Pregnancy Due Date: How It's Calculated
The due date is one of the first things expectant parents want to know — and one of the most misunderstood. Despite its precision (a specific calendar date), a due date is really an estimate with a margin of about 2 weeks in either direction. Only about 5% of babies arrive on their exact due date.
How Due Dates Are Calculated
Method 1: Last Menstrual Period (LMP)
The most common method uses Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the estimate is adjusted. A 35-day cycle would add 7 days to the standard calculation.
Method 2: Ultrasound Dating
First-trimester ultrasound (before 14 weeks) is generally considered the most accurate dating method, with a margin of error of about ±5–7 days. The ultrasound measures the crown-rump length (CRL) of the embryo. If the ultrasound date differs from the LMP date by more than 7 days, most providers will use the ultrasound date.
Method 3: Conception Date
If you know the exact date of conception (common with fertility treatments), add 266 days. This eliminates the guesswork about ovulation timing but is only applicable when conception is precisely known.
Calculate your due date
Use CalcViral's Pregnancy Due Date Calculator to estimate your due date, current week, trimester, and upcoming milestones.
The Three Trimesters
- First trimester (weeks 1–12): Major organ development. Highest risk of miscarriage. Common symptoms include nausea, fatigue, and breast tenderness.
- Second trimester (weeks 13–26): Often the most comfortable period. Baby begins to move (quickening around week 18–22). Anatomy scan at week 20.
- Third trimester (weeks 27–40): Rapid weight gain. Baby's lungs mature. Full term is defined as 37–42 weeks.
Why Due Dates Are Imprecise
A “full term” pregnancy ranges from 37 to 42 weeks, a five-week window. Factors affecting timing include genetics (first-time mothers tend to deliver later), maternal age, the baby's sex (boys tend to go slightly longer), and individual biological variation. The due date is best understood as the middle of a probable delivery window, not a precise prediction.
Disclaimer: This information is for educational purposes only and does not substitute for prenatal care. Always consult your healthcare provider for personalized medical guidance.