Pregnancy Due Date Calculator
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
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