Pregnancy Due Date Calculator: Estimate Your EDD with Naegele’s Rule

An estimated due date (EDD) is the calendar target clinicians use to plan prenatal visits, screening windows, and leave. It is not a promise that a baby will arrive that day — only about 5% do. Still, a clear EDD and gestational age let you answer “how many weeks pregnant am I?” without doing the 280-day arithmetic by hand.

The Pregnancy Due Date Calculator runs entirely in your browser. Enter the first day of your last menstrual period (LMP), a conception date, or an ultrasound dating scan. The tool applies Naegele’s rule, adjusts for cycle length when needed, and returns the due date, estimated conception date, current trimester, weeks remaining, and a short list of milestone dates.

Nothing is uploaded. There is no account. You can copy the result set and take it to a prenatal appointment as a starting point — not as a substitute for clinical dating.

Why Use a Pregnancy Due Date Calculator?

• Same formula clinicians start from. Naegele’s rule sets the EDD at 280 days (40 weeks) from the first day of the LMP. That is the standard first estimate before an ultrasound revises it.
• Three dating methods. Use LMP when the last period is known, conception date after IVF or a well-timed cycle, or an ultrasound scan when the clinic already reported gestational age.
• Cycle-length adjustment. A 21–45 day cycle is not treated as 28 days. The calculator shifts the due date by (cycle length − 28) so a longer cycle does not silently push the EDD too early.
• Gestational age, not just a date. Results show weeks + days (for example 22w 3d), pregnancy progress as a percent of 280 days, and weeks remaining — or how far past the EDD you are.
• Trimester and milestone map. First trimester ends at 13w 0d, second at 27w 0d. The tool also marks viability (24 weeks) and early full term (37 weeks).
• Private by design. Dates stay on the device. That matters for health data you may not want in a third-party account.

Key Features

• Ultrasound weeks must be between 4 and 42; extra days on the scan are 0–6 so the input matches how a report is written (12w 3d).
• An LMP in the future is rejected. Gestational age is measured from LMP to today, so a future period date would invent a negative pregnancy.
• The progress bar caps at 100%. After 40 weeks the result switches to “past due by” instead of inventing a second trimester.

How to Use

1. Open the Pregnancy Due Date Calculator and pick a method: first day of last period, conception date, or ultrasound scan.
2. Enter the reference date. In LMP mode, set average cycle length if it is not 28 days. In ultrasound mode, enter the gestational weeks (and days) printed on the scan report.
3. Click Calculate. Check that the estimated due date, conception date, and gestational age look consistent with what you already know.
4. Read the trimester card and milestone list. Note the end of the current trimester and the 24-week and 37-week dates if you are planning travel or leave.
5. Copy the result block if you want a plain-text snapshot for a notes app or a question for your midwife or obstetrician.

Reset clears the form. Recalculate after a new scan — ultrasound dating often replaces the LMP estimate in the first trimester.

Understanding Naegele’s Rule and Gestational Age

Pregnancy dating uses gestational age, which starts on the first day of the LMP, not on the day of conception. A typical ovulation is about 14 days after LMP, so at “4 weeks pregnant” fertilization is only about two weeks old. That convention is why full term is 40 weeks and why Naegele’s rule is “LMP + 280 days,” not “conception + 266 days” — both describe the same 38 weeks of embryonic life plus the two weeks before ovulation.

LMP dating. Add 280 days to the first day of bleeding. If the cycle is longer than 28 days, ovulation likely happened later, so the calculator adds the extra days to the EDD. If the cycle is shorter, it subtracts. The supported range is 21–45 days — outside that, clinical advice matters more than a generic formula.

Conception dating. Useful after IVF, IUI, or a well-tracked ovulation kit. The tool sets LMP = conception − 14 days, then applies 280 days. That keeps gestational age on the same scale as an LMP-based chart even when you never recorded a period.

Ultrasound dating. A first-trimester crown–rump length is usually more accurate than memory of the LMP. Enter the scan date and the gestational age the sonographer reported. The calculator subtracts those weeks and days from the scan date to recover an implied LMP, then runs Naegele’s rule. Later scans have a wider error margin; many clinics lock the EDD after the dating scan and do not keep changing it.

Trimesters. This tool uses 13w 0d and 27w 0d as the trimester boundaries — a common educational split (weeks 1–13, 14–27, 28–40). Some posters use 12/28 or 13/26. The dates are planning aids, not legal definitions of trimester.

Why the exact day is rare. Length of the luteal phase varies. Implantation timing varies. Fetal growth is a range, not a point. The EDD is the middle of a window: term is often discussed as 37–42 weeks. Treat the calendar day as the center of that window.

Practical Use Cases

Planning prenatal appointments from an LMP

You know the first day of your last period and a roughly 30-day cycle. Enter that LMP, set cycle length to 30, and read the EDD and current week. Use the first-trimester end date to see whether the nuchal or dating scan still falls in the usual window. If your clinic already gave a different EDD, keep theirs — this page is for a first estimate when you do not have one yet.

IVF or known conception date

A transfer or positive ovulation test gives a conception (or transfer) date that is more reliable than a period. Switch to conception mode. Gestational age will still be reported in obstetric weeks so lab reference ranges and “week 12” leaflets stay comparable with everyone else’s chart.

Reconciling an ultrasound report

The report says 8w 2d on 3 March. Enter that scan date and 8 weeks 2 days. Compare the calculated EDD with the date printed on the report. A one- or two-day difference is usually rounding. A large gap means a mistyped week or a report that used a different convention — ask the clinic rather than averaging the two dates yourself.

Leave, travel, and “how many weeks am I?”

The weeks-remaining figure and the 37-week early-full-term milestone are useful for airline policies and parental-leave forms that ask for an EDD, not a trimester name. Copy the result so the same numbers appear in every form. Recalculate after the official dating scan if the clinic updates the EDD.

Best Practices

• Use the first day of bleeding, not the last. Naegele’s rule is anchored on day 1 of the LMP.
• Prefer a first-trimester ultrasound when it exists. After that, do not keep “correcting” the EDD with every later growth scan unless your clinician does.
• Set cycle length only when you know it. Guessing 35 days because one cycle was long will slide the due date by a week.
• Enter ultrasound age exactly as printed. 12w 4d is not the same as 12 weeks. The extra days change the implied LMP.
• Treat the result as an estimate. The on-page disclaimer is there for a reason: most births miss the exact EDD.
• Keep health decisions with a clinician. Dating, viability, and induction thresholds are clinical calls. This calculator does not diagnose or schedule care.

Ready for a first estimate? Open the Pregnancy Due Date Calculator, pick LMP, conception, or ultrasound, and generate a due date plus gestational age in the browser — no signup, no upload.

Related Tools You Might Like:

• Date Calculator — add or subtract days and compare two calendar dates
• Age Calculator — exact age in years, months, and days from a birth date
• Week Number Calculator — ISO week numbers and the week of any date

Happy dating — and remember the EDD is a window, not a deadline.