Due Date Calculator
Calculate your pregnancy due date using your last menstrual period (LMP) with adjustments for different cycle lengths.
Pregnancy Information
First day of your last menstrual period
Typical range: 21-35 days (average: 28 days)
Due Date Results
Enter your last menstrual period date to calculate your due date
What is Due Date Calculator?
A due date calculator estimates when your baby is likely to be born based on your last menstrual period (LMP). This calculation follows Naegele's rule, the medical standard used by healthcare providers worldwide.
How Due Dates Are Calculated
Naegele's Rule Formula:
Due Date = LMP + 280 days
(Adjusted for cycle length: +/- days if cycle is not 28 days)
Cycle Length Adjustments
While the standard calculation assumes a 28-day cycle, many women have different cycle lengths. This calculator adjusts the due date based on your average cycle length:
- Shorter cycles (21-27 days): Due date moved earlier
- Longer cycles (29-35 days): Due date moved later
- 28-day cycle: Standard calculation (no adjustment)
Delivery Window
Only about 5% of babies are born exactly on their due date. Most are born within the full-term window:
- Early Term: 37-38 weeks (considered safe for delivery)
- Full Term: 39-40 weeks (optimal timing)
- Late Term: 41-42 weeks (monitoring may increase)
- Post-term: 42+ weeks (induction often considered)
Accuracy and Limitations
LMP-based due date calculations are accurate within 1-2 weeks for women with regular cycles. Factors that can affect accuracy include:
- Irregular menstrual cycles
- Uncertainty about LMP date
- Breastfeeding or recent birth control use
- PCOS or other hormonal conditions
When Ultrasound Dating May Be Preferred
First-trimester ultrasounds (6-13 weeks) can provide more accurate dating if: your cycles are irregular, you're unsure of your LMP date, or there's a significant discrepancy between LMP and ultrasound dating.