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Due Date Calculation Formula:
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The due date is calculated from the Last Menstrual Period (LMP) because it provides a standardized and easily identifiable reference point for pregnancy dating. Since the exact date of conception is often unknown, LMP serves as a reliable starting point for estimating gestational age.
The calculator uses the standard obstetric formula:
Where:
Explanation: This calculation method, known as Naegele's Rule, assumes a 28-day menstrual cycle and ovulation occurring on day 14. The 280 days represents 40 weeks of gestation.
Details: LMP-based due date calculation remains the standard in obstetrics because it's consistently measurable, provides early pregnancy dating, and serves as a baseline for comparing ultrasound measurements throughout pregnancy.
Tips: Enter the first day of your last menstrual period. The calculator will automatically add 280 days to provide your estimated due date. Ensure the date entered is accurate for the most reliable result.
Q1: Why 280 days specifically?
A: 280 days represents 40 weeks of pregnancy, which is the standard duration used in obstetrics based on average human gestation periods.
Q2: What if my cycle isn't 28 days?
A: For women with longer or shorter menstrual cycles, the due date may need adjustment. Typically, add or subtract days based on how your cycle length differs from 28 days.
Q3: How accurate is LMP-based dating?
A: LMP dating is generally accurate within ±2 weeks. First-trimester ultrasound dating is more precise and may be used to adjust the due date if there's significant discrepancy.
Q4: Why not use conception date?
A: Most women don't know their exact conception date, while LMP is usually well-documented and provides a consistent starting point for all pregnancies.
Q5: When should the due date be recalculated?
A: If first-trimester ultrasound measurements differ from LMP-based dating by more than 5-7 days, healthcare providers typically adjust to the ultrasound dating.