BCBetter Calculators

Due Date Calculator

Calculate your estimated due date, weeks pregnant, trimester, and days remaining.

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Enter your values and click Calculate

How It Works

This calculator uses Naegele's rule, the standard formula for estimating due dates. The formula is: EDD = LMP + 280 days, adjusted for cycle length. The 280 days (40 weeks) represents the average human gestation period measured from the first day of the last menstrual period. The cycle adjustment adds or subtracts days for cycles longer or shorter than 28 days — for example, a 32-day cycle pushes the EDD forward by 4 days since ovulation occurs later. Weeks pregnant is calculated by dividing total days pregnant by 7. Trimester boundaries: first trimester is weeks 1–13 (days 1–97), second trimester is weeks 14–27 (days 98–195), third trimester is weeks 28–40+ (days 196+). The EDD is an estimate — ultrasound dating, especially in the first trimester, is more accurate than LMP-based calculation.

Examples

Early Pregnancy (8 Weeks)
LMP started 56 days ago with a standard 28-day cycle.
Result: Approximately 8 weeks pregnant, first trimester, with about 196 days until the due date.
Second Trimester (20 Weeks)
LMP started 140 days ago with a 30-day cycle.
Result: Approximately 20–21 weeks pregnant, second trimester, with about 140 days until the due date.
Third Trimester (32 Weeks)
LMP started 224 days ago with a 26-day cycle.
Result: Approximately 32 weeks pregnant, third trimester, with about 52 days until the due date.

Frequently Asked Questions

How accurate is this due date calculator?
The calculator uses Naegele's rule, the standard clinical formula. It is accurate within a few days for women with regular cycles. However, ultrasound dating in the first trimester (ideally at 8–12 weeks) is more accurate because it directly measures fetal size rather than relying on LMP recall. Only about 5% of babies arrive on their exact due date; most births occur within two weeks before or after the EDD.
What if my cycle is irregular?
For women with irregular cycles, LMP-based calculation is less reliable. Enter your average cycle length if you can estimate one, but know the result will be less precise. Your doctor or midwife will use first-trimester ultrasound to establish your dating more accurately. If your cycles vary by more than 5–7 days month to month, ultrasound dating is strongly preferred over this formula.
What does the cycle length adjustment do?
Standard Naegele's rule assumes a 28-day cycle where ovulation occurs around day 14. If your cycle is longer (e.g., 35 days), ovulation occurs around day 21 — 7 days later than assumed. The calculator adds those extra days to the EDD. For shorter cycles (e.g., 21 days), ovulation occurs earlier, so the EDD is moved earlier. This adjustment brings the estimate closer to the actual conception date.
When should I see a doctor or midwife?
As soon as you have a positive pregnancy test — ideally within the first 8 weeks. Your first prenatal appointment typically includes a confirmation ultrasound, blood tests, and establishment of your official due date. Early prenatal care is associated with better outcomes for both mother and baby. This calculator provides an estimate but is not a substitute for professional medical care.

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