Free Pregnancy Due Date Calculator — Naegele's Rule + Cycle Adjustment

What This Calculator Does

This tool gives you an Estimated Due Date (EDD) — the day your pregnancy reaches the textbook 40-week mark — along with the supporting context most pregnancy timelines need: your current gestational age in weeks and days, which trimester you are in, the full-term window (weeks 37 through 42 of gestation), and an estimate of the conception date. You can compute the timeline from either of two starting points: the first day of your last menstrual period (LMP), which is the standard clinical anchor, or a known conception date, which is what you would use after an IVF transfer or when ovulation tracking pinpointed the day.

It is worth saying clearly up front: only about 4-5% of babies are actually born on their estimated due date. The EDD is a midpoint, not a deadline. The vast majority of pregnancies deliver somewhere in the three-week window between weeks 37 and 41, and that is the number that matters for planning — the due date is shorthand, the due window is reality.

Naegele’s Rule and the 280-Day Standard — The Math

The math behind every LMP-based due date calculator — including this one and the wheel your OB pulls out of a drawer — is Naegele’s rule, named for the 19th-century German obstetrician Franz Karl Naegele. The formula is disarmingly simple:

Two assumptions are baked into that 280-day number, and understanding them is the difference between trusting the result blindly and using it well. First, the formula assumes a 28-day menstrual cycle with ovulation on day 14 — meaning conception happens 14 days after the LMP, and pregnancy from conception lasts about 266 days (38 weeks). Add the 14 days back and you get 280 days from LMP. Second, the formula assumes the user knows their LMP date precisely — for many people, especially those with irregular cycles or recent hormonal contraception, that is a guess. Both assumptions matter, and both are why early-trimester ultrasound is a more reliable dating tool when the LMP is uncertain.

Cycle-Length Adjustment for Non-28-Day Cycles

Real cycles range from about 21 to 45 days, and the textbook 28-day cycle is a population average, not a personal guarantee. If your cycle is longer than 28 days, ovulation happens later in the cycle (the luteal phase after ovulation is roughly fixed at 14 days, so cycle-length variability shows up before ovulation). A later ovulation means a later conception, which means a later due date by the same number of days.

That is why this calculator includes a cycle-length input. Internally it adds cycleLength − 28 days to the standard 280 to produce a personalized due date. Two examples:

• 21-day cycle(short): adjustment is −7 days. EDD = LMP + 273 days. The baby’s estimated arrival is one week earlier than naïve Naegele.
• 35-day cycle (long): adjustment is +7 days. EDD = LMP + 287 days. The estimated arrival is one week later.

If you do not know your cycle length, leave it at 28 — that is the population default and what every clinician will use until an ultrasound refines the date. If your cycles vary widely month to month, treat any LMP-based estimate (here or in a clinic) as provisional until you get a first-trimester dating scan.

Gestational Age vs Fetal Age

These two numbers sound interchangeable but they are not, and confusion between them accounts for a surprising amount of pregnancy-week anxiety.

• Gestational ageis measured from the first day of the LMP. It is what clinicians use, what pregnancy apps display, and what this calculator reports as “X weeks Y days.” A “12-week pregnancy” means 12 weeks of gestational age.
• Fetal age (also called conceptional age or embryonic age) is measured from the actual day of conception — about 14 days later. So a fetus at 12 weeks gestational age is about 10 weeks fetal age.

The 2-week gap exists because medicine standardized on the LMP as the anchor point a century ago, before ultrasound. The LMP is observable; conception almost never is. Using LMP also means we count two weeks during which the patient was technically not yet pregnant — a quirk that confuses every first-time parent. This calculator follows the clinical convention: when we say “you are 8 weeks 3 days,” that is gestational age.

The Three Trimesters

The 40-week pregnancy is conventionally divided into three trimesters, each with its own clinical concerns and milestones. The boundaries are not crisp biological transitions — they are useful conventions for organizing prenatal care:

• First trimester (weeks 1-12). Embryonic and early fetal development. Highest risk window for miscarriage (most loss happens in the first 10 weeks). Common experience: nausea, fatigue, breast tenderness. Clinical milestones: confirmation of pregnancy, dating ultrasound (ideally 8-12 weeks for accuracy), first-trimester screening (NT scan, NIPT) around 11-13 weeks.
• Second trimester (weeks 13-26). Often the most comfortable stretch — nausea generally resolves, energy returns, the bump becomes visible. Clinical milestones: anatomy scan around 18-22 weeks (and the option to learn fetal sex), quickening (first felt movement) typically 18-20 weeks, glucose screening for gestational diabetes around 24-28 weeks.
• Third trimester (weeks 27 onward). Rapid fetal growth, increasing maternal physical strain. Clinical milestones: weekly visits from week 36, group B strep swab around 36-37 weeks, monitoring of fetal position and movement, eventual labor in the 37-42 week window. Babies born before 37 weeks are preterm; those born after 42 weeks are post-term and induction is typically discussed.

How to Use This Calculator

1. Pick the method. Default is LMP — use this if you know the first day of your most recent period. Switch to Conception only if you have a confidently known conception date (IVF transfer day, single-act conception, or ovulation-tracked timing).
2. Enter the reference date in YYYY-MM-DD format. For LMP this is the first day of bleeding, not the last day, not the middle.
3. (LMP method only.) Enter your average cycle length in days. If you are unsure or your cycles vary, leave it at 28 and treat the result as provisional.
4. Read the estimated due date, the current gestational age, which trimester you are in, and the full-term window. The full-term window — weeks 37 to 42 — is the realistic delivery range.
5. Treat the result as a planning anchor, not a contract with biology. Any ultrasound your clinician performs, especially one done in the first trimester, will almost always be more accurate than LMP-based math.

Three Worked Examples

Three scenarios with concrete dates — drop any of them into the calculator above to see the full output.

Example 1 — Standard 28-day cycle

A patient’s LMP is April 1, 2026. Cycle length is the textbook 28 days. Naegele’s rule with no cycle adjustment gives EDD = April 1 + 280 days = January 6, 2027. Estimated conception (LMP + 14 days) is approximately April 15, 2026. Full-term window: roughly December 16, 2026 through January 20, 2027 (37-42 weeks). At a clinic visit on June 1, 2026, gestational age would read 8 weeks 5 days — first trimester, with the dating scan and first-trimester screening up next.

Example 2 — Long cycle (35 days)

Same LMP — April 1, 2026 — but the patient has consistent 35-day cycles. The cycle adjustment is 35 − 28 = +7 days, so EDD = April 1 + 287 days = January 13, 2027. Estimated ovulation is around day 21of the cycle (April 22, 2026), not day 14. Without the cycle adjustment, this person’s due date would be miscalculated by a full week — and a week matters when discussions of induction or post-term monitoring start. This is why the cycle-length input is not cosmetic.

Example 3 — Conception date known (IVF)

After an IVF embryo transfer, the conception date is effectively known. Suppose the confirmed conception date is March 1, 2026. Switch the calculator to the Conception method and EDD = March 1 + 266 days = November 22, 2026. For paperwork or charts that still ask for an LMP, the equivalent is approximately conception minus 14 days = February 16, 2026. IVF dates are the gold standard — they almost always supersede ultrasound dating because the biology was directly observed in a lab rather than inferred from menstrual history.

Common Mistakes

• Using the last day of the last period instead of the first day. LMP means the first day of bleeding. Using the last day shifts the EDD several days late and throws off every gestational-age readout. If you menstruate for 5 days April 1 through April 5, the LMP for the formula is April 1.
• Forgetting the cycle-length adjustment.If your cycle is 32 or 35 days, the standard Naegele’s rule will under-count by 4 to 7 days. Many free pregnancy calculators silently assume 28 days and never expose the input — read the fine print or use a tool (like this one) that lets you enter your real cycle.
• Trusting the LMP when cycles are irregular. If your last few cycles ran 21, 38, 27, 41 days, no LMP-based estimate is reliable. In that case, the first-trimester ultrasound dating scan (typically 8-12 weeks) becomes the authoritative timeline and should override any calculator output, including this one.
• Reading the EDD as a hard delivery date. Only about 4-5% of babies are born on the EDD itself. About 80% deliver between 37 and 41 weeks, with the median at around 40 weeks 0 days for first-time parents and slightly earlier for subsequent pregnancies. The due date is a midpoint, not a verdict.
• Confusing gestational age with fetal age.A “10-week pregnancy” (gestational age) means an 8-week-old fetus (fetal age). Apps and clinicians use gestational age; pop-science articles sometimes drift between the two. When in doubt, assume gestational age.
• Using a conception-method calculation when conception is uncertain. Unless you had IVF, ovulation testing on the day, or a single intercourse exposure, “conception date” is itself a guess derived from cycle math. Defaulting to LMP mode is almost always more honest.

When This Calculator Decides For You

Math like this rarely sits in a vacuum — the timeline drives a series of practical decisions, and the more accurately you anchor the dates, the better those decisions go. Common ones:

1. When to schedule the dating scan. Ultrasound is most accurate for dating between 8 and 12 weeks. Knowing your gestational age tells you whether to book the scan this week or wait two more.
2. When first-trimester screening makes sense. NT scans and NIPT have a gestational-age window (roughly 10-13 weeks for NIPT, 11-13 weeks for NT). Miss the window, miss the test.
3. Maternity-leave planning. Most leave policies anchor on the EDD, not the actual delivery date. If your EDD is January 6, you cannot wait until January 5 to file paperwork.
4. Travel and exercise decisions. Most airlines restrict travel after 36 weeks; many clinicians advise caution after 28-32 weeks for long-haul flights. Knowing the week makes the rule readable.
5. Tracking fetal movement. Counting fetal kicks becomes meaningful from around 28 weeks. Before that the baby is too small and movement too sporadic for the count to flag concerns.

The Limits of LMP-Based Dating

Naegele’s rule is over 200 years old. It was a great approximation in 1812 — and it is still a fine starting estimate today — but modern obstetric practice does not consider an LMP-based EDD authoritative. Here is why.

First, the rule embeds the 28-day cycle assumption that we already discussed; many women do not have a 28-day cycle. Second, it assumes the user reliably remembers the first day of their last period, which surveys consistently show is wrong about 30-50% of the time (off by days or even weeks). Third, it ignores variability in implantation timing and early embryo development that ultrasound can directly visualize. For these reasons, professional guidance (ACOG, RCOG, and most national bodies) is to use a first-trimester crown-rump length (CRL) ultrasound to confirm or adjust the EDD when LMP-based dating differs by more than 7 days from the scan-based estimate. After 13 weeks, dating becomes less precise and the LMP-based EDD is harder to override.

The practical takeaway: use this calculator to plan, to understand the timeline, and to track gestational age between visits. Do not use it to override clinical dating, to decide when to induce, or to make any decision your obstetrician would make differently. A free web calculator is a map; the ultrasound, the bloodwork, and the patient in front of the clinician are the territory.

For broader context around pregnancy: pre-pregnancy and gestational weight tracking intersect with the BMI calculator, since pre-pregnancy BMI shapes recommended weight-gain ranges across the trimesters. Nutritional needs change too — the calorie / TDEE calculator can help baseline daily calories, on top of which most clinicians add roughly 340 kcal/day in the second trimester and 450 kcal/day in the third. If you are still trying to conceive and want to time it well, the ovulation calculator is the sister tool to this one. And once the baby is here, the age calculator takes over for tracking weeks, months, and milestones from birth onward.