Your estimated due date (EDD) is calculated using Naegele’s Rule: the first day of your last menstrual period plus 280 days. This assumes a 28-day cycle and ovulation on day 14. The calculator adjusts automatically for longer or shorter cycles.
📅 Due Date Calculator
How Due Dates Are Calculated
Naegele’s Rule dates to 1812 and remains the global standard. The formula: EDD = LMP + 280 days, adjusted for cycle length. A woman with a 35-day cycle ovulates approximately 7 days later than the 28-day model predicts, so her EDD is LMP + 287 days. If your cycle is irregular, an early ultrasound (8–12 weeks) provides a more accurate date — crown-rump length measurement in the first trimester can date a pregnancy to within ±5 days, more precise than any calendar calculation.
Why Your Scan Date May Differ From Your Calculator Result
It’s common for the 10–14 week dating scan to give a different EDD than the LMP calculation, sometimes by 1–2 weeks. This usually reflects actual ovulation timing differing from the assumed day 14, not an error by either method. When the scan date and LMP date disagree by more than 7 days in the first trimester, most hospitals use the scan date as the official EDD. The scan is measuring actual embryo size rather than modelling your cycle.
What ‘Due Date’ Actually Means in Practice
Only 5% of babies arrive on their exact EDD. The more accurate concept is a delivery window: early term is 37–38 weeks; full term is 39–40 weeks; late term is 41 weeks; post-term is 42 weeks. Most spontaneous labours occur between 38–41 weeks. If you reach 41 weeks, your provider will increase monitoring and likely recommend induction by 42 weeks, when placental function begins to decline and stillbirth risk rises measurably.
Key Milestones From Your Due Date
- 8–10 weeks: first antenatal appointment; embryo visible on ultrasound
- 10–14 weeks: first trimester screening — nuchal translucency scan, NIPT/blood tests
- 12 weeks: miscarriage risk drops significantly; most families share the news
- 18–20 weeks: anatomy scan — all organs and structures checked in detail
- 24 weeks: viability milestone — survival outside the womb becomes possible with intensive care
- 28 weeks: third trimester begins; kick counting starts; GBS test at 35–36 weeks
- 37 weeks: ‘full term’ — baby considered ready for birth
- 40 weeks: your estimated due date
- 41–42 weeks: post-dates monitoring and induction discussion
Frequently Asked Questions
My scan date is 10 days different from my calculator result — which is right?
The scan. When a first-trimester scan differs from LMP-based dating by more than 7 days, clinical guidelines recommend using the scan date. Crown-rump length at 8–12 weeks dates a pregnancy to within ±5 days — considerably more accurate than calendar calculation. Your midwife will update your official EDD based on the scan.
What if I don’t know my last period date?
An early ultrasound establishes your due date without LMP information. Between 8–12 weeks, the crown-rump length gives the most accurate estimate. After 14 weeks, head circumference, abdominal circumference, and femur length are used — accuracy reduces somewhat as pregnancy advances, but a reasonable estimate is still possible.
Can I choose to be induced before 40 weeks?
Elective induction at 39 weeks is increasingly discussed after the 2018 ARRIVE trial found it did not increase — and may modestly reduce — caesarean rates in low-risk women. Many providers now offer this as an option. Induction before 37 weeks requires a medical indication. Discuss your individual circumstances and preferences with your obstetrician or midwife.
Medical disclaimer: This tool is for informational purposes only and does not replace medical advice. Always consult your OB, pediatrician, or healthcare provider with any concerns about your baby’s health or development.
Related Reading
- Your body at 1 week pregnant: what’s really happening
- 12 weeks pregnant: safe to share the news?
- 20 weeks pregnant: halfway there — anatomy scan guide
Found this helpful? Sign up to the LylyMama newsletter — honest, evidence-based guidance for every stage of pregnancy and parenting.
