How accurate is a due date calculator?

Due date math is a mix of conventions (LMP + 280 days), ultrasound measurements, and specialty rules like IVF dating. This page walks through the ideas so you can ask sharper questions at visits.

Calculator toolkit on this site

Use these tools while you read—keep the same LMP or ultrasound anchor your clinician documented.

Pregnancy due date calculator · Due date by last period · Pregnancy week calculator · How many weeks pregnant · FAQ hub · How to calculate due date (blog)

Methods, accuracy, and examples

Calculators are precise at arithmetic but blind to your uterus: wrong LMP, irregular ovulation, or mistyped ultrasound fields change outputs.

Clinical accuracy improves when early ultrasound refines or replaces naive LMP dating—especially with long cycles.

Treat any printed EDD as a planning midpoint; birth timing remains individualized.

How to use this page with your pregnancy timeline

Screening tests are offered on schedules that depend on dating accuracy. If your estimated due date moves, ask whether previously booked labs or imaging should be re-timed rather than assuming the old calendar still fits.

If English is not your first language, ask your clinic for translated materials or an interpreter for key visits. Consent conversations deserve clarity, especially around induction and cesarean decision points.

Medical responsibility reminder

This article is educational. It does not diagnose, treat, or triage emergencies. Always follow your licensed obstetric clinician, midwife, or local emergency guidance.

Try the free pregnancy due date calculator

Switch between LMP, conception, and ultrasound modes, see your week and trimester, and save a snapshot for your next visit. Educational estimates only—always confirm with your clinician.

Open main calculator Due date by LMP Pregnancy week calculator

Guides & week pages

FAQ deep dives

Prefer a hub view? Browse the pregnancy FAQ index or open the main calculator.

Questions about this topic

Short answers for quick reading. Explore linked guides for depth.

Is an app due date “official”?

Not automatically. Your clinician documents official dating using policies that may differ from consumer app defaults.

Why do LMP and ultrasound disagree sometimes?

Ovulation timing, irregular cycles, and measurement averages can shift the best anchor. Early ultrasound often clarifies dating when LMP is fuzzy.

How do I count weeks at home?

Use our pregnancy week calculator and how many weeks pregnant tool, then confirm at your appointment.

Educational content only—not medical advice. Last reviewed for clarity: May 2026.