Pregnancy Due Date Calculator

Pregnancy Due Date Calculator

Calculate gestational due dates and clinical milestone timelines instantly using local browser memory.

Mathematical Formula & Standards

Naegele's Rule, formulated by German obstetrician Franz Karl Naegele in 1812, is deceptively simple:

1. 1Standard Naegele's Rule (28-day cycle):

EDD = LMP + 280 days (40 weeks)

Or equivalently: Add 1 year, subtract 3 months, add 7 days to the LMP date.

1. 2Cycle Length Adjustment:

For cycles other than 28 days, the estimated ovulation date shifts:

EDD_adjusted = EDD_standard + (Cycle Length - 28) days

A woman with a 35-day cycle ovulates approximately 7 days later than a 28-day cycle woman, shifting the EDD forward by 7 days.

1. 3Trimester Milestones:
2. 2End of First Trimester: LMP + 13 weeks (91 days)
3. 3End of Second Trimester: LMP + 27 weeks (189 days)
4. 4Full Term: LMP + 37 weeks (259 days) — most physicians consider delivery at or after this point "term"

[LMP Date + Cycle Length (days)]
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[Naegele's Rule: LMP + 280 + (Cycle - 28) days]
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[Estimated Due Date + Trimester Milestone Dates]

Input Methodology

1. 1Enter LMP Date: Input the first day of your last menstrual period in YYYY-MM-DD format.
2. 2Enter Cycle Length: Input your average menstrual cycle length in days (default: 28).
3. 3Review Results: See your estimated due date, adjusted for your cycle length.
4. 4Note Milestones: Check the first and second trimester end dates for planning purposes.

Pregnancy Due Date Calculator — Naegele's Rule Estimator

If the first day of your Last Menstrual Period (LMP) was October 1st, Naegele's Rule calculates your estimated delivery date (EDD) as July 8th of the following year (adding 280 days). However, because cycle lengths vary from the standard 28-day model, this calculator adjusts your due date based on your personal cycle spacing, outlining the dates for key gestational milestones.

• Due Date Seems Much Earlier or Later Than Expected: Verify the LMP date is entered in YYYY-MM-DD format. Entering dates in other formats (MM/DD/YYYY) may cause an incorrect calculation.
• Adjusted EDD Differs From Doctor's Estimate: Your OB/GYN may have used ultrasound dating rather than LMP-based calculation, which is more accurate if done in the first trimester.
• Tool Shows Invalid Date: Confirm your LMP date is a real calendar date entered correctly.

Why is pregnancy counted from the LMP and not conception?

Because the exact date of conception is rarely known. LMP dating gives a reliable, observable reference point. The 40-week count includes approximately 2 weeks before conception actually occurs (the follicular phase of the cycle).

What does "full term" mean?

Full term pregnancy is defined as 39–40 weeks of gestation. Deliveries at 37–38 weeks are "early term." Deliveries at 41 weeks are "late term." Post-term is 42+ weeks. These distinctions affect delivery planning and monitoring decisions.

How different can the ultrasound EDD be from the LMP EDD?

A discrepancy of 5–7 days is common and clinically acceptable. If the ultrasound and LMP dates disagree by more than 7 days, most OB/GYNs will adjust the official EDD to the ultrasound estimate.

Is my reproductive health data stored anywhere?

No. All calculations run client-side in your local browser. Your LMP date is never uploaded.

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