How is pregnancy due date calculated?

A pregnancy due date is an estimate of when a baby might arrive, not a guaranteed birthday. Most clinics begin with your last menstrual period (LMP), add about 280 days (40 weeks), then refine timing with ultrasound or special rules when needed.

Why most calculators start with your last period

When people ask how pregnancy due date is calculated, the most common classroom answer is: find the first day of your last menstrual period (LMP), then add 280 days (40 weeks). That shortcut is sometimes called a “Naegele-style” estimate because it mirrors what many clinics still teach on paper pregnancy wheels.

What that math quietly assumes

This approach assumes a 28-day cycle with ovulation around day 14. Real cycles vary. If you ovulate earlier or later, the true conception window shifts—even if your LMP date is perfectly accurate. That is one reason your official due date on a clinic letterhead may differ slightly from a free online pregnancy due date calculator.

For a dedicated LMP workflow (with trimester context), you can also use our due date calculator by LMP page, which opens directly on the LMP tab.

How ultrasound dating changes the estimate

Ultrasound does not “guess” a due date magically—it measures fetal size and compares it to published growth curves, then assigns a gestational age. In early pregnancy, measurements are often more consistent week-to-week than later scans.

When clinicians prioritize ultrasound

Many practices use first-trimester ultrasound dating when cycles are irregular, LMP is uncertain, or the ultrasound date and LMP date disagree by more than a set threshold. Policies differ by country and hospital, but the big idea is the same: pick the method that best matches reality for your pregnancy.

If you are comparing numbers at home, read our FAQ on whether ultrasound can change a due date next.

Conception dating: a second common pathway

If you have a narrower idea of when fertilization happened, some tools add about 266 days (38 weeks) from an estimated conception date. That is a different anchor than LMP, and it can produce a different estimated due date even when both inputs feel “right.”

Our main calculator lets you switch modes so you can see how LMP, conception, and ultrasound inputs compare—useful before you ask, “Which dating rule is my clinic using?”

Examples and quick calculations

Example A (LMP style): If LMP started on January 10, a 280-day estimate points toward October 17 (dates shift with leap years and local date rules—always verify in-tool).

Example B (why weeks feel “off”): If ovulation ran late in the cycle, you might subjectively feel “less pregnant” than LMP-based week counts suggest early on. That mismatch is a normal reason clinicians adjust dating.

Common misconceptions

• Mistake: treating the due date like a deadline. Many healthy births occur across a broader window around the estimate.
• Mistake: using spotting as LMP. Clinicians usually want the first day of meaningful period flow for the cycle before pregnancy.
• Mistake: mixing apps without checking assumptions. Some apps default to LMP while others emphasize IVF rules—compare labels, not just final dates.

Planning tips (non-medical)

• Bring your dating questions to the first prenatal visit: ask how your clinic assigns an “official” estimated due date.
• If you track cycles digitally, screenshot the relevant cycle around conception—it can help reconstruct LMP.
• Use our tools to prepare questions, not to delay care if you have urgent symptoms.

Related guides and tools

Use these internal links to keep learning—each FAQ is written to stand alone, and the calculators help you turn reading into concrete numbers for your next appointment.

Pregnancy due date calculator (home) · Due date by LMP · Pregnancy week calculator · How many weeks pregnant · Blog: calculate due date · Blog index · About

Last reviewed for clarity: May 2026. Always follow your own clinician’s dating, screening schedule, and urgent-care instructions.