Why use this calculator
Knowing your estimated due date helps you plan prenatal care, prepare for delivery, and track important pregnancy milestones throughout each trimester.
How to use it
Enter the date of your last menstrual period and your average cycle length. The calculator uses Naegele's rule, adjusted for cycle length, to estimate your due date and trimester dates.
The formula
Naegele's Rule: Due Date = LMP + 280 days + (Cycle Length - 28) days. First Trimester: Weeks 1-12. Second Trimester: Weeks 13-27. Third Trimester: Weeks 28-40.
Worked examples
LMP January 1, 2025 with 28-day cycle
Due date October 8, 2025
LMP March 15, 2025 with 32-day cycle
Due date December 24, 2025 (adjusted +4 days)
When people use it
- Planning prenatal appointments and ultrasounds
- Tracking pregnancy milestones and fetal development
- Preparing maternity leave and work schedules
- Estimating when to have the nursery ready
Tips
- Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after
- An early ultrasound (6-9 weeks) provides the most accurate dating
- Cycle length adjustment matters if your cycles are regularly longer or shorter than 28 days
Questions people ask
- How accurate is Naegele's rule?
- It provides a reasonable estimate for most pregnancies. However, only about 5% of babies arrive on the exact due date. Most are born within a 2-week window around it.
- What if I do not know my last period date?
- An early ultrasound can estimate gestational age and due date. Consult your healthcare provider for dating options.
- Does cycle length really affect the due date?
- Yes. Longer cycles typically mean later ovulation, which can push the due date back by the same number of extra days.