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Pick LMP, known conception, IVF transfer, or ultrasound dating from your strongest record.
Enter the calendar date from the selected record, e.g. 2026-01-10.
Enter 20-40 days; 28 keeps the standard Naegele baseline.
days
Slide 20-40 days to test how cycle timing moves the estimated due date.
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Select day 3, day 5, or day 6 to match the transfer note.
Enter report age as completed weeks plus 0-6 extra days, e.g. 8 weeks 3 days.
weeks days
Slide completed scan weeks, then fine-tune the extra days from 0 to 6.
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Enter 266-294 days; 280 is the standard 40-week baseline.
days
Slide 38-42 weeks when matching a provider-set planning length.
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Enter -14 to +14 days; use 0 when not matching an assigned EDD.
days
Slide -14 to +14 days to mirror an official adjustment without changing the source record.
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Choose first-pregnancy or follow-up schedule for the exported visit plan.
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Checkpoint Gestation Date Status Planning note Copy
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Window Gestation Date span Status Planning focus Copy
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Tags: Date Time, Pregnancy
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A due date is less like an appointment with labor and more like the calendar point used to organize a pregnancy timeline. It anchors gestational age, scan windows, screening ranges, maternity leave planning, support coverage, and late-pregnancy follow-up. Because most pregnancy care is scheduled by weeks and days of gestation, a small dating change can move several later milestones.

Obstetric dating counts from the first day of the last menstrual period, or from an equivalent day reconstructed from another record. That convention means a pregnancy measured as 40 weeks includes roughly two weeks before fertilization for a typical 28-day cycle. The convention can feel odd at first, but it lets menstrual dates, conception dates, IVF transfer records, and ultrasound reports land on one shared week scale.

Different records answer slightly different questions. A menstrual date tells where the cycle started. A conception or ovulation record narrows the fertilization window. IVF notes can be more exact because the transfer date and embryo age are documented. Ultrasound dating estimates gestational age from fetal measurements, and earlier dating scans are generally more dependable for setting or confirming the estimated due date than later scans.

Dating record What it anchors Common caution
Last menstrual period Cycle day 1 on the obstetric calendar Irregular cycles or uncertain bleeding dates can shift ovulation away from the 28-day assumption.
Known conception Approximate fertilization timing Ovulation, fertilization, and implantation are close together but not the same event.
IVF transfer Transfer date plus embryo age Day 3, day 5, and day 6 transfers need different back-calculations.
Ultrasound dating Gestational age measured on the scan date Later scans are useful clinically, but they are less precise for resetting the due date.
Pregnancy timeline from dating anchor to term thresholds A horizontal gestation timeline showing the LMP-equivalent anchor, estimated due date, early term, full term, and post-term threshold. LMP-equivalent anchor gestational day 0 EDD 40w planning point 37w 39w 42w Early term Full term Post-term One due date sits inside a wider term window

The estimated due date is best read as the center of planning, not as a guarantee. Term pregnancy is commonly discussed in bands: early term begins at 37 weeks, full term at 39 weeks, late term at 41 weeks, and post-term at 42 weeks. Those labels help explain why a due date, a full-term date, and a post-dates follow-up date are related but not interchangeable.

Calendar dating cannot show fetal growth, placental function, labor readiness, or whether a pregnancy needs extra monitoring. When a maternity clinician assigns an official estimated due date, that date should lead care decisions, especially if it differs from a personal calculation.

How to Use This Tool:

Start with the strongest dated record you have, then use the outputs as a planning aid alongside your maternity notes.

  1. Choose Dating method: Last menstrual period (LMP), Known conception date, IVF embryo transfer date, or Ultrasound dating.
  2. Enter the requested Reference date. The date must be a real past or current record, because future dates cannot anchor an existing pregnancy timeline.
  3. For LMP dating, set Cycle length. A 28-day cycle keeps the standard baseline; shorter or longer cycles move the estimate by the difference from 28 days.
  4. For IVF, match the transfer note with a day 3, day 5, or day 6 embryo. For ultrasound dating, enter the scan report age as completed weeks plus 0 to 6 extra days.
  5. Use Advanced only when you need a what-if or provider-matching adjustment. Standard gestation spans 266 to 294 days, EDD adjustment spans -14 to +14 days, and Visit cadence changes only the average-risk checkup schedule.
  6. Read Dating Snapshot first, then use Pregnancy Calendar, Checkup Pace, Pregnancy Week Position, and Delivery Runway Map for milestone dates, upcoming windows, chart views, and exports.

Interpreting Results:

Estimated due date is the headline result. LMP-equivalent anchor shows the day 0 date used to build every later row, which is useful when comparing LMP, conception, IVF, and scan-based estimates. Dating confidence describes how dependable the date source is for calendar dating; it does not describe pregnancy health.

Current gestational age, Current stage, and term badges update relative to today. If the anchor moves, the trimester label, full-term date, care windows, and post-term threshold move with it. Treat warnings as prompts to check the source date, cycle assumptions, scan age, or advanced adjustments before using the calendar for planning.

Pregnancy Calendar lists fixed milestones such as the dating scan window, anatomy scan window, full-term start, late-term start, and post-term threshold. Checkup Pace is an average-risk schedule guide, with different row sets for a first pregnancy and a follow-up pregnancy. Local practice, symptoms, complications, twins or multiples, and clinician preference can change appointment timing.

Pregnancy Week Position and Delivery Runway Map are visual summaries. They help show where today sits against the EDD and which checkpoints are closest, but they should not be used to decide when to seek care. Bleeding, severe pain, reduced fetal movements, fluid leakage, severe headache, high blood pressure concerns, or clinician instructions should override any calendar estimate.

Technical Details:

Gestational age is a day count from an LMP-equivalent anchor. LMP dating uses the menstrual start date directly. Conception dating subtracts 14 days so the result lands on the obstetric week scale. IVF dating subtracts 14 days plus the embryo age on transfer. Ultrasound dating subtracts the reported scan gestational age from the scan date.

Once the anchor is known, the estimated due date is calculated by adding a gestation length. The default length is 280 days, which corresponds to 40 weeks from the LMP-equivalent anchor. LMP mode can add or subtract days for cycle length, and an explicit EDD adjustment can shift the final date without changing the original record.

Formula Core:

The same day arithmetic drives the EDD, current gestational age, term labels, milestone rows, and chart positions.

EDD = Anchor+(gestation days+cycle adjustment+EDD adjustment) Current gestational age = Today-Anchor LMP cycle adjustment = cycle length-28

For an LMP date of 10 January 2026, 280 gestation days, no EDD offset, and a 28-day cycle, the EDD is 17 October 2026. If only the cycle length changes to 32 days, the cycle adjustment is +4 days and the EDD moves to 21 October 2026.

Method Anchor rule Confidence logic
LMP LMP date becomes day 0; cycle length affects the final EDD only in this mode. Moderate by default, lower when cycle length is outside 21 to 35 days.
Known conception Conception date minus 14 days becomes the LMP-equivalent anchor. High, with a note that implantation timing can still vary slightly.
IVF transfer Transfer date minus 14 days minus embryo age becomes the anchor. Very high because the transfer record fixes the dating window tightly.
Ultrasound Scan date minus scan gestational age becomes the anchor. High through 13w 6d, moderate through 21w 6d, and lower after about 22 weeks.

Timing Boundaries:

Pregnancy weeks are stored as whole gestational days. Boundary labels switch when the day count reaches the listed threshold.

Boundary Gestational day Week label Effect
First trimester complete 97 13w 6d The next day starts the second-trimester label.
Third trimester begins 196 28w 0d The current stage moves into the third trimester.
Early term begins 259 37w 0d The term badge changes from preterm window to early term.
Full term begins 273 39w 0d The full-term window begins.
Late term begins 287 41w 0d Post-dates planning usually becomes more active.
Post-term threshold 294 42w 0d The strongest follow-up warning is shown.

Care Window Model:

The appointment rows are average-risk planning windows. They do not diagnose risk and do not replace a local care plan.

Profile Included rows Main difference
Average-risk first pregnancy Booking, dating scan, 16w, anatomy scan, 25w, 28w, 31w, 34w, 36w, 38w, 40w, and 41w windows. Adds extra first-pregnancy reviews at 25 and 31 weeks.
Average-risk follow-up pregnancy Booking, dating scan, 16w, anatomy scan, 28w, 34w, 36w, 38w, 40w, and 41w windows. Uses a leaner follow-up schedule after the anatomy scan.

Limitations and Privacy:

Pregnancy dating is informational. It should support planning conversations, not replace medical judgment.

  • Use the clinician-assigned EDD for care decisions when it differs from a personal estimate.
  • First-trimester dating usually gives the strongest ultrasound anchor; later ultrasound dates can reflect growth variation as well as gestational age.
  • Average-risk care windows do not account for twins or multiples, medical conditions, prior pregnancy history, local protocols, abnormal test results, or individualized monitoring.
  • The calculation runs in the browser, and entered pregnancy dates are not sent to a server for the date arithmetic. Downloaded files are created from the results shown on the page.
  • Seek care based on symptoms, fetal movement concerns, bleeding, pain, fluid leakage, high blood pressure warning signs, or your clinician's instructions rather than waiting for a calendar checkpoint.

Worked Examples:

LMP example: An LMP date of 10 January 2026 with a 28-day cycle gives an Estimated due date of 17 October 2026. The LMP-equivalent anchor remains 10 January 2026, and estimated conception appears around 24 January 2026.

Cycle-length example: Keeping the same LMP date but changing cycle length to 32 days adds four days to the EDD. The estimate moves from 17 October 2026 to 21 October 2026, and later milestones move by the same amount.

IVF example: A day-5 embryo transfer on 20 February 2026 subtracts 19 days from the transfer date. The LMP-equivalent anchor becomes 1 February 2026, and the default 280-day EDD becomes 8 November 2026.

Ultrasound example: A scan on 15 March 2026 reporting 8w 3d subtracts 59 days from the scan date. The reconstructed anchor becomes 15 January 2026, and the default EDD becomes 22 October 2026.

FAQ:

Why does pregnancy dating start before conception?

Gestational age uses an LMP-equivalent day 0. In a typical 28-day cycle, ovulation and conception are roughly two weeks after that date, so conception-based and IVF-based records are shifted back onto the same week scale.

Which dating method should I choose?

Use the strongest documented record for this pregnancy. IVF transfer and known conception records are tightly dated, first-trimester ultrasound is useful when menstrual dates are uncertain, and LMP works best when the period date and usual cycle length are reliable.

Why did cycle length change only the LMP result?

Cycle length is an ovulation-timing adjustment for LMP dating. Conception, IVF, and ultrasound methods already carry their own dating anchor, so the cycle-length field does not shift those modes.

What should I do if my clinician gives a different due date?

Follow the clinician-assigned EDD for care. You can use the EDD adjustment control to mirror that date for personal planning, but the official medical record should guide scans, tests, monitoring, and delivery discussions.

Does the checkup schedule apply to high-risk pregnancies?

No. Checkup Pace is an average-risk planning guide. High-risk pregnancies, multiples, complications, prior history, local policy, or new symptoms can require a different schedule.

Glossary:

LMP
Last menstrual period, usually counted from the first full day of bleeding.
EDD
Estimated due date, the calendar date reached by adding the selected gestation length to the pregnancy dating anchor.
Gestational age
Pregnancy age measured in completed weeks and extra days from the LMP-equivalent anchor.
LMP-equivalent anchor
The reconstructed day 0 used to align LMP, conception, IVF, and ultrasound records onto one obstetric timeline.
Dating confidence
A label describing the date source's strength for calendar estimation, not a measure of fetal health or pregnancy risk.
Post-term
The pregnancy stage beginning at 42w 0d.

References: