Method 1: Naegele's Rule (from last menstrual period)
Due Date = First Day of Last Menstrual Period + 280 days (40 weeks)
If your last period started on 1 January 2026, your due date would be approximately 8 October 2026. Obstetric convention counts pregnancy from the first day of the last period rather than from conception.
Adjusting for non-28-day cycles
For a 35-day cycle, ovulation typically occurs around day 21 — seven days later than standard — shifting the due date forward by seven days.
Method 2: From conception date
Due Date = Conception Date + 266 days (38 weeks)
This method is often more accurate for women with irregular cycles who know their ovulation date.
Method 3: IVF transfer date
- Day 5 blastocyst transfer: Due date = transfer date + 261 days
- Day 3 embryo transfer: Due date = transfer date + 263 days
How accurate is a due date?
Only about 5% of babies are born on their exact due date. Normal delivery spans weeks 37 to 42. First trimester ultrasound (before 12 weeks) is the most accurate dating method, typically accurate to within 5-7 days.
Key milestones in your pregnancy
- Week 6: Heartbeat typically visible on ultrasound
- Week 12: End of first trimester — miscarriage risk drops significantly
- Week 20: Anatomy scan
- Week 24: Viability milestone
- Week 37: Full term begins
- Week 40: Your estimated due date