Estimated Due Date Calculator

Enter the first day of your last menstrual period (LMP) to calculate your estimated due date, current week, trimester, and pregnancy progress.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter First Day of Last Menstrual Period (LMP)
Input the exact date of the first day of your last menstrual period. This is the standard starting point for due date calculations.
2
Review Your Results
Instantly see your Estimated Due Date, current pregnancy week, trimester, and other key pregnancy milestones.

Example Calculation

An expectant parent wants to find out their baby's estimated due date and current pregnancy status based on their last menstrual period.

First Day of Last Menstrual Period (LMP) (date)

2025-01-15

Results

Wednesday, October 22, 2025

Tips

Consult Your Healthcare Provider

While this calculator provides an estimate, always confirm your due date with a licensed healthcare provider. Early ultrasounds (8-12 weeks) are often used to refine or confirm the LMP-based due date with greater accuracy.

Understand the 'Due Window'

Only about 5% of babies are born on their exact due date. Consider your due date as a 'due window' of two weeks before and two weeks after the estimated date, as most healthy full-term births occur within this range.

Track Early Ultrasound Dates

If you have an early ultrasound, note the estimated due date provided by your doctor. This date is often considered more accurate than LMP, especially if your menstrual cycles are irregular or you're unsure of your LMP.

Charting Your Pregnancy Journey: The Estimated Due Date Calculator

The Estimated Due Date Calculator is an invaluable tool for expectant parents, offering a clear roadmap of their pregnancy journey. By simply inputting the first day of your last menstrual period (LMP), it quickly provides your estimated due date, current pregnancy week, trimester, and days remaining. This information is crucial for planning prenatal care, preparing for your baby's arrival, and understanding the developmental milestones ahead. In 2025, with modern prenatal care emphasizing early and accurate dating, this calculator serves as an excellent first step for every expectant family.

Why Accurate Due Date Estimation is Vital

Accurate due date estimation is vital for both medical professionals and expectant parents. For healthcare providers, it helps monitor fetal growth, schedule critical screenings, and make informed decisions about interventions if a pregnancy extends beyond term. For parents, a reliable due date allows for practical preparations, from setting up the nursery to planning maternity leave, and reduces anxiety about the unknown. While only about 5% of babies arrive on their exact due date, the estimate provides a crucial benchmark for the 40-week gestational period, ensuring everyone is aligned with the expected timeline.

How to Determine Your Estimated Due Date

This calculator uses the widely accepted Naegele's Rule to determine your estimated due date (EDD) and other key pregnancy milestones:

due date = LMP date + 280 days
days elapsed = today - LMP date
weeks elapsed = days elapsed / 7
trimester = calculated based on weeks elapsed

Here, LMP date is the first day of your last menstrual period. The due date is 40 weeks (280 days) from the LMP. Days elapsed and weeks elapsed track your current progress, and the trimester is determined by which stage of pregnancy you are in.

💡 Understanding your fertility cycle is key to conception. Our Ovulation Date Calculator can help pinpoint your most fertile days.

Worked Example: Calculating a Baby's Due Date

Let's calculate the estimated due date for an expectant parent, assuming their Last Menstrual Period (LMP) started on January 15, 2025.

Input LMP Date: January 15, 2025.
Calculate Estimated Due Date: Add 280 days to the LMP.
- Estimated Due Date = January 15, 2025 + 280 days = October 22, 2025
Determine Current Week and Trimester (assuming today is January 15, 2025, the LMP date):
- Days Elapsed = 0
- Current Week = Week 1
- Trimester = First Trimester
Days Remaining Until Due Date:
- Days Until Due = 280

Based on these inputs, the Estimated Due Date is Wednesday, October 22, 2025.

💡 For a more detailed look at the optimal time for conception, our Ovulation & Fertile Window Calculator helps identify your most fertile days.

Clinical Significance of Gestational Age and Trimesters

Gestational age, calculated from the last menstrual period (LMP), is of paramount clinical significance for monitoring fetal development and tailoring prenatal care. Healthcare providers use it to schedule crucial screenings, interpret growth charts, and anticipate potential complications. A full-term pregnancy is precisely defined as 37-40 weeks. Deliveries occurring before 37 weeks are classified as preterm, while those after 42 weeks are post-term, both carrying increased risks. Each trimester has distinct developmental milestones and recommended clinical interventions: the first trimester (weeks 1-13) focuses on organogenesis and screening for chromosomal abnormalities; the second (weeks 14-27) involves detailed anatomy scans; and the third (weeks 28-40+) prepares for birth, monitoring for growth and maternal well-being.

Scenarios Where LMP-Based Due Dates May Be Inaccurate

While the Last Menstrual Period (LMP) method is a standard for due date calculation, it can be less reliable in certain situations. Women with irregular menstrual cycles, which can vary significantly in length, may have an ovulation date that deviates from the standard day 14 assumption, leading to an inaccurate EDD. Similarly, recent use of hormonal birth control can mask or alter natural cycle patterns, making LMP an unreliable starting point. Uncertainty about the exact first day of the last menstrual period, perhaps due to light bleeding or memory recall, also contributes to inaccuracy. In these cases, early ultrasound measurements, typically performed between 8-12 weeks of gestation, are considered more accurate. These scans measure the crown-rump length (CRL) of the fetus, providing an EDD with a margin of error of ±5-7 days, significantly more precise than the ±14 days often associated with LMP-based estimates.

Frequently Asked Questions

How does an Estimated Due Date Calculator work?

An Estimated Due Date Calculator typically works by adding 280 days (or 40 weeks) to the first day of your last menstrual period (LMP). This method, known as Naegele's Rule, is based on the assumption of a 28-day menstrual cycle with ovulation occurring on day 14. The calculator then uses this date to determine current pregnancy week, trimester, and days remaining, providing a comprehensive overview of the pregnancy timeline.

Why is the Last Menstrual Period (LMP) used for due date calculations?

The Last Menstrual Period (LMP) is used for due date calculations because it provides a consistent and easily identifiable starting point for most pregnancies. While conception typically occurs around two weeks after the LMP, the LMP offers a more reliable reference than trying to pinpoint the exact moment of fertilization. This convention allows healthcare providers to standardize gestational age tracking and prenatal care schedules.

What is the average duration of a full-term pregnancy?

The average duration of a full-term pregnancy is typically considered to be 40 weeks (280 days) from the first day of the last menstrual period. However, a full-term birth is clinically defined as occurring between 37 weeks 0 days and 40 weeks 6 days of gestation. Babies born within this window are generally considered to have developed adequately for life outside the womb.

Can the estimated due date change during pregnancy?

Yes, the estimated due date can change during pregnancy, especially if the initial calculation was based solely on LMP and there's uncertainty about the date or irregular cycles. Early ultrasounds (between 8 and 12 weeks of gestation) are often used to confirm or adjust the due date, as they provide a more accurate measurement of fetal size and development. Subsequent ultrasounds are less likely to change the due date unless there are significant discrepancies.