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.
📐 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.
Sources & Methodology
Calculations are based on the most current publicly available data from authoritative government and industry sources:
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 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.
How is a Pregnancy Due Date Calculated?
The estimated due date (EDD) is most commonly calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the due date adjusts accordingly — our calculator makes this correction automatically.
Why 40 Weeks From LMP if Conception Happens at Week 2?
This is one of the most common points of confusion. Pregnancy is counted from the LMP, not from conception — because the LMP is a known date while conception is estimated. By the time a missed period is noticed (week 4–5 of pregnancy), conception actually occurred 2 weeks earlier. So gestational age is always about 2 weeks more than fetal age.
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