Understanding Your Due Date Estimate
Your due date calculator gave you a specific date—but only about 5% of babies arrive exactly on their due date. This guide explains what your estimated due date actually means, how it's calculated, and why you should think of it as a target window rather than a precise date.
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View on AmazonA due date is an estimate of when your pregnancy will reach 40 weeks. It's calculated from the first day of your last menstrual period (LMP), even though conception typically occurs about two weeks later. Most babies are born within two weeks of their due date—either before or after—making the due date a midpoint in a likely delivery window rather than a precise prediction.
How Due Dates Are Calculated
Different methods produce similar but not always identical results:
| Method | How It Works | Best For |
|---|---|---|
| Last Menstrual Period (LMP) | Adds 280 days (40 weeks) to the first day of last period | Regular 28-day cycles |
| Naegele's Rule | LMP date minus 3 months plus 7 days | Quick mental calculation |
| Early Ultrasound | Measures embryo/fetus size to estimate gestational age | Most accurate before 12 weeks |
| Conception Date | Adds 266 days (38 weeks) to known conception date | IVF or known ovulation |
The Most Important Insight
Your due date is the middle of a five-week window. Full-term pregnancy ranges from 37 weeks to 42 weeks. About 80% of babies are born between 38 and 42 weeks. Only 5% arrive on their actual due date. Planning should account for delivery anytime within this window.
First-time mothers tend to deliver slightly later than the due date on average, while subsequent pregnancies often come slightly earlier.
When Babies Actually Arrive
Understanding delivery statistics helps set realistic expectations:
Born on exact due date
Born within 2 weeks of due date
First babies arrive after due date
Before Due Date
About 25% of babies arrive before their due date but after 37 weeks. This is considered full-term and typically requires no special intervention.
After Due Date
About 50% of first-time mothers deliver after their due date. Most providers discuss induction options if pregnancy extends past 41-42 weeks.
Planning Considerations Based on This Estimate
- Cash flow: Plan for maternity leave to potentially start 2 weeks before your due date and extend 2 weeks after. Hospital costs, copays, and initial baby expenses should be budgeted before the 37-week mark when delivery becomes more likely.
- Time tradeoffs: Complete essential preparations (nursery, car seat installation, hospital bag) by 36-37 weeks. Items on your registry should be ordered by week 34 to allow shipping and returns time.
- Long-term impact: The due date helps determine maternity leave planning, when to stop working, and childcare start dates. Build flexibility into arrangements—booking childcare for "6 weeks after due date" provides buffer for late arrivals.
Why Your Due Date Might Change
It's common for due dates to be adjusted, especially after early ultrasound:
Common Reasons for Due Date Adjustments
- Ultrasound dating: If first-trimester ultrasound shows different size than expected by LMP, the due date may be adjusted
- Irregular cycles: Women with cycles longer or shorter than 28 days often have adjusted due dates
- Uncertain LMP: If the last period date isn't certain, ultrasound measurements take precedence
- IVF/fertility treatment: Known transfer or conception dates provide more precise timing
Early ultrasound (before 12 weeks) is most accurate for dating because early growth rates are consistent across pregnancies.
How to Use Your Due Date
Plan for a window, not a date
Be ready for delivery anytime between 37 and 42 weeks. Have essentials in place by week 36.
Use it for milestone tracking
Due date helps calculate weeks pregnant, timing for prenatal tests, and when to expect fetal movement and other milestones.
Communicate flexibly with work
Tell employers you're due "in early March" rather than "March 15th" to set appropriate expectations about your availability.
Don't over-share the exact date
Sharing a specific date invites constant "is the baby here yet?" messages. A month or "late March" reduces pressure.
Calculate Your Due Date
Enter your last period date or conception date to estimate your due date.
Use Due Date CalculatorFrequently Asked Questions
How accurate is my due date?
Due dates are accurate within about 2 weeks for most pregnancies. Only 5% of babies arrive on the exact due date. Early ultrasound-confirmed dates tend to be more accurate than LMP-based calculations.
Why is pregnancy counted from the last period, not conception?
Most women know their last period date but not their exact conception date. This method has been used for centuries and medical milestones are calibrated to it. Actual conception typically occurs around day 14 of the cycle.
What happens if I go past my due date?
Going past your due date is common, especially for first pregnancies. Most providers monitor more closely after 40 weeks and discuss induction options between 41-42 weeks, as risks increase with prolonged pregnancy.
Can I influence when my baby arrives?
Despite popular beliefs, there's limited evidence that walking, spicy food, or other methods reliably induce labor. Babies generally arrive when they're ready. Medical induction may be recommended for health reasons.