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Pregnancy Calculator

Calculate your due date, current pregnancy week, and trimester based on your last menstrual period or conception date.

Due Date Calculation

Naegele's Rule

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Pregnancy Week

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days
Typical range: 21-35 days

Understanding Pregnancy Dating

Pregnancy is typically calculated from the first day of your last menstrual period (LMP), not from conception. This means you're considered 'two weeks pregnant' at the time of conception, since ovulation typically occurs around day 14 of a 28-day cycle.

A full-term pregnancy is 40 weeks (280 days) from LMP, or 38 weeks from conception. This calculator uses Naegele's Rule, the standard method used by healthcare providers.

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40 Week Journey

Full-term pregnancy spans 40 weeks from LMP, divided into three trimesters.

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LMP Dating

Medical professionals date pregnancy from LMP, adding ~2 weeks before conception.

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5% Accuracy

Only about 5% of babies arrive on their exact due date. Most arrive within 2 weeks.

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Ultrasound Dating

Early ultrasounds can adjust due dates if measurements differ significantly.

The Three Trimesters

Pregnancy is divided into three trimesters, each with distinct developmental milestones:

1️⃣

First Trimester (Weeks 1-12)

Major organ development occurs. You may experience morning sickness, fatigue, and breast tenderness. By week 12, the baby is about 2 inches long and has all major organs forming.

2️⃣

Second Trimester (Weeks 13-27)

Often called the 'honeymoon period'—nausea typically subsides. You'll feel baby movements (quickening) around weeks 16-20. Baby grows to about 14 inches and develops fingerprints.

3️⃣

Third Trimester (Weeks 28-40)

Rapid weight gain for baby. You may experience Braxton Hicks contractions, back pain, and difficulty sleeping. Baby's lungs mature and they gain about half a pound per week.

Week-by-Week Development

Key developmental milestones throughout pregnancy:

WeekBaby SizeKey DevelopmentWhat You May Feel
4 Poppy seed Implantation occurs Missed period, fatigue
8 Raspberry Heart beating, limbs forming Morning sickness peaks
12 Lime Reflexes develop, gender determined Nausea may decrease
16 Avocado Can hear sounds, eyes move Energy returns, may feel movement
20 Banana Halfway point, vernix forms Definite movements felt
24 Corn Lungs developing, responds to sounds Visible kicks, Braxton Hicks may start
28 Eggplant Eyes can open, brain developing rapidly Shortness of breath, heartburn
32 Squash Bones hardening, practicing breathing Frequent urination, trouble sleeping
36 Honeydew Baby 'drops' into pelvis Easier breathing, pelvic pressure
40 Watermelon Full term, ready for birth Contractions, nesting instinct

Due Date Accuracy

Due dates are estimates, not guarantees. Understanding the range helps set realistic expectations:

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Statistical Reality

Only about 4-5% of babies arrive on their exact due date. About 80% are born within 2 weeks (before or after) of the due date.

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Term Definitions

Early term: 37-38 weeks. Full term: 39-40 weeks. Late term: 41 weeks. Post-term: 42+ weeks. These distinctions matter for infant health outcomes.

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First-Time Mothers

First pregnancies tend to go slightly longer than subsequent ones. First-time mothers are more likely to go past their due date.

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Cycle Length Matters

Standard calculations assume a 28-day cycle. If your cycle is longer, your due date may be later. If shorter, it may be earlier.

Important Prenatal Milestones

Key appointments and tests during pregnancy:

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First Prenatal Visit (Weeks 6-8)

Confirm pregnancy, establish due date, initial blood work, discuss medical history. May include early ultrasound to confirm viability.

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First Trimester Screening (Weeks 11-14)

NT scan (nuchal translucency) and blood tests to screen for chromosomal conditions. Optional but recommended.

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Anatomy Scan (Weeks 18-22)

Detailed ultrasound checking all major organs, spine, heart. Can often determine baby's sex if desired.

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Glucose Test (Weeks 24-28)

Screening for gestational diabetes. You'll drink a glucose solution and have blood drawn to check sugar processing.

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Group B Strep Test (Weeks 36-37)

Swab test for GBS bacteria. If positive, antibiotics during labor prevent transmission to baby.

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Weekly Visits (Weeks 36-40)

More frequent monitoring as due date approaches. Checks for cervical changes, baby position, and signs of labor.

When to Call Your Provider

Contact your healthcare provider immediately if you experience:

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Bleeding

Any vaginal bleeding, especially bright red or accompanied by cramping. Light spotting can be normal early on, but always report it.

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Fluid Leaking

Gush or steady leak of fluid—could indicate ruptured membranes (water breaking) even before labor starts.

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Severe Pain

Intense abdominal pain, especially if one-sided or constant. Could indicate ectopic pregnancy or placental issues.

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High Fever

Temperature over 101°F (38.3°C) could indicate infection that may need treatment.

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Decreased Movement

After week 28, if you notice significantly less baby movement than usual. Try drinking cold water and lying down first.

How to use this pregnancy calculator

  1. Pick a calculation method using the three-button toggle: Last Period (LMP), Conception Date, or Known Due Date. LMP is the standard medical approach when you know the first day of your last menstrual period.
  2. Enter the date in the matching field. For LMP mode, use a calendar or period-tracking app to find the first day of bleeding — not the last day. For Conception Date mode, use the date of ovulation or a confirmed conception event.
  3. If you're in LMP mode, set Average Cycle Length. The default 28 days fits most users, but enter 21–45 days if your cycles are reliably shorter or longer — the calculator shifts ovulation and your due date accordingly.
  4. Click Calculate Due Date to see your estimated due date, current gestational week and day, trimester, days pregnant, days remaining, and the dates your first and second trimesters end.

Examples

Basic: LMP with a 28-day cycle

A user whose last period started on January 1 and who has a textbook 28-day cycle.

ResultEstimated due date around October 8, 2026 (LMP + 280 days). Estimated conception around January 15, 2026. First trimester ends around March 26, 2026; second trimester ends around July 9, 2026.

Naegele's rule adds 280 days to the LMP: January 1 + 280 days = October 8. Ovulation is assumed at day 14, so conception lands two weeks after the LMP. The calculator backs out the trimester boundaries from the same LMP — week 13 starts at LMP + 84 days and week 28 starts at LMP + 189 days — which is why your gestational age is always two weeks ahead of your 'time since conception.'

Intermediate: longer 32-day cycle

A user with a regular 32-day cycle whose LMP was March 10, 2026.

ResultEstimated due date around December 18, 2026 — four days later than a standard 28-day cycle would predict.

Standard Naegele's rule assumes a 28-day cycle with ovulation on day 14. With a 32-day cycle, ovulation shifts about four days later (day 18), so conception and the due date shift by the same amount. The calculator applies this cycle adjustment automatically: 280 + (32 − 28) = 284 days from LMP. If your cycle is irregular, an early dating ultrasound is more reliable than cycle math.

Edge case: dating from a known conception date

A user who conceived via IVF and knows the exact transfer-derived conception date of August 5, 2026.

ResultEstimated due date around April 28, 2027 (conception + 266 days). LMP is back-calculated to roughly July 22, 2026.

When you know the conception date, the calculator uses the 266-day fetal-age count instead of the 280-day gestational-age count from LMP. It back-dates the LMP by 14 days so that the rest of the timeline — trimester boundaries, current week, days remaining — still reports gestational age the way clinicians do. This conception-date mode is most useful for IVF or for cases with a single, well-documented intercourse date.

How it works

The calculator uses Naegele's rule, the dating method introduced by 19th-century German obstetrician Franz Karl Naegele and still recommended by ACOG and the WHO: estimated due date (EDD) = first day of LMP + 280 days, equivalent to 40 weeks. Because day 1 of pregnancy is counted from LMP rather than conception, you are already 'two weeks pregnant' at the moment of ovulation.

If you enter LMP, the tool adds 280 days for a standard 28-day cycle. If your cycle is longer or shorter, it shifts the due date by (cycle length − 28) days because ovulation — and therefore conception — moves with cycle length. If you enter a conception date, it adds 266 days (40 weeks − 2 weeks). If you enter a known due date, it works backward to recover the implied LMP and conception date.

From the LMP, the calculator derives gestational age in completed weeks and days. Trimester boundaries follow the common clinical split: first trimester through week 13, second trimester weeks 14–27, third trimester weeks 28 onward. It also reports when the first and second trimesters end and when 'full term' (39 weeks) begins so you can plan appointments.

Dating accuracy is a screening estimate, not a guarantee. First-trimester ultrasound crown-rump length measurements are accurate to within 5–7 days and are generally preferred over LMP dating when they disagree by more than that. Your prenatal provider will confirm or adjust the EDD; this tool is for orientation between visits.

When to use this calculator

  • Confirming a suspected pregnancy. After a positive home pregnancy test, enter your LMP to see how many weeks along you are and roughly when to schedule your first prenatal visit (typically 6–8 weeks).
  • Planning prenatal appointments. Use the trimester-end dates and current week to anticipate windows for the NT scan (11–14 weeks), anatomy scan (18–22 weeks), glucose tolerance test (24–28 weeks), and GBS swab (36–37 weeks).
  • Tracking week-by-week development. Recalculate each week to see your gestational age advance and align it with the week-by-week development table for fetal milestones and common symptoms.
  • Adjusting for an irregular cycle. If your cycle is reliably shorter or longer than 28 days, enter the actual cycle length so the due date reflects when you likely ovulated, not the textbook average.
  • Working from a known conception date. Use the Conception Date mode for IVF pregnancies or cases with a documented ovulation/intercourse date — it skips the 'two-week shift' from LMP.
  • Reverse-checking a clinician-given due date. Enter the due date your provider gave you into the Known Due Date mode to derive the implied LMP and current week, useful when LMP records were lost or imprecise.

Common mistakes

  • MistakeEntering the last day of your period instead of the first day.
    FixLMP refers to the first day of bleeding of your most recent menstrual period — not the last day, not when bleeding became heavy. Using the wrong day shifts your due date by several days.
  • MistakeTreating the estimated due date as the day labor will start.
    FixOnly about 4–5% of babies arrive on their EDD. Most spontaneous labors occur between 37 and 42 weeks. Plan with a range, not a single date.
  • MistakeIgnoring cycle length when cycles are reliably 32 or 35 days.
    FixStandard Naegele's rule assumes a 28-day cycle. Enter your actual average cycle length so the calculator shifts ovulation and the EDD accordingly — a four-day cycle difference moves the due date by four days.
  • MistakeMixing up gestational age and fetal age.
    FixGestational age (counted from LMP) is what clinicians use and is what this calculator reports as 'current week.' Fetal age (counted from conception) is about two weeks less. When people ask 'how far along are you?' they almost always mean gestational age.
  • MistakeOverruling an early ultrasound with LMP math.
    FixA first-trimester crown-rump-length ultrasound is more accurate than LMP for dating. If your scan disagrees with this calculator by more than ~7 days, the ultrasound EDD is the one your provider will use.
  • MistakeUsing this calculator instead of consulting a prenatal care provider.
    FixThis tool gives an estimate. Schedule a prenatal visit as soon as you suspect pregnancy. Your provider's confirmation, dating ultrasound, and ongoing care — not a web calculator — should guide any decision about your pregnancy.

Frequently asked questions

When is my due date?

Your estimated due date is 40 weeks (280 days) from the first day of your last menstrual period if your cycle is around 28 days. For example, an LMP of January 1 gives an EDD of about October 8. The calculator above runs this math automatically and adjusts if you enter a different cycle length or a conception date.

Are LMP and conception date different?

Yes. LMP is the first day of your last menstrual period — typically about two weeks before ovulation in a 28-day cycle. Conception happens around ovulation. Pregnancy dating uses LMP because it's a clearly remembered date for most people, which is why the count starts two weeks before fertilization actually occurred.

What is gestational age versus fetal age?

Gestational age is the count from your LMP and is what doctors, ultrasound reports, and this calculator report as 'current week.' Fetal age (also called embryonic or post-conception age) is counted from conception and is about two weeks less. So at 12 weeks gestational, the fetus is about 10 weeks post-conception.

When does the second trimester start?

The most common clinical convention is that the second trimester begins at week 14 (after completing week 13) and ends at the close of week 27. The third trimester begins at week 28 and runs through delivery. Some sources mark the boundary at week 13 or use slightly different cutoffs — this calculator uses the week 14 / week 28 convention.

How accurate is the due date?

Only about 4–5% of babies are born on their exact EDD. Most healthy pregnancies deliver between 37 and 42 weeks. Dating by LMP is accurate to within 1–2 weeks for someone with regular cycles; a first-trimester ultrasound (before 14 weeks) tightens that to within 5–7 days and is preferred when the two disagree.

Why is pregnancy counted from LMP and not conception?

LMP is a known, observable event for most people, while the exact day of conception usually is not. ACOG, the WHO, and most national health systems standardized on LMP-based dating so gestational age can be compared consistently between patients, clinics, and studies. The system 'wastes' the first two weeks by design — that's the trade-off for a date you can actually pinpoint.

What's the chance of going past my due date?

About one in three first pregnancies and one in six subsequent pregnancies continue past 40 weeks. Going one or two weeks past the EDD is common and usually safe. Most providers begin discussing induction options between 41 and 42 weeks because the risk of complications rises after 42 weeks (post-term).

When should I see a doctor or midwife?

Schedule your first prenatal appointment as soon as you have a positive pregnancy test — most providers see you between 6 and 10 weeks. Earlier visits may be recommended if you have a history of miscarriage, ectopic pregnancy, chronic illness, or if you are over 35. This calculator is not a substitute for clinical care — use it for orientation, then book a prenatal provider for definitive dating and ongoing care.

Can my due date change after the first ultrasound?

Yes, and that's normal. If a first-trimester crown-rump-length measurement differs from your LMP-based date by more than 5–7 days, your provider will usually re-date the pregnancy to match the ultrasound, which is more accurate this early. After about 14 weeks, ultrasound dating becomes less precise, so the EDD established in the first trimester is generally kept for the rest of the pregnancy.

Is this calculator different from the pregnancy due date calculator?

The sibling pregnancy-due-date calculator focuses specifically on producing the EDD. This pregnancy calculator gives the same EDD plus the broader timeline: current gestational week and day, trimester, days pregnant, days remaining, conception date, and trimester-end dates. Use whichever fits the question you're answering.

Sources

Methodology

This calculator implements Naegele's rule (EDD = LMP + 280 days) with three input modes. In LMP mode it adds 280 days plus a cycle-length adjustment of (cycle − 28) days so ovulation tracks your actual cycle. In Conception Date mode it adds 266 days (40 weeks minus the two-week LMP-to-ovulation offset). In Known Due Date mode it back-derives the LMP by subtracting 280 days. From the LMP it computes gestational age in completed weeks and days, classifies trimester (weeks 1–13 first, 14–27 second, 28+ third), and reports days pregnant, days remaining, estimated conception, and the dates that the first and second trimesters end (LMP + 84 days and LMP + 189 days). All output is a screening estimate; definitive dating uses first-trimester ultrasound and clinical confirmation, and any pregnancy-related decision should involve a qualified prenatal care provider.

Learn more about our methodology →

Pro Tips

  • Bookmark this calculator for quick access in the future
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  • Try different scenarios to compare outcomes
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