Due Date, Pregnancy Week, and Conception Date Calculators: How Estimates Differ

Key points

What to take from this guide

• A due date is commonly estimated from the first day of the last menstrual period, not from the day conception happened.
• Pregnancy week is gestational age as of a chosen date, so it answers a different question than a due-date calculator.
• Conception-date estimates are rough windows and should not be used for medical, legal, or paternity certainty.

Guide section

The short answer

A due date calculator estimates the end of a 40-week pregnancy from the first day of the last menstrual period, usually with a cycle-length adjustment. A pregnancy week calculator estimates gestational age as of a chosen date. A conception date calculator back-calculates a rough likely timing window from an estimated due date.

Those results are related, but they are not interchangeable. Pregnancy weeks are commonly counted from the last menstrual period, which is usually about two weeks before conception. Clinical dating can also revise calculator estimates when ultrasound, assisted reproduction dates, cycle history, or medical records point to a better date.

• Use the due date calculator when starting from last menstrual period and cycle length.
• Use the pregnancy week calculator when asking how far along as of today or another date.
• Use the conception date calculator only for a rough timing estimate from a due date.
• Use clinician-provided dates for care decisions and official records.

Guide section

Why the dates look offset

The usual pregnancy-week count starts with the first day of the last menstrual period. That convention means early pregnancy dating includes time before fertilization. This is why a pregnancy week estimate can look about two weeks ahead of a conception estimate.

A due date is a guide for tracking pregnancy progress, not a prediction that birth will happen on that exact day. ACOG notes that due dates are often confirmed with ultrasound, and clinical teams may compare ultrasound dating with LMP-based dating before selecting the estimated due date.

• LMP-based dating starts at the first day of the last menstrual period.
• Conception estimates are usually later because ovulation and fertilization happen after the cycle starts.
• Ultrasound and clinical history can change how a due date is assigned.
• Once a clinical due date is selected, care teams usually keep a documented date for consistency.

Guide section

Worked example

Say the last menstrual period started May 1, 2026 and the average cycle length is 28 days. The due date calculator estimates February 5, 2027 by using the common 280-day pregnancy convention from the first day of the last period.

As of May 26, 2026, the pregnancy week calculator shows about 3 weeks and 4 days from the last menstrual period. The conception date calculator, starting from the February 5, 2027 due date, estimates conception around May 15, 2026. The pregnancy week and conception estimate are offset because they answer different questions.

• Last menstrual period: May 1, 2026.
• Cycle length: 28 days.
• Estimated due date: February 5, 2027.
• As of May 26, 2026: about 3 weeks and 4 days pregnant by LMP dating.
• Rough conception estimate from that due date: around May 15, 2026.

Guide section

Which timing tool to use

Start from the information you actually have. If you know the first day of the last menstrual period, use the due date or pregnancy week calculator. If you have a clinician-provided due date and want to understand the rough conception timing behind it, use the conception date calculator cautiously.

Use the date calculator only for simple calendar spacing, such as counting days between appointments. It should not replace pregnancy-specific dating logic or clinician guidance.

• Starting from LMP: due date calculator.
• Starting from LMP plus an as-of date: pregnancy week calculator.
• Starting from estimated due date: conception date calculator.
• Counting calendar gaps only: date calculator.

Guide section

Common mistakes

The biggest mistake is assuming pregnancy weeks are counted from conception. In ordinary gestational-age dating, they are counted from the last menstrual period, so week counts and conception estimates are intentionally offset.

Another mistake is treating a calculator result as proof. Irregular cycles, uncertain LMP, recent contraception changes, assisted reproduction, early ultrasound findings, bleeding, pain, pregnancy loss history, or urgent symptoms can all make calculator-only dating inappropriate.

• Using a conception estimate as an exact date.
• Expecting the due date to predict the exact birth date.
• Using irregular cycles as if they were regular 28-day cycles.
• Using calculator results for paternity, viability, medical, or legal certainty.
• Ignoring clinician-provided dates after ultrasound or assisted reproduction dating.

Worked example

One pregnancy timing example, three estimates

The same calendar facts can produce a due date, a gestational-age count, and a rough conception estimate.