Pregnancy Due Date Calculator: How It's Calculated, Why It's Usually Wrong, and What Matters More

• Your due date is calculated as LMP (Last Menstrual Period) + 280 days — but only 4–5% of babies are born on their exact due date. The realistic delivery window is 38–42 weeks.
• The due date calculation assumes a 28-day cycle with ovulation on day 14 — women with longer or irregular cycles will have inaccurate estimates without ultrasound correction.
• A first-trimester ultrasound (before 13 weeks) is the most accurate way to confirm or adjust your due date — more accurate than LMP calculation.
• Your estimated due date (EDD) determines when you're considered term (37 weeks), when you may be induced (41–42 weeks), and how your pregnancy is tracked — so getting it right matters medically, not just logistically.

Use our free Pregnancy Due Date Calculator to find your estimated due date, current week of pregnancy, and key milestone dates — based on LMP, conception date, or first ultrasound.

How the Due Date Is Calculated: Naegele's Rule

The standard method for calculating an estimated due date (EDD) is Naegele's Rule, developed by German obstetrician Franz Naegele in 1812 and still used worldwide.

Formula: EDD = LMP + 280 days (40 weeks)

Alternative formula: EDD = LMP + 9 months + 7 days

Example: LMP (Last Menstrual Period) = January 1 EDD = January 1 + 280 days = October 8 Or: January 1 + 9 months + 7 days = October 8 ✓

The logic behind 280 days: Pregnancy is counted from the first day of the last menstrual period — not from conception. Since ovulation typically occurs around day 14 of a 28-day cycle, you're actually "2 weeks pregnant" before you've even conceived. The full 280 days (40 weeks) from LMP = approximately 38 weeks of actual fetal development.

This convention was established because women know their LMP date reliably. Conception date is rarely known with certainty — sperm can survive 3–5 days in the reproductive tract, so even a known "conception date" may be off by days.

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Why Your Calculated Due Date May Be Wrong

Naegele's Rule assumes:

• A 28-day menstrual cycle
• Ovulation on exactly day 14
• Fertilisation on the same day as ovulation

For women whose cycles don't match these assumptions, the LMP-based calculation can be significantly off.

Longer cycles (e.g., 35-day cycle): Ovulation occurs around day 21, not day 14. Conception is 7 days later than Naegele assumes. Adjusted EDD = Standard EDD + 7 days

Shorter cycles (e.g., 21-day cycle): Ovulation around day 7 — conception 7 days earlier than assumed. Adjusted EDD = Standard EDD − 7 days

Irregular cycles: Calculating from LMP alone is unreliable. This is where the first-trimester ultrasound becomes essential.

A good pregnancy due date calculator asks for:

• LMP date AND cycle length (not just LMP)
• Or conception date if known
• Or first ultrasound date and gestational age at that scan

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First Trimester Ultrasound: The Most Accurate Dating Method

A dating ultrasound performed between 6 and 13 weeks gestation is the gold standard for accurate due date calculation.

How it works: The sonographer measures the CRL (Crown-Rump Length) — the distance from the top of the baby's head to the bottom of its spine. This measurement correlates precisely with gestational age in early pregnancy, when all fetuses grow at nearly identical rates (unlike later in pregnancy when individual variation increases).

Accuracy of CRL dating:

• Before 9 weeks: ±5 days accuracy
• 9–13 weeks: ±7 days accuracy
• After 13 weeks: ±2–3 weeks accuracy (significantly less reliable)

This is why Indian and global guidelines recommend a first trimester scan before 13+6 weeks for accurate dating — not just for structural assessment.

When to trust the ultrasound date over LMP: Most obstetricians follow this protocol:

• If ultrasound and LMP dates agree within 5–7 days → use the LMP date
• If difference is greater than 7 days → use the ultrasound date
• If LMP is unknown or unreliable → always use the ultrasound date

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The Realistic Delivery Window: Not a Due "Date"

Only 4–5% of babies are born on the exact due date. The EDD is a statistical midpoint — not a prediction.

Normal delivery distribution:

• Before 37 weeks: preterm (11.1% globally)
• 37–38 weeks 6 days: early term (27.7%)
• 39–40 weeks 6 days: full term (57.5%) — optimal
• 41–41 weeks 6 days: late term (6.4%)
• 42+ weeks: post-term (<3%)

The vast majority of normal, healthy pregnancies deliver between 38 and 42 weeks. A window of 4 weeks, not a date.

What "term" means:

• Preterm: < 37 weeks (requires NICU readiness)
• Early term: 37–38 weeks (baby may need more support)
• Full term: 39–40 weeks (optimal for most outcomes)
• Late term: 41–41+6 weeks
• Post-term: ≥ 42 weeks (increased monitoring, induction often considered)

Understanding this range matters because many parents (and some poorly informed providers) treat the due date as a target — causing unnecessary anxiety when pregnancy continues past it, and sometimes unnecessary inductions.

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Key Pregnancy Milestone Dates: What the Calculator Should Show

A comprehensive pregnancy due date calculator doesn't just give the EDD — it maps the full 40-week journey.

First Trimester (Weeks 1–13):

• Week 6–7: Heartbeat typically visible on ultrasound
• Week 8–10: All major organs have formed
• Week 11–13: First trimester screening (nuchal translucency scan, NIPT)
• Week 13: End of first trimester — miscarriage risk drops significantly

Second Trimester (Weeks 14–27):

• Week 16–18: Quickening (first fetal movements felt — typically later for first pregnancies)
• Week 18–20: Anatomy scan (detailed structural ultrasound)
• Week 20: Halfway point of pregnancy
• Week 24: Viability threshold — survival outside womb becomes possible with intensive care
• Week 24–28: Glucose tolerance test (GDM screening in India — often done at 24–28 weeks)

Third Trimester (Weeks 28–40+):

• Week 28: Third trimester begins; Rh-negative mothers receive anti-D injection
• Week 32–34: Growth scan if clinically indicated
• Week 35–37: Group B Strep test (in countries that screen)
• Week 37: "Term" begins — baby is considered ready for birth
• Week 39–40: Optimal delivery window for planned births
• Week 41: Many providers begin discussing or recommending induction
• Week 42: Most guidelines recommend induction to avoid post-maturity complications

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Calculating Gestational Age When You Don't Know Your LMP

Some women don't track their periods or have irregular cycles. Three alternative methods:

From conception date (IVF or confirmed ovulation): EDD = Conception date + 266 days (38 weeks)

IVF is the most precise case — the day of egg retrieval is known, so EDD = retrieval date + 266 days. IVF twins and singleton dates are calculated identically from retrieval.

From known ovulation date (basal body temperature charting or LH surge): EDD = Ovulation date + 266 days

From ultrasound at known gestational age: EDD = Ultrasound date + (remaining days to 280 from confirmed gestational age)

Example: Ultrasound at 10 weeks (70 days) confirms 10 weeks gestation. EDD = Ultrasound date + (280 − 70) = Ultrasound date + 210 days

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Pregnancy Due Date in India: The Doctor Visit Timeline

FOGSI (Federation of Obstetric and Gynaecological Societies of India) recommends this antenatal visit schedule:

Visit	Timing	Key Checks
1st visit	As soon as pregnancy confirmed	Confirm pregnancy, establish EDD, blood work
2nd visit	11–13+6 weeks	NT scan, NIPT option, dating confirmation
3rd visit	16 weeks	Second trimester blood tests
4th visit	18–20 weeks	Anomaly scan (morphology/anatomy)
5th visit	24–28 weeks	GDM test, growth assessment
6th visit	28–32 weeks	Blood pressure, fetal position
7th visit	34–36 weeks	Presentation, birth planning
8th visit	38–40 weeks	Cervical assessment, delivery readiness

Your EDD anchors every one of these visits — the timing of each scan and test is calculated from it. An incorrect EDD means tests happen at wrong gestational ages, potentially missing or misinterpreting findings.

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Due Date for Twin Pregnancies

Twins are calculated the same way from LMP — but the expected delivery timing is different.

Average delivery timing:

• Dichorionic diamniotic (DCDA) twins: 37–38 weeks (elective delivery often planned at 37–38 weeks)
• Monochorionic diamniotic (MCDA) twins: 36–37 weeks (higher risk, earlier delivery)
• Monochorionic monoamniotic (MCMA) twins: 32–34 weeks (highest risk, very early planned delivery)

The EDD (40 weeks) still applies as the theoretical endpoint, but twin pregnancies are typically not allowed to reach 40 weeks. Your calculator should flag twin pregnancies with the adjusted typical delivery window.

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Common Misconceptions About Due Dates

Misconception 1: "My doctor changed my due date after the ultrasound — something is wrong." Changing the EDD based on ultrasound data is standard practice when there's a significant discrepancy from LMP. It means the dating is now more accurate — not that anything is wrong with the pregnancy.

Misconception 2: "I'm overdue at 40 weeks." 40 weeks is the EDD — not a deadline. You're "late term" at 41 weeks and "post-term" at 42 weeks. Being at your EDD with a healthy pregnancy is entirely normal.

Misconception 3: "I need an induction at 40 weeks." Induction is typically recommended at 41–42 weeks in low-risk pregnancies. Evidence for routine induction at 40 weeks is mixed. Discuss with your OB based on your specific risk factors.

Misconception 4: "Premature labour at 37 weeks is fine." 37 weeks is "early term," not full term. Babies born at 37–38 weeks have slightly higher rates of breathing difficulties, feeding problems, and NICU admission than babies born at 39–40 weeks. Where possible, planned deliveries before 39 weeks should have clinical justification.

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FAQ

How is the due date calculated in pregnancy?

The estimated due date (EDD) = first day of your last menstrual period + 280 days (40 weeks), using Naegele's Rule. This assumes a 28-day cycle. For longer or shorter cycles, the calculator adjusts the date. First trimester ultrasound is the most accurate confirmation method.

How accurate is the calculated due date?

The calculated date has accuracy of ±1–2 weeks from LMP alone. First trimester ultrasound improves this to ±5–7 days. Only 4–5% of babies are born on the exact EDD — the realistic delivery window is 38–42 weeks.

What if I don't know my last period date?

Your doctor will use the first trimester ultrasound CRL (Crown-Rump Length) measurement to establish gestational age and calculate the EDD. This is equally valid and often more accurate than LMP-based calculation for women with irregular cycles.

Can I change my due date?

Your OB may revise the EDD based on first trimester ultrasound if it differs by more than 7 days from LMP calculation. This is standard practice. The date may only be officially changed once — further ultrasounds don't change the established EDD.

What week of pregnancy am I in?

Count from the first day of your last menstrual period. If your LMP was 10 weeks and 3 days ago, you are in week 10, day 3 of pregnancy. Our calculator shows current gestational week, day, and trimester automatically.

Your Due Date Is a Midpoint, Not a Deadline

Understanding your due date as a statistical estimate — not a fixed endpoint — reduces anxiety, helps you plan realistically, and prevents the common mistake of medically unnecessary interventions based on arbitrary calendar milestones.

Track your milestones, attend your scans, and know that a healthy full-term pregnancy can deliver anywhere from 38 to 42 weeks and be completely normal.

Use our free Pregnancy Due Date Calculator to find your EDD, current gestational week, key milestone dates, and trimester timeline — from LMP, conception date, or first ultrasound.

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