clinical screening windows (such as the 18–22 week anatomy ultrasound) follow standard obstetric timelines, individual clinical schedules and diagnostic testing protocols are determined by your personal healthcare provider.

Educational and Informational Estimate Only

This tool generates mathematical estimates based on user-entered dates. It does not provide medical diagnoses, viability assessments, or clinical management recommendations, and is not a substitute for professional prenatal care.

Comparisons

LMP-Based Dating vs. First-Trimester Ultrasound Dating

While LMP provides an immediate, accessible starting date, early ultrasound biometry remains the clinical gold standard for verifying gestational age.

LMP Formula Dating (Naegele's Rule)First-Trimester Ultrasound (CRL)
Clinical FoundationMathematical formula: LMP + 280 days (ACOG Committee Opinion No. 700 baseline)Direct biometric measurement of fetal crown-rump length (CRL)
Cycle DependencyRequires reliable recall and assumes regular 28-day cycle with day-14 ovulationIndependent of menstrual cycle length, regularity, or ovulation timing
Typical Accuracy WindowApproximates delivery window within ±2 weeks for regular cyclesAccurate within ±5 to 7 days when performed up to 13 6/7 weeks of gestation
Clinical RoleInitial estimated due date established at first appointmentAuthoritative clinical dating method used to confirm or revise the official EDD

Which should you use?

ACOG Committee Opinion No. 700 recommends that when a reliable first-trimester ultrasound measurement differs from LMP-derived dating by more than 5 days (at <=8 6/7 weeks) or 7 days (at 9 0/7 to 13 6/7 weeks), the ultrasound measurement should determine the clinical due date.

Gestational Age (Obstetric) vs. Fetal Age (Conception)

Obstetric charting measures pregnancy from the last menstrual period, whereas embryological development begins at conception.

Gestational Age (Menstrual Age)Fetal / Conceptional Age
Starting BenchmarkFirst day of last menstrual period (LMP)Date of fertilization / conception
Total Duration40 completed weeks (280 days)38 completed weeks (266 days)
Week Count at ConceptionApproximately 2 weeks 0 days pregnant0 weeks 0 days (fertilization point)
Medical UseUniversal standard in clinical obstetric charting and medical recordsUsed primarily in embryology and assisted reproductive technology (ART)

Which should you use?

Because conception occurs approximately two weeks after the start of a standard menstrual cycle, gestational age is always approximately two weeks greater than the actual biological age of the developing fetus.

FAQs

An estimated due date (EDD) represents a 40-week statistical benchmark rather than a guaranteed delivery date. Clinical studies indicate that only approximately 4% to 5% of spontaneous births occur on the exact estimated due date, with the vast majority delivering within the surrounding two-to-three week window.

How is my estimated due date calculated?

The baseline method is Naegele's rule: add 280 days (40 completed weeks) to the first day of your last normal menstrual period (LMP), or add 266 days (38 completed weeks) to a known conception date. This formula assumes a 28-day cycle with ovulation on cycle day 14. In clinical practice, guidelines from the American College of Obstetricians and Gynecologists (ACOG Committee Opinion No. 700) recommend verifying or adjusting this date using early first-trimester ultrasound biometry (crown-rump length) when available.

How many weeks pregnant am I?

Obstetric dating calculates gestational age from the first day of your last menstrual period (LMP), not from the moment of conception. Because ovulation and conception occur roughly 14 days into a cycle, you are considered approximately 2 weeks pregnant at the time of fertilization, and roughly 4 weeks pregnant by the time of a missed period. This calculator uses today's date and your calculated LMP baseline to report your current gestational age in completed weeks.

What week does each trimester start and end?

In this calculator weeks are completed weeks since the first day of your last period (10 days in is 1 week, 3 days), so the First Trimester covers 0 to 12 completed weeks (up to 12 weeks, 6 days); the Second Trimester covers 13 to 26 weeks; and the Third Trimester begins at 27 weeks and runs through delivery. The milestone dates are the days you reach 13, 20, 27, 37 and 40 completed weeks. Clinical guidance from ACOG and the Society for Maternal-Fetal Medicine (SMFM) further subdivides term delivery into Early Term (37 0/7 to 38 6/7 weeks), Full Term (39 0/7 to 40 6/7 weeks), Late Term (41 0/7 to 41 6/7 weeks), and Postterm (42 0/7 weeks and beyond).

How accurate is an estimated due date?

An estimated due date is a clinical benchmark rather than a precise delivery appointment. Only approximately 4% to 5% of spontaneous births occur on the exact calculated due date, though the majority occur within the surrounding term window between weeks 37 and 42. Early first-trimester ultrasound (before 14 weeks) provides the most accurate clinical dating window (within +/- 5 to 7 days).

Can I calculate my due date if I conceived through IVF?

Yes, but dating is calculated from the embryo transfer or retrieval date rather than the menstrual cycle. For a fresh cycle or egg retrieval date, add 266 days (38 weeks). For a 3-day embryo transfer, add 263 days. For a 5-day blastocyst transfer, add 261 days. Fertility clinics provide an authoritative clinical date, but these calculations provide an accurate standard reference.

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