Why the Estimated Due Date Is Exactly That — an Estimate
The EDD (Estimated Due Date) calculated from the last menstrual period assumes a 28-day cycle with ovulation on day 14. In practice, cycle length varies significantly between individuals, which shifts the actual conception date — and therefore the true gestational age — by several days. Only about 5% of babies are born on their calculated due date. Around 80% are born within two weeks either side.
The EDD is a planning tool and a clinical reference point, not a prediction. Understanding its limitations helps expectant parents approach the final weeks of pregnancy with realistic expectations rather than anxiety when the due date passes.
Why Your EDD May Change After an Ultrasound Scan
The LMP-based due date calculated here is the starting reference. Most obstetricians adjust this date after the first dating ultrasound scan, which provides a more accurate gestational age estimate.
- •First trimester dating scan (8–14 weeks): Crown-rump length (CRL) measurement is the most accurate method for dating a pregnancy — accurate to within ±5–7 days. If the scan-based EDD differs from the LMP-based EDD by more than 7 days, most clinicians revise the due date to align with the scan.
- •Second trimester scan (18–22 weeks): The anomaly or mid-pregnancy scan provides several measurements (head circumference, abdominal circumference, femur length) but is less accurate for dating — accurate to only ±14 days. After 22 weeks, due dates are rarely revised because scan measurements have high natural variability.
- •Irregular cycles and PCOS: Women with cycles significantly longer or shorter than 28 days, or with polycystic ovary syndrome (PCOS) causing irregular ovulation, may find that LMP-based dating produces a due date that diverges meaningfully from the dating scan. The scan date is the more reliable clinical reference in these cases.
Full Term, Early Term, and Late Term — What the Range Means
"Full term" is not a single date — it is a range. Medical guidelines define gestational term categories to guide clinical decisions about induction, monitoring, and delivery timing.
| Term Classification | Gestational Age |
|---|---|
| Extremely preterm | Before 28 weeks |
| Very preterm | 28–32 weeks |
| Preterm | 32–37 weeks |
| Early term | 37–39 weeks |
| Full term | 39–41 weeks |
| Late term | 41–42 weeks |
| Post-term | Beyond 42 weeks |
Medical classifications as per ACOG (American College of Obstetricians and Gynaecologists) guidelines. Consult your healthcare provider for personalised advice.
Practical Planning Around Your Estimated Due Date
- ✓Plan leave to begin at 37–38 weeks rather than the exact EDD — since labour can begin any time from 37 weeks onwards, starting maternity leave 2–3 weeks before the EDD ensures adequate rest before delivery regardless of when it occurs.
- ✓Notify your employer and HR department by the time you are 25 weeks pregnant (second trimester) — most maternity leave regulations require advance notice, and the period from 25 to 37 weeks is the optimal window for processing.
- ✓Prepare your hospital bag and birth plan by Week 36 — delivering before the due date is common, and being unprepared at 37 weeks because you expected 40 is a source of unnecessary stress.
- ✓Understand that 'overdue' in everyday language is different from medical post-term — most OBs do not consider any action necessary until 41–42 weeks, even if the calculated due date has passed.
- ✓Keep a record of your scan-based EDD separately from your LMP-based EDD if they differ — insurance claims, maternity leave calculations, and medical records may each use a different reference date, and knowing both prevents confusion.