Understanding Pregnancy Duration
A full-term pregnancy lasts between 37 and 42 weeks, though the standard assumption is 280 days (40 weeks) from LMP. Only about 4% of babies arrive on their exact due date, so it's more realistic to think in terms of a due month rather than a single day.
Medical professionals measure pregnancy duration from the start of menstruation because many women don't know their exact conception date. This dating convention means you're already considered 2 weeks pregnant when your period is 2 weeks late—the egg was likely fertilised around the midpoint of your previous cycle.
Throughout pregnancy, your due date may shift based on ultrasound findings, especially early scans. If growth measurements suggest the baby is ahead or behind the initial estimate, your clinician may adjust the date. This is normal and reflects the inherent uncertainty in predicting labour onset.
Due Date Calculation Methods
Two primary methods calculate your due date. If you know your LMP, Naegele's rule adds 280 days. If you're certain of conception, add 266 days instead (since ovulation and conception typically happen 14 days after LMP begins).
The standard 28-day cycle assumption built into Naegele's rule works well for regular cycles. However, if your cycle is longer (e.g., 30 or 35 days), you may conceive later, making the conception-date method more precise if available.
Due Date (from LMP) = First day of LMP + 280 days + (cycle length − 28)
Due Date (from conception) = Conception date + 266 days
Due Month Start = Due Date − 14 days
Due Month End = Due Date + 14 days
Days Until Due Date = Due Date − Today's date
Pregnancy Progress (%) = (280 − Days Remaining) ÷ 280 × 100
LMP— First day of your last menstrual period (onset of bleeding)Cycle length— Your average menstrual cycle duration in days (typically 21–35)Conception date— The exact date fertilisation occurred (usually 14 days after LMP)Days Remaining— Number of days from today until the estimated due date
What Is the Due Month?
Because babies rarely arrive on the calculated due date, clinicians use the concept of a "due month"—the period spanning 2 weeks before and 2 weeks after your estimated due date. Approximately 80% of babies are born within this 4-week window.
For example, if your due date is May 1st, your due month runs from April 17th to May 15th. This range reflects the natural variation in labour onset. Your healthcare provider will monitor you closely once you reach 37 weeks (the start of term delivery) and will not let pregnancy continue beyond 42 weeks without intervention due to increased risks.
Last Menstrual Period vs. Conception Dating
Naegele's Rule (LMP method) assumes a 28-day cycle with ovulation on day 14. It's simple and works well for women with predictable periods, but accuracy depends on remembering your LMP accurately and having a regular cycle. If your cycles are irregular or longer than 28 days, this method may overestimate by several days.
Conception-date method is more direct if you know exactly when fertilisation occurred. Women who track ovulation through basal temperature, cervical mucus, or ovulation tests may know this date precisely. Sperm can survive up to 7 days in the reproductive tract, so intercourse within 5 days before ovulation can result in pregnancy.
Early pregnancy ultrasound (before 13 weeks) is the gold standard for confirming dates. These scans measure the embryo or fetus and can adjust your due date if discrepancies exist, particularly if the scan differs from your calculated date by more than 3–5 days.
Key Considerations for Due Date Accuracy
Several factors can affect how reliable your calculated due date is and how pregnancy progresses.
- Irregular cycles shift your due date — If your menstrual cycle is longer than 28 days, ovulation and conception happen later than Naegele's rule assumes. A 35-day cycle means you might conceive on day 21 instead of day 14, pushing your due date forward. Always enter your average cycle length into the calculator if it differs from the standard.
- Early ultrasound is the most accurate check — Dating scans in the first trimester (before week 14) are accurate to within ±3–5 days. Later scans are less precise. If your scan reveals a discrepancy, trust the imaging over your LMP estimate, especially if you're uncertain about your cycle.
- Due dates shift during pregnancy — Your estimated due date may change as pregnancy advances. Doppler scans, biometry measurements, and fundal height assessments can prompt adjustments. This is expected and doesn't indicate a problem—it simply reflects new information about how your baby is growing.
- Physical symptoms don't predict labour timing — Early pregnancy signs like nausea, fatigue, or Braxton-Hicks contractions vary widely and don't reliably forecast when labour will start. Rely on clinical assessment and your calculated dates rather than symptom timing.