lmp edd calculator

What is an LMP EDD calculator?

An LMP EDD calculator is a pregnancy due date tool that estimates your expected date of delivery (EDD) based on the first day of your last menstrual period. It uses a standard obstetric method called Naegele’s rule, which assumes a 28-day cycle and adds 280 days (40 weeks) to the LMP date.

Because many people do not have a perfectly regular 28-day cycle, this calculator also lets you adjust cycle length. That makes the estimate more personalized and often more realistic for planning appointments, prenatal screening timelines, and maternity leave prep.

How this due date calculation works

Standard method

For a 28-day cycle:

• Estimated Due Date = LMP + 280 days
• Estimated conception date is often around LMP + 14 days

Cycle length adjustment

If your cycle is longer or shorter than 28 days, ovulation timing shifts. This calculator applies:

• Adjusted EDD = LMP + 280 + (cycle length − 28)

Example: if your cycle is 31 days, the calculator adds 3 extra days.

Why your “due date” is an estimate

Even with excellent date tracking, a due date is still an estimate. Only a small percentage of babies are born on the exact calculated date. Most births happen within a window around that date.

• Normal delivery timing can vary by days or even weeks
• Ovulation and implantation can happen earlier or later than average
• Early ultrasound may refine the pregnancy dating

How to use this calculator accurately

1) Enter the correct LMP date

Use the first day of bleeding for your last normal period, not the day your period ended.

2) Use a realistic cycle length

If your cycle varies, use your average from the last 3–6 months.

3) Interpret results as planning guidance

The output is useful for milestones such as trimester transitions and appointment timing, but your clinician’s dating (especially with first-trimester ultrasound) should be considered the clinical standard.

Pregnancy milestones this calculator supports

Along with your estimated due date, this tool provides meaningful time markers:

• Current gestational age in weeks and days
• Estimated conception date
• End of first trimester (13w6d)
• End of second trimester (27w6d)
• Approximate start of full term (39 weeks)

When LMP dating may be less reliable

LMP-based dating can be off when cycles are irregular or ovulation timing is unpredictable. You should expect stronger reliance on ultrasound dating if any of the following apply:

• Irregular periods
• Recent hormonal contraception changes
• Breastfeeding-related cycle changes
• PCOS or other ovulatory disorders
• Uncertain LMP date

Frequently asked questions

Is this a conception date calculator too?

It includes an estimated conception date based on ovulation timing from your cycle length. It is an estimate, not proof of exact fertilization timing.

Is ultrasound more accurate than LMP?

In early pregnancy, ultrasound dating is often more accurate, especially if your cycle is irregular or LMP is uncertain.

Can my due date change?

Yes. Providers may revise the EDD if early ultrasound measurements differ meaningfully from LMP-based dating.

Bottom line

This LMP EDD calculator is a practical way to estimate your due date and track where you are in pregnancy today. It is best used as an educational and planning tool. For medical decisions, prenatal testing windows, and official pregnancy dating, follow your healthcare provider’s guidance.