Pregnancy Due Date Calculator
On the left, choose the first day of your last period and your average cycle length; the right side updates your estimated due date, gestational age, trimester, time remaining, and progress instantly, with no calculate button.
Input
Choose the first day of your last period to instantly calculate the due date and gestational age.
Result
Choose the first day of your last period to instantly calculate the due date and gestational age.
- Gestational age
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- Trimester
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- Time remaining
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- Estimated conception
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The due date is an estimate based on the Naegele rule (LMP + 280 days) adjusted for your cycle length. Only about 4 to 5 percent of babies arrive on the exact due date, and an early first-trimester ultrasound (CRL) can date the pregnancy more precisely. This tool is for reference only and is not a medical diagnosis. Follow your prenatal provider for clinical decisions.
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How to estimate your due date and gestational age from your last period
This pregnancy due date calculator estimates your due date and current gestational age from just two inputs: the first day of your last menstrual period (LMP) and your average cycle length. Choose the date and cycle on the left, and the right side lays out your estimated due date (EDD), gestational age in weeks and days, trimester, time remaining, estimated conception date, and a 0 to 40 week progress meter, with no calculate button.
The math is built on the Naegele rule used in obstetrics: add 280 days (40 weeks) to the first day of your last period, then adjust for how far your average cycle differs from 28 days. Gestational age is counted from the last period, not from conception, so it runs about two weeks ahead of the actual ovulation date. Keeping that in mind prevents a common misread of the weekly count.
A due date marks the center of a range, not a guaranteed delivery day. Only about 4 to 5 percent of babies arrive on the exact date, and 37 to 42 weeks is a normal full-term window. If an early ultrasound has re-dated the pregnancy, that estimate can be more accurate, so treat this tool as a quick reference around your prenatal visits.
Due date, gestational age, and trimester are different
The result numbers each mean something different. Keep these apart.
- Estimated due date: the day delivery is expected (center of a range).
- Gestational age: how many weeks and days along you are today.
- Trimester: first, second, or third stage of pregnancy.
- Time remaining: how many days are left until the due date.
The three steps behind the due date
The calculator estimates the due date in this order.
- Step 1: treat the first day of your last period as day 0.
- Step 2: add 280 days (40 weeks) — the Naegele rule.
- Step 3: if your cycle is not 28 days, add (cycle minus 28) days.
Why average cycle length matters
The Naegele rule assumes a 28-day cycle. A different cycle shifts ovulation and the due date.
- A longer cycle (say 35 days) means later ovulation, so the date moves later (about +7 days).
- A shorter cycle (say 21 days) moves the date earlier (about -7 days).
- If you do not know your cycle, leaving it at 28 days is fine.
Reading your gestational age
Gestational age is menstrual age, counted from your last period.
- It is read as completed weeks plus days, for example 12 weeks 3 days.
- At 4 weeks you are usually only just past a missed period.
- Actual conception happens about two weeks after week 0.
How trimesters are grouped
Trimesters are split by completed weeks; visits and body changes differ by stage.
- First trimester: roughly weeks 0 to 13.
- Second trimester: roughly weeks 14 to 27.
- Third trimester: roughly week 28 to birth.
What the conception estimate means
The conception date is a separate reference from gestational age.
- It is estimated as last period plus (cycle minus 14) days.
- If you know the exact conception date, conception plus 266 days is closer.
- Ovulation tests or other signs, if you have them, take priority.
Reading time remaining and progress
The meter and time-remaining show where you are in the pregnancy.
- The progress meter maps 0 to 40 weeks (280 days) onto 100 percent.
- Time remaining shows days to the due date as weeks, days, and a D-day.
- Past the due date it switches to a D-plus count of days overdue.
Why an ultrasound date can differ
A clinic date and this estimate often differ, which is normal.
- Early ultrasound dates the pregnancy from fetal size (CRL).
- Late ovulation or irregular cycles throw off period-based dating.
- A re-dated ultrasound estimate is usually the more accurate one.
What this calculator does not do
Knowing the limits keeps you from over-reading the result.
- It does not predict or confirm the real delivery day.
- It does not diagnose fetal health, risks, or test results.
- It does not replace medical advice (reference calculation only).
Frequently asked questions
How is the due date calculated from my last period?
It uses the Naegele rule, the most common method. Your estimated due date (EDD) is the first day of your last menstrual period (LMP) plus 280 days (40 weeks). This tool also adjusts for your average cycle length, so a cycle longer than 28 days pushes the date later and a shorter cycle pulls it earlier (by cycle minus 28 days).
Will my baby actually arrive on the due date?
The due date is the middle of an estimated range, not a fixed day. Only about 4 to 5 percent of babies are born on the exact date, and delivery any time from 37 to 42 weeks is considered full term.
What if my cycle is not 28 days?
Change the average cycle length to your real value. The Naegele rule assumes a 28-day cycle, so a 35-day cycle moves the due date about 7 days later and a 21-day cycle about 7 days earlier. This accounts for ovulation happening later or earlier in longer or shorter cycles.
Why is gestational age counted from the last period?
Obstetric gestational age is measured from the first day of your last period, not from conception. Ovulation and fertilization usually happen about two weeks later, so at 4 weeks of pregnancy you are typically only just past a missed period. This calculator uses the same convention.
What if I know my conception or ovulation date?
If you know the conception date, conception plus 266 days is closer to the due date. This tool works from the last period but also shows an estimated conception date (about LMP plus cycle minus 14 days). If you tracked ovulation directly, use that.
My ultrasound due date is different. Which one should I trust?
An early ultrasound (roughly 8 to 13 weeks) dates the pregnancy from the fetal crown-rump length (CRL) and is often more accurate than a period-based estimate, especially with irregular cycles. If your provider re-dated your pregnancy by ultrasound, follow that date.
Can I use this result for medical decisions?
No. The due date, gestational age, trimester, and conception estimate are reference values from a public formula, not a diagnosis. Confirm your prenatal schedule, risk factors, and delivery plan with your obstetric provider.
Basis: Naegele rule (last period + 280 days) with an average-cycle adjustment. Gestational age is menstrual age (from LMP). Reference date 2026-07-16. Reference estimate, not a medical diagnosis.