2026 Obstetric Standards

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Due Date Calculator

Calculate your estimated due date using LMP, ultrasound, conception date, or IVF transfer. Get a complete pregnancy timeline with trimester milestones, term windows, and gestational age tracking.

Pregnancy Information

2026 Obstetric Standards
28 days

Adjusts calculation for cycles different from 28 days.

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Your Pregnancy Timeline

Enter your pregnancy information to see your due date
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The 4% Reality

Only 4% of babies are born on their actual due date. The "due date" is really the middle of a 5-week delivery window (37-42 weeks). Most healthy babies arrive between 39-41 weeks, so think of your EDD as a target range, not a deadline.

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LMP vs. Ultrasound Dating

First-trimester ultrasounds are considered the gold standard for dating if they differ from LMP calculations by more than 7 days. Early ultrasounds (6-9 weeks) are accurate to within ±3-5 days, while LMP-based dates assume regular 28-day cycles.

Due Date Calculator: Your Gestational Timeline Engine

A due date isn't a deadline—it's the statistical center of a delivery window. These insights reveal the counterintuitive truths about pregnancy dating that most parents-to-be never learn.

Pregnancy Dating Intelligence

The 4% Reality

•Your due date is a target, not a deadline.
•Only 4% of babies arrive on their exact due date. The other 96% arrive across a 5-week window (37-42 weeks). Stop thinking "due date" and start thinking "due month." This mental shift reduces anxiety and sets realistic expectations.

The Dating Paradox

•Earlier ultrasounds are MORE accurate, not less.
•At 7 weeks, embryos measure identically regardless of genetics—making dating accurate to ±3 days. By 36 weeks, "small" babies measure 5 lbs while "large" babies measure 9 lbs. More data doesn't mean better accuracy; earlier data does.

The 39-Week Threshold

•Why doctors won't schedule elective deliveries before 39 weeks.
•Brain development accelerates dramatically between weeks 37-39. A baby born at 37 weeks has significantly higher NICU admission rates than one born at 39 weeks. Those final 2 weeks aren't just "bonus time"—they're neurologically critical.

The First Baby Delay

•First pregnancies really do go longer—it's biology, not myth.
•First-time mothers average 39w 5d versus 39w 2d for subsequent pregnancies. Your body has never done this before—the cervix needs more time to ripen, and the uterus needs more practice contracting effectively.

Due Date Calculator: Complete Pregnancy Dating & Gestational Age Guide

Calculate your estimated due date using LMP, ultrasound, conception date, or IVF transfer. Get a complete pregnancy timeline with trimester milestones, term classifications, and preparation guidance.

What This Calculator Does & Who It's For

Calculator Purpose & Outputs

  • Outputs You'll Receive:
    Estimated Due Date (EDD): Your projected 40-week delivery date. Current Gestational Age: How far along you are in weeks and days. Trimester Milestones: End dates for 1st trimester (13w 6d) and 2nd trimester (27w 6d). Term Window: Early term, full term, and late term dates. Baby Size Comparison: Week-by-week visualization. Days Remaining: Countdown to your due date.
  • Supported Calculation Methods:
    LMP: Standard method with cycle length adjustment. Conception Date: For users who know their fertilization date. Ultrasound: Gold standard—input scan date and measured gestational age. IVF Transfer: Precise calculation for 3-day embryos or 5-day blastocysts.
  • Ideal Users:
    Newly pregnant: Establish your complete timeline. Irregular cycles: Use ultrasound method for accurate dating. IVF patients: Get precise calculations based on known transfer date. Healthcare planning: Schedule prenatal appointments around milestone dates.
  • Accuracy by Method:
    LMP: ±2 weeks (assumes regular cycles and day-14 ovulation). First-trimester ultrasound: ±3-5 days (most accurate). Conception date: ±1-2 days if timing is certain. IVF: Most precise due to known fertilization date.
This due date calculator determines your estimated delivery date using four different methods, then provides a complete gestational timeline. It's designed to give you more than just a single date—you'll get milestone tracking, term window classification, and week-by-week context.

Due Date Calculation Formulas

The Four Dating Methods Explained

  • LMP Method (Naegele's Rule):
    EDD = LMP + 280 days + (Cycle Length - 28)

    Standard: LMP January 1 with 28-day cycle → EDD October 8.
    Adjusted: LMP January 1 with 35-day cycle → EDD October 15 (+7 days).

  • Conception Date Method:
    EDD = Conception Date + 266 days

    This equals 38 weeks—the actual pregnancy duration from fertilization. The 2-week difference from gestational age accounts for the LMP-to-ovulation period.

  • Ultrasound Method:
    EDD = Scan Date + (280 - (Weeks × 7 + Days))

    Example: Scan on March 1 showing 8w 3d (59 days) → EDD = March 1 + 221 = October 8.

  • IVF Transfer Method:
    EDD (5-day blastocyst) = Transfer Date + 261 days
    EDD (3-day embryo) = Transfer Date + 263 days

    The 2-day difference accounts for embryo development stage at transfer.

Each method calculates your due date differently based on the starting information available. Understanding the math helps you verify your results and discuss dating with your provider.

Understanding Gestational vs. Fetal Age

The 2-Week Offset Explained

  • Gestational Age (Medical Standard):
    Counted from day 1 of your last period. When your provider says you're "12 weeks pregnant," this is gestational age. All prenatal milestones, trimester divisions, and due date calculations use this convention. At "4 weeks gestational age," a pregnancy test becomes reliable—but conception occurred only 2 weeks prior.
  • Fetal Age (Embryonic Age):
    Counted from actual fertilization—2 weeks younger than gestational age. At "12 weeks pregnant" (gestational), your baby is approximately 10 weeks old from conception. This distinction explains why early developmental milestones (heartbeat at "6 weeks") seem earlier than expected.
  • Why LMP Became the Standard:
    Historically, women knew when their period started but not when they conceived. This reliable date became the medical standard. Even now that conception can sometimes be identified precisely (IVF, tracking), the 40-week gestational system persists for consistency across all pregnancies.
  • How IVF Handles This:
    IVF patients know their exact fertilization date. Gestational age is calculated by adding 2 weeks to the embryo age at transfer. A 5-day blastocyst transferred on day 0 is considered 2 weeks 5 days gestational age that same day—aligning with the LMP-based system.
Pregnancy dating uses "gestational age" counted from your last menstrual period—about 2 weeks before conception. This creates a consistent but sometimes confusing system that affects how milestones are reported.

The ACOG Term Classification System

When Is Baby Ready? The 5-Week Delivery Window

  • Early Term (37w 0d – 38w 6d):
    Baby is considered mature but may benefit from additional development. Brain and lung development continue significantly during weeks 37-39. Babies born in this window have slightly higher NICU admission rates. Clinical guidance: Avoid elective deliveries unless medically indicated.
  • Full Term (39w 0d – 40w 6d):
    Optimal window for delivery. Brain development reaches a critical maturity threshold, and lung surfactant production is complete. Clinical guidance: ACOG recommends waiting until at least 39 weeks for all elective inductions and scheduled C-sections. This is the "due date" window.
  • Late Term (41w 0d – 41w 6d):
    Still within normal variation but requires closer monitoring. Placental function begins declining, and amniotic fluid may decrease. Clinical guidance: Non-stress tests or biophysical profiles typically recommended. Induction discussion is common.
  • Post-Term (42w 0d and beyond):
    Increased risk of complications including stillbirth, meconium aspiration, and macrosomia (large baby). Clinical guidance: Most providers strongly recommend delivery by 42 weeks. Only 5-10% of accurately dated pregnancies reach this point.
The American College of Obstetricians and Gynecologists (ACOG) classifies "term" pregnancy as a spectrum with distinct clinical implications for each stage.

When and Why Ultrasound Changes Your Due Date

The 7-Day Rule and Dating Accuracy by Trimester

  • First Trimester (Before 14 Weeks):
    Accuracy: ±3-5 days. Crown-rump length (CRL) measurements are highly standardized because early embryo growth is nearly identical regardless of genetics. The 7-day rule: If ultrasound differs from LMP by more than 7 days, providers typically change the due date to match the scan. This is the clinical standard.
  • Second Trimester (14-28 Weeks):
    Accuracy: ±10-14 days. Genetic variation in fetal size begins affecting measurements—some healthy babies are simply bigger or smaller. Due dates are only changed if discrepancy exceeds 10-14 days. The anatomy scan at 20 weeks primarily confirms dating, not establishes it.
  • Third Trimester (28+ Weeks):
    Accuracy: ±2-3 weeks. Size variation makes precise dating impossible. A "small" baby measuring 5 lbs and a "large" baby measuring 9 lbs could both be healthy at 36 weeks. Due dates are never changed based on third-trimester scans—growth tracking replaces dating.
  • Why Earlier Is Better:
    Paradoxically, earlier ultrasounds produce more accurate dating. At 7 weeks, all embryos measure essentially the same (±2mm). This uniformity decreases as genetic and environmental factors introduce variation. If dating matters (scheduling, monitoring), push for an early scan.
Ultrasound accuracy varies dramatically by gestational age. Understanding when providers adjust dates—and why—helps you anticipate potential changes to your timeline.

Practical Planning and Preparation

Setting Realistic Expectations

  • Delivery Timing Statistics:
    First-time mothers: Average delivery at 39 weeks 5 days. Most common window: 39-41 weeks. Second+ pregnancies: Average delivery at 39 weeks 2 days, often 38-40 weeks. Twins: Average delivery at 36 weeks. Key insight: Going past your due date is normal—intervention typically isn't discussed until 41 weeks.
  • Preparation Timeline:
    By 36 weeks: Car seat installed, nursery ready. By 37 weeks: Hospital bag packed (early term begins). By 38 weeks: Childcare arrangements finalized for other children. By 39 weeks: Work handoffs complete if possible. Mental prep: Expect baby anytime from 37-42 weeks.
  • When to Call Your Provider:
    Before 37 weeks: Regular contractions (possible preterm labor). Any time: Decreased fetal movement, water breaking, heavy bleeding, severe abdominal pain. After due date: Discuss monitoring schedule and induction timing at 41-week appointment.
  • Managing Due Date Anxiety:
    Avoid telling everyone your exact due date—say "mid-October" instead of "October 15." This reduces the "any day now?" messages that start at 38 weeks. Remember: your baby hasn't read the calendar. The due date is a statistical construct, not a biological deadline.
Understanding the statistical reality of due dates helps you prepare mentally and practically. Planning for a window rather than a date reduces stress and ensures readiness.

FAQ

? How do I calculate my due date from my last period?

Add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle isn't 28 days, adjust by adding or subtracting the difference. Example: 32-day cycle = add 4 extra days to your due date. This is Naegele's Rule, the standard medical calculation used since 1812.

? Can my due date change after an ultrasound?

Yes, if an early ultrasound (before 14 weeks) differs from your LMP-based date by more than 7 days, most providers will adjust your due date to match the scan. This is called the "7-day rule." Later ultrasounds are less reliable for changing dates due to natural size variation between babies.

? What percentage of babies are actually born on their due date?

Only about 4%. The due date is a statistical midpoint, not a prediction. About 80% of babies arrive within 2 weeks of the EDD (38-42 weeks). Think of your due date as the center of a 5-week delivery window rather than a specific target.

? How is an IVF due date calculated?

For a 5-day blastocyst transfer: add 261 days to your transfer date. For a 3-day embryo transfer: add 263 days. IVF due dates are among the most accurate because the exact fertilization timing is known—no guessing about ovulation or conception date.

? What's the difference between gestational age and fetal age?

Gestational age counts from your last period (the medical standard), while fetal age counts from conception. There's a 2-week difference—at "12 weeks pregnant" (gestational), your baby is actually ~10 weeks old from fertilization. All medical milestones use gestational age.

? When is my baby considered "full term"?

Full term is 39-40 weeks—the optimal window for delivery. "Early term" (37-38 weeks) means baby is mature but may benefit from more time. "Late term" (41 weeks) is still normal but requires monitoring. ACOG recommends waiting until at least 39 weeks for elective deliveries.

? Why do first babies often arrive late?

First-time mothers deliver at an average of 39 weeks 5 days versus 39 weeks 2 days for subsequent pregnancies. The cervix and uterus are "untested"—they've never dilated or contracted through labor before. This typically adds 2-3 days compared to later pregnancies.

? When should I have my hospital bag packed?

By 37 weeks (early term), when baby could safely arrive. Have childcare arrangements finalized by 38 weeks, and complete work handoffs by 39 weeks if possible. Mentally prepare for delivery anytime between 37-42 weeks—that's your realistic arrival window.
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Informational Use: These calculations (BMI, Calories, etc.) are based on standard statistical formulas and are intended for general reference and goal-setting purposes only.

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