How Due Dates Are Calculated
Pregnancy due dates are estimated 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 occurring on day 14. If you know your conception date, the calculator subtracts 14 days to estimate the LMP equivalent and then applies the standard 280-day calculation.
Understanding Trimesters
| Trimester | Weeks | Key Developments |
|---|
| First | 1-12 | Major organs form, heartbeat begins |
| Second | 13-26 | Movement felt, gender identifiable |
| Third | 27-40 | Rapid growth, lung maturation |
Important Notes
Only about 5% of babies are born on their exact due date. A normal full-term pregnancy ranges from 37 to 42 weeks. Your healthcare provider may adjust your due date based on ultrasound measurements, especially in the first trimester when dating is most accurate. Always consult with your OB-GYN or midwife for personalized prenatal guidance.
Frequently Asked Questions
How accurate is a due date calculator?
Due date calculators provide an estimate based on average pregnancy length. Only about 5% of babies arrive on their exact due date. Most births occur within 1-2 weeks before or after the estimated date.
What is Naegele's Rule?
Naegele's Rule is the standard method for calculating due dates: add 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day menstrual cycle with ovulation on day 14.
What if my cycle is longer or shorter than 28 days?
If your cycle is regularly longer or shorter than 28 days, your due date may shift. For example, a 35-day cycle means ovulation likely occurred around day 21 instead of day 14, pushing the due date about a week later.
When does each trimester start and end?
The first trimester spans weeks 1-12, the second trimester covers weeks 13-26, and the third trimester runs from week 27 until delivery (typically around week 40).
Is conception date or LMP more accurate?
If you know your exact conception date (through IVF or ovulation tracking), it can be more accurate. However, most providers use LMP since conception date is often uncertain. First-trimester ultrasound is the most accurate dating method.