Finding out your due date is one of the first and most exciting milestones of pregnancy. But how exactly is it calculated, and how accurate is it?
How Due Dates Are Calculated
The standard method is Naegele’s Rule, which estimates the due date as 40 weeks (280 days) from the first day of your last menstrual period (LMP).
Formula: LMP date + 280 days = estimated due date
If you know your conception date, the due date is approximately 266 days (38 weeks) from conception.
Our due date calculator handles both methods automatically and shows your weekly pregnancy timeline.
Due Date Accuracy
Only about 4% of babies are born exactly on their due date. Most arrive within a two-week window around the estimated date:
- 37-38 weeks: Early term (about 25% of births)
- 39-40 weeks: Full term (about 55% of births)
- 41 weeks: Late term (about 10% of births)
- 42+ weeks: Post term (about 5% of births)
First-time pregnancies tend to go slightly longer than subsequent pregnancies.
Ultrasound Dating
Early ultrasound (between 8-14 weeks) is the most accurate method for dating a pregnancy. It measures the baby’s crown-rump length and can adjust the due date if there’s a significant discrepancy with the LMP-based date.
Later ultrasounds are less reliable for dating because babies grow at different rates in the second and third trimesters.
Pregnancy Timeline: Trimesters
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | Week 1-12 | Conception, heartbeat detected, morning sickness |
| Second | Week 13-27 | Baby begins moving, gender can be determined, quickening felt |
| Third | Week 28-40 | Rapid growth, baby positions for birth, Braxton Hicks contractions |
Our due date calculator breaks down each week with key milestones.
Factors That Can Affect Due Date
- Irregular menstrual cycles make LMP-based dating less accurate
- IVF pregnancies have precisely known conception dates
- Multiple pregnancies (twins, triplets) often deliver earlier
- Maternal age and health history may influence delivery timing
Preparing for Your Due Date
While the exact date is uncertain, you can prepare:
- Schedule maternity leave starting around 38 weeks
- Pack a hospital bag by 36 weeks
- Install the car seat by 37 weeks
- Have a birth plan discussed with your healthcare provider
- Prepare for the possibility of going past your due date
Summary
Your due date is an estimate, not a guarantee. Use our due date calculator to get your estimated date, track weekly milestones, and prepare for your baby’s arrival.