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How to calculate your pregnancy due date (LMP & Naegele’s rule)

A plain-language guide to estimating a due date from your last menstrual period, when ultrasound dating matters, and how to confirm timing with your care team.

· Updated

Most families first hear a due date after a positive pregnancy test and a dating conversation at the first prenatal visit. This article explains the common LMP-based estimate used by many pregnancy due date calculators, why it is only an estimate, and how your clinician may adjust dating using ultrasound or other rules.

Start with the first day of your LMP

LMP means the first day of bleeding from your last menstrual period before this pregnancy—not spotting alone, and not the last day of bleeding. If you are unsure, your clinic can help you pick a consistent date for paperwork.

Apply the common 280-day (40-week) estimate

Many educational tools add 280 days (40 weeks) from LMP as a quick “Naegele-style” estimate. This assumes a typical 28-day cycle with mid-cycle ovulation on average. If your cycles are shorter or longer, LMP-based dating can be less accurate.

Compare with ultrasound dating when available

Early first-trimester ultrasound measurements are often used to refine or replace LMP-based dating, especially when cycles are irregular or when LMP does not match early growth measurements.

Confirm your official due date with your clinician

Your official estimated due date is a clinical decision. Bring your notes, cycle history, and any ultrasound reports so your team can explain which dating method they are using for you.

Medical disclaimer

This website provides general educational information and estimated dates only. It does not provide medical diagnosis, treatment, or personalized clinical recommendations. Always follow the guidance of your licensed obstetric care team, midwife, or physician for decisions about your pregnancy.

If you have pain, bleeding, severe headache, vision changes, decreased fetal movement, or any urgent symptoms, seek immediate in-person medical care.

How to estimate a due date from your last menstrual period

Educational overview of LMP-based dating and when to confirm results with your obstetric clinician.

Typical reading time: about three minutes.

  1. 1

    Identify the first day of your LMP

    Use the first day of regular-flow bleeding from the cycle before this pregnancy—not spotting alone, and not the last day of bleeding.

  2. 2

    Add 280 days (40 weeks) for a common estimate

    Many educational calculators add 280 days from LMP as a quick “Naegele-style” estimate. This assumes a typical 28-day cycle with mid-cycle ovulation on average.

  3. 3

    Compare with ultrasound dating if available

    If you have an early ultrasound, compare the estimate to LMP-based dating. Your clinician may adopt ultrasound dating when it is more reliable for your situation.

  4. 4

    Confirm your official due date with your clinician

    Bring your notes to prenatal care and ask how your clinic assigns the official estimated due date and what window is considered full term for you.

Frequently asked questions

Why do clinicians start counting pregnancy from LMP?
LMP is an observable anchor that is easy to document across patients. Dating from the first day of bleeding standardizes communication, even though conception usually happens later in the cycle.
When should I trust ultrasound dating instead of LMP?
Early first-trimester ultrasound dating is often used to refine or replace LMP-based dating when cycles are irregular, bleeding history is unclear, or there is a meaningful mismatch between LMP and measurements.
Is an online due date the same as my official due date?
Not necessarily. Your clinician may choose a different official due date based on ultrasound, IVF dating rules, medical history, or local guidelines. Use online estimates as educational planning aids, not a diagnosis.

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