Pregnancy Due Date Calculator

How the Pregnancy Due Date Calculator Works

This pregnancy due date calculator estimates when your baby may arrive based on typical pregnancy length and the dates you provide. It is designed for informational planning only and does not replace advice from your midwife, obstetrician, or other healthcare professional.

There are three main ways you might see a due date estimated in everyday use:

• Last menstrual period (LMP) – counting from the first day of your last menstrual period.
• Conception date – counting from the day sperm and egg likely met (often based on ovulation tracking or fertility treatment dates).
• Known due date – using a date already given by your healthcare provider or ultrasound and working backwards.

Most calculators, including this one, assume:

• Pregnancy lasts about 40 weeks (280 days) from LMP, and
• About 38 weeks (266 days) from conception.

The tool also lets you enter your average cycle length. This helps adjust the estimate if your cycle is longer or shorter than the common 28-day example.

Key Formulas Behind the Calculator

When you use your LMP, a common approach is a version of Naegele’s rule. In simple terms, it adds one year, subtracts three months, and adds seven days to the first day of your last menstrual period.

In day-count form, the basic idea can be expressed as:

If you are using a conception date, the calculator instead adds 266 days:

For people whose cycles are not 28 days long, a simple adjustment is often made based on cycle length:

This reflects that ovulation often occurs about 14 days before the next period, so a longer cycle may mean a slightly later ovulation and due date, and a shorter cycle the opposite.

Interpreting Your Results

After you enter your date and (optionally) your average cycle length, the calculator estimates:

• Estimated due date – the main calendar date when your baby might be born.
• Current gestational age – roughly how many weeks and days pregnant you are today.
• Trimester timing – approximate start dates for the second and third trimesters.

It is normal if your healthcare provider gives a slightly different date, especially after an early ultrasound. Think of the result as a target week rather than a guaranteed specific day. Only a small percentage of babies are born exactly on their due date; many arrive in the two weeks before or after.

You can use this estimate to plan prenatal appointments, discuss parental leave, and get a general feel for the pregnancy timeline week by week.

LMP vs. Conception Date vs. Known Due Date

Different starting points can sometimes give slightly different numbers. The table below summarizes how each method is typically used.

If different methods do not match exactly, your provider’s estimate (especially when based on an early ultrasound) usually takes priority.

Worked Example

Imagine your last menstrual period started on 1 March 2025, and your average cycle length is close to 28 days.

1. Enter 1 March 2025 as your LMP.
2. Leave the cycle length at 28 days if that fits you.
3. The calculator adds 280 days to 1 March 2025 to estimate a due date around 6 December 2025.
4. If today is 15 May 2025, the calculator also shows that you are roughly 10 weeks and a few days pregnant.

If your cycle tends to be 32 days instead, the tool may shift the estimate a few days later to reflect later ovulation.

Pregnancy Timeline Overview

Pregnancy is often described in weeks and divided into three trimesters:

• First trimester: from conception to the end of week 12.
• Second trimester: weeks 13 to 27.
• Third trimester: week 28 until birth.

Knowing your estimated week helps you understand typical milestones, such as when an anatomy scan may be scheduled or when you might start feeling more movement. Your provider can give you a personalized timeline based on your health and pregnancy.

Assumptions and Limitations of This Calculator

This tool uses standard obstetric conventions and several simplifying assumptions. It is important to understand where those assumptions may not fit real life perfectly.

• Single pregnancy assumption: The estimates are based on one baby. Pregnancies with twins or higher multiples can have different typical timelines.
• Average gestation length: The 40-week and 38-week figures are averages. Many healthy pregnancies end before or after the estimated date.
• Cycle length range: The cycle length adjustment is most helpful for cycles roughly between about 21 and 35 days. Far outside this range, results are more approximate.
• Irregular cycles and ovulation: If you ovulate much earlier or later than average, any due date based on LMP or assumed ovulation may be less accurate.
• No medical diagnosis: The calculator does not screen for complications and does not provide medical recommendations.
• Provider estimates may differ: Healthcare professionals may adjust your due date based on ultrasound measurements, your medical history, and their clinical judgment.

Always discuss your estimated due date and any concerns about timing, symptoms, or fetal movement with a qualified healthcare professional.

When to Contact a Healthcare Professional

Regardless of what the calculator shows, reach out to a healthcare provider if you:

• Think you might be pregnant and have not yet had a confirmation visit.
• Do not remember your last menstrual period and are unsure how far along you might be.
• Notice concerning symptoms such as heavy bleeding, severe pain, or a sudden change in fetal movement at any stage.
• Have questions about medications, existing health conditions, or previous pregnancy complications.

This tool is intended to support conversations and planning, not to make any decisions about your care on its own.

FAQ

Can my due date change?

Yes. Many people receive an initial estimate based on LMP, which is then adjusted after an early ultrasound. Your provider may change the official due date if measurements suggest a different gestational age.

Do I need a 28-day cycle to use this calculator?

No. The cycle length field lets you adjust the estimate if your cycles are longer or shorter than 28 days. However, the more irregular your cycle, the more approximate any LMP-based due date will be.

What if I do not remember my last period?

If you cannot recall your LMP, you can try using a conception date (if you know it) or speak with a healthcare professional, who may use an ultrasound to estimate gestational age and set a due date.

Is this calculator medically reviewed?

The methods used here follow widely accepted obstetric conventions for estimating due dates. Always verify your specific dates and care plan with your own healthcare professional.