Due Date Calculator (by LMP)

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Category Women\'s Health

First Day of Last Period

Average Cycle Length (Days)

28 Days

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About

The Last Menstrual Period (LMP) calculation is the global standard for initial pregnancy dating. It provides a non-invasive reference point available to most patients before an ultrasound is performed. However, the accuracy of this method relies heavily on the regularity of the menstrual cycle.

This tool incorporates a Cycle Correction coefficient. Standard Naegele's Rule assumes a perfect 28-day cycle with ovulation on day 14. If a user's average cycle is longer (e.g., 32 days), ovulation is delayed, and the fetus is biologically younger than the standard count suggests. This calculator adjusts the Estimated Date of Delivery (EDD) to account for these physiological variations, preventing unnecessary induction for "post-dates" that are simply due to a longer cycle.

Formulas

The corrected formula extends the standard Naegele's Rule:

EDD = LMP + 280 days + [CycleLength − 28]

Example: For a 31-day cycle:

EDD = LMP + 280 + 3 days

Reference Data

Cycle Length	Ovulation Day (Approx)	Correction Factor
21 Days	Day 7	−7 Days
28 Days	Day 14	0 Days (Standard)
30 Days	Day 16	+2 Days
35 Days	Day 21	+7 Days
40 Days	Day 26	+12 Days

Frequently Asked Questions

If your cycles vary significantly (e.g., 25 days one month, 40 the next), the LMP method becomes unreliable. In such cases, the date should be treated as a rough estimate until confirmed by a dating ultrasound between 8 and 12 weeks.

280 days is 40 weeks. This number is derived from adding 9 calendar months and 7 days to the LMP (Naegele's Rule). It includes the 2 weeks before ovulation, meaning the fetus is only 38 weeks old at birth.

If the date is unknown, this calculator cannot be used effectively. An ultrasound scan is required to measure the Fetal Crown-Rump Length (CRL) to estimate gestational age.