Pregnancy Due Date Calculator
Estimates expected date of delivery (EDD), current gestational week, and key milestone trimesters via Naegele's clinical rule.
How This Calculation Formula is Formulated
Naegele's Rule is the standard clinical obstetrical method used to calculate estimated date of delivery (EDD). Human pregnancy averages 280 days (40 completed weeks, or 10 lunar months) from the first day of the last normal menstrual period (LMP) for a standard 28-day cycle.
Variable Definitions & Measurement Units
LMP on January 1st with Standard 28-Day Cycle
Trimester Chronology and Gestational Accuracy
1st Trimester: Week 1 through Week 13 (organogenesis, embryonic development). 2nd Trimester: Week 14 through Week 27 (rapid anatomical growth). 3rd Trimester: Week 28 through Week 40+ (fetal lung maturation and weight gain).
Clinical studies show that only ~4% to 5% of babies are born precisely on their estimated due date; however, approximately 85% to 90% arrive within a 2-week window (between 37 and 42 completed gestational weeks).
Frequently Asked Questions About Pregnancy Due Date Calculator
Why is gestational age counted from the last period instead of conception?
Because the exact date of ovulation and fertilization is rarely known with certainty, while the onset of menstruation is an observable physical milestone.
What if my menstrual cycle is longer than 28 days?
If your cycle is 32 days (4 days longer than average), ovulation occurs 4 days later; the calculator automatically adds 4 days to your estimated due date.
Which is more accurate: LMP due date or first-trimester ultrasound?
A crown-rump length (CRL) measurement during an early first-trimester ultrasound (between 8 and 13 weeks) has an accuracy margin of ±3 to 5 days and is clinically prioritized if it deviates significantly from LMP.
When is a pregnancy considered 'Full Term'?
According to ACOG (American College of Obstetricians and Gynecologists), early term is 37w0d–38w6d, full term is 39w0d–40w6d, and late term is 41w0d–41w6d.
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