Pregnancy Due Date Calculator
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Calculate your estimated due date from your last period or conception date. Shows weeks pregnant, trimester, and key milestone dates.
Quick answer: Estimated due date (EDD) = last menstrual period (LMP) + 280 days, per Naegele's Rule (assumes a 28-day cycle). Only about 4–5% of babies arrive on their exact due date — most are born within two weeks before or after.
📐 Formula
EDD = LMP + 280 days (Naegele's Rule, 28-day cycle). Adjusted: EDD = LMP + 280 + (cycle − 28) days
How to Use the Due Date Calculator
Enter the first day of your last period
Input the date when your most recent menstrual period began — not when it ended. This is the standard starting point for Naegele's Rule.
Enter your average cycle length
The default is 28 days. If your cycle is consistently longer or shorter, adjust this number. A 35-day cycle shifts the due date 7 days later; a 21-day cycle moves it 7 days earlier.
Review your due date and trimesters
The calculator shows your estimated due date, the current gestational week if you are already pregnant, and the start dates of each trimester.
Confirm with your healthcare provider
An ultrasound before 12 weeks is the most accurate dating method and will be used by your provider to confirm or adjust the estimated due date.
How the Due Date Formula Works
The standard method for calculating estimated due date (EDD) is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For cycles other than 28 days, the due date adjusts by the difference: a 32-day cycle adds 4 days to the standard EDD; a 25-day cycle subtracts 3 days.
The 40-week figure counts from the LMP — meaning you are already considered 2 weeks pregnant at the time of conception. Actual gestational age from fertilisation is approximately 38 weeks (266 days), but obstetric convention uses LMP for consistency and because the exact date of conception is rarely known.
First Trimester (Weeks 1–12)
The first trimester covers conception through week 12. This period sees the most rapid developmental change: the neural tube forms by week 6, the heart begins beating around week 6–7, and all major organs are established by week 10. The risk of miscarriage is highest in the first trimester (approximately 10–15% of confirmed pregnancies), declining sharply after the first ultrasound confirms a heartbeat. Most healthcare providers schedule the first prenatal appointment at 8–10 weeks.
Dating Accuracy: Calculator vs Ultrasound
The due date calculator provides a reliable estimate when LMP and cycle length are known accurately, but ultrasound dating is more precise — particularly before 12 weeks when the embryo's crown-to-rump length measurement has a margin of error of only ±5 days. If there is a discrepancy of more than 7 days between calculator and ultrasound dates, the ultrasound date is typically used. After 20 weeks, ultrasound dating becomes less reliable due to normal variation in fetal size, making early dating essential for accurate gestational age throughout pregnancy.
Pregnancy Trimester Breakdown
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First Trimester | Weeks 1–13 | Organ formation, morning sickness, first ultrasound |
| Second Trimester | Weeks 14–26 | Movement felt, anatomy scan (20 weeks), energy returns |
| Third Trimester | Weeks 27–40 | Rapid growth, nesting, preparation for birth |
How Accurate is the Due Date?
Only about 4–5% of babies are born on their exact due date. Most arrive within two weeks before or after. A first-trimester ultrasound (before 12 weeks) is the most accurate dating method — it's accurate to within 5–7 days. Second and third trimester ultrasounds are less accurate for dating (±2–3 weeks).
A baby born between 37–42 weeks is considered full term. Before 37 weeks is preterm; after 42 weeks is post-term.
Factors That Can Affect Your Due Date
- Cycle length: Longer cycles (35+ days) push the due date forward; shorter cycles move it earlier
- Irregular periods: LMP-based calculation is less reliable; ultrasound dating preferred
- IVF: Exact conception date is known — add 266 days for precise EDD
- Previous pregnancies: Don't affect due date calculation but may affect typical delivery timing
How the Due Date Is Calculated by Hand: Worked Example (Naegele's Rule)
Take a last menstrual period (LMP) date of April 13, 2026. Naegele's rule adds 280 days (40 weeks) to the LMP date: April 13, 2026 + 280 days = January 18, 2027.
The 280-day count starts from the LMP, not from conception — which is why the "40 weeks of pregnancy" figure is slightly misleading. Ovulation and conception typically occur around day 14 of the cycle, so true fetal development at the calculated due date is closer to 38 weeks from conception, even though the pregnancy is conventionally described in 40 LMP-based weeks throughout.
How is gestational age at any point calculated from this date?
Gestational age is simply the number of days elapsed since the LMP, expressed in weeks and days. Checking on July 6, 2026 against the April 13 LMP: the elapsed period is 84 days, or 12 weeks, 0 days — placing the pregnancy at the very end of the first trimester on that date.
How Reliable Is a Calculated Due Date, and What Can Shift It?
Why do only about 5% of babies arrive on the exact calculated due date?
Naegele's rule assumes a standard 28-day cycle with ovulation on day 14 — an average, not a universal constant. Full-term delivery is normally defined as any point from 37 to 42 weeks, a five-week window, which is why the "due date" is better understood as the statistical center of a wide normal range than as a specific predicted delivery day.
How does cycle length change the calculation's accuracy?
A person with a consistently 35-day cycle (rather than the assumed 28 days) ovulates later in their cycle, meaning the standard LMP-based formula overestimates gestational age and calculates a due date earlier than the pregnancy's true timeline — often by roughly a week for a cycle that long. This is one of the most common reasons an early ultrasound date differs from the LMP-calculated date, and clinically, ultrasound dating in the first trimester is generally considered more reliable than LMP-based calculation alone when the two disagree by more than about 5–7 days.
Why does first-trimester ultrasound dating override the LMP calculation when they conflict?
Fetal growth is remarkably consistent in the first trimester regardless of cycle length or ovulation timing, making crown-rump length measurement a more precise dating method than a calculation that assumes a "textbook" 28-day cycle. Clinical guidelines commonly favor an early ultrasound date over LMP-based dating specifically because it sidesteps the cycle-length assumption that Naegele's rule depends on.
Frequently Asked Questions
Add 280 days (40 weeks) to the first day of your last menstrual period. This is Naegele's Rule, the standard method used by most healthcare providers. It assumes a 28-day cycle.
If your cycle is longer (e.g. 35 days), your due date is pushed forward by 7 days. If shorter (e.g. 21 days), it moves earlier by 7 days. This calculator adjusts automatically.
Only about 4–5% of babies are born on their exact due date. Most arrive within 2 weeks either side. An ultrasound in the first trimester (before 12 weeks) gives the most accurate dating.
Yes. If an early ultrasound (before 12 weeks) differs from the LMP-calculated date by more than about 7 days, most providers will use the ultrasound date instead, since crown-rump length measurement is more precise than LMP-based calculation in early pregnancy.
A baby born between 37 and 42 weeks is considered full term. Birth before 37 weeks is preterm; birth after 42 weeks is post-term.
Sources & Methodology
Calculations are based on the most current publicly available data from authoritative government and industry sources: