PREGNANCY DATING

Pregnancy Due Date Calculator

Calculate your estimated due date (EDD), current gestational age, and key clinical trimester milestones using ACOG standard obstetric guidelines.

LMP is standard in clinical obstetric practice.
Select the first day of your last menstrual cycle.
Adjusts Naegele's formula for non-28 day cycles.
Estimated Due Date & Gestational Progress ACOG Standard
Estimated Due Date (40 Weeks)
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Current Gestational Age: —
Current Trimester
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Gestational stage
End of First Trimester
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13 Weeks Completed
End of Second Trimester
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27 Weeks Completed
Early Full-Term Date
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37 Weeks Completed
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Clinical Understanding: Only about 4% to 5% of babies are born on their exact estimated due date. A normal full-term pregnancy spans between 37 weeks 0 days and 41 weeks 6 days. First-trimester ultrasound biometrics (Crown-Rump Length) can refine this date if there is a discrepancy.

How Obstetric Due Dates Are Determined

In clinical obstetrics, pregnancy duration is conventionally calculated as 280 days (40 completed weeks) from the first day of the last menstrual period (LMP). This method—first established by German obstetrician Franz Naegele in the 19th century—assumes a 28-day cycle with ovulation occurring on Day 14.

Adjusting for Individual Cycle Lengths

If your typical cycle is longer or shorter than 28 days, standard Naegele's rule must be adjusted. For example, if your cycle is 32 days, ovulation typically occurs around Day 18 (4 days later), shifting your estimated due date forward by 4 days.

When Ultrasound Supersedes LMP Dating

According to the American College of Obstetricians and Gynecologists (ACOG Committee Opinion No. 700), if a first-trimester ultrasound measurement differs from your LMP-calculated due date by more than 5 days (up to 8 6/7 weeks) or more than 7 days (from 9 0/7 to 13 6/7 weeks), the ultrasound dating replaces the LMP date as the official clinical due date.

Authoritative Medical References