Due Date Calculator

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

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Select the LMP Month
Choose the month your last menstrual period (LMP) began. This is the starting point for most due date calculations.
2
Enter the LMP Day
Input the day of the month your LMP started. Accuracy here is key for a precise estimate.
3
Specify the LMP Year
Enter the four-digit year your LMP began. The calculator supports dates from 2000 onwards.
4
Adjust for Cycle Length (days)
Input your average menstrual cycle length. A 28-day cycle is standard, but adjustments are made for shorter or longer cycles.
5
Review your results
The calculator will display your estimated due date, current week, trimester, and other key pregnancy milestones.

Example Calculation

A woman discovers she is pregnant and wants to estimate her due date based on her last menstrual period, which began on January 15, 2025, with a regular 28-day cycle.

LMP Month

January

LMP Day

LMP Year

2025

Cycle Length (days)

28 days

Results

October 22, 2025

Tips

First Trimester Ultrasound for Accuracy

For the most accurate due date, especially if your cycle length varies, aim for an early dating ultrasound between 8 and 12 weeks of gestation. This can be more precise than LMP-based estimates by up to 5-7 days.

Understand Naegele's Rule Basis

Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is typically 32 days, your due date will be shifted forward by 4 days (32-28). Conversely, a 24-day cycle would shift it back 4 days.

Due Date is an Estimate

Only about 4% of babies are born on their exact due date. Most full-term pregnancies occur between 37 and 42 weeks, so your due date is a target, not a guarantee. Focus on the 'due window' rather than the specific day.

Estimating Your Pregnancy Journey with the Due Date Calculator

The Due Date Calculator provides an easy way to estimate your baby's arrival, current gestational week, and trimester based on your last menstrual period (LMP) and average cycle length. This tool uses the widely accepted Naegele's Rule, adjusting for individual cycle variations to provide a personalized estimate. For expectant parents in 2025, knowing your due date is a fundamental step in planning for prenatal care, preparing for the baby's arrival, and understanding the progression of pregnancy.

Why Estimating Your Due Date Is Crucial for Prenatal Care

Estimating your due date is a cornerstone of prenatal care, providing a crucial timeline for monitoring fetal development and maternal health. Healthcare providers rely on this date to schedule important screenings, such as genetic tests and anatomy scans, ensuring they occur within optimal windows. It also helps track the baby's growth against expected milestones and plan for the timing of interventions if complications arise. An accurate due date minimizes unnecessary inductions or extended monitoring by clearly defining the "full-term" period, typically between 39 and 40 weeks, according to the American College of Obstetricians and Gynecologists (ACOG).

How Naegele's Rule Determines Your Due Date

The Due Date Calculator primarily uses Naegele's Rule, a standard method for estimating a pregnancy's expected delivery date. This rule is based on the assumption that a typical pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period (LMP) and that ovulation occurs on day 14 of a 28-day cycle.

The basic formula for Naegele's Rule is:

Due Date = LMP + 280 days

However, the calculator refines this by accounting for individual cycle length:

Adjustment = Cycle Length - 28 days
Due Date = LMP + (280 + Adjustment) days

This means if your cycle is longer than 28 days, your due date shifts later; if it's shorter, it shifts earlier. The calculation then determines current gestational age, trimester, and days remaining by comparing the LMP and due date to the current date.

💡 As your due date approaches, tracking the regularity and intensity of contractions becomes important. Our Contraction Frequency Calculator can help monitor labor progression.

Estimating a Due Date for a First-Time Mother

Imagine a first-time mother whose last menstrual period began on January 15, 2025. She typically has a very regular 28-day menstrual cycle. She wants to use this information to estimate her due date.

Identify the first day of the Last Menstrual Period (LMP): January 15, 2025.
Determine the cycle length adjustment: Her cycle is 28 days, so the adjustment is 28 - 28 = 0 days.
Add 280 days (plus adjustment) to the LMP: January 15, 2025 + 280 days = October 22, 2025

Based on Naegele's Rule, her estimated due date is October 22, 2025. This allows her and her healthcare provider to begin planning her prenatal care schedule.

💡 For early genetic screening, procedures like Chorionic Villus Sampling (CVS) have specific timing windows. Our CVS Date Calculator can help determine if you are within the recommended timeframe.

Key Milestones in Pregnancy Trimesters

Pregnancy is typically divided into three trimesters, each marked by unique developmental milestones for the fetus and distinct physiological changes for the pregnant individual. The first trimester (weeks 1-12) is a period of rapid development, where major organs begin to form, and the embryo transforms into a fetus. It's crucial for folic acid intake (400 mcg daily) to prevent neural tube defects. The second trimester (weeks 13-27) often brings increased energy and is when most people feel the first fetal movements. The fetus grows substantially, and gender can often be determined by ultrasound. The third trimester (weeks 28-40) is the final stretch, focusing on fetal growth and maturation, particularly of the lungs and brain. Weight gain guidelines suggest 25–35 lbs for individuals with an average BMI, with a significant portion occurring in this trimester.

Understanding Due Date Accuracy and Variability

While a calculated due date provides a valuable benchmark, it's essential to understand its inherent variability. Only about 4% of babies are born on their exact estimated due date. Most full-term pregnancies naturally fall within a "due window" of 37 to 42 weeks of gestation. Factors like variations in ovulation timing, cycle length irregularities, and even maternal health can influence the actual delivery date. Healthcare organizations like the American College of Obstetricians and Gynecologists (ACOG) emphasize that ultrasound dating in the first trimester (between 8-12 weeks) is generally the most accurate method, often within a 5-7 day margin, as it directly measures fetal size rather than relying on assumptions about the menstrual cycle.

Frequently Asked Questions

How is a pregnancy due date calculated?

A pregnancy due date is most commonly calculated using Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a standard 28-day menstrual cycle. Adjustments are made for cycle lengths that are shorter or longer than 28 days, shifting the due date accordingly to account for earlier or later ovulation.

Is my due date always accurate?

Your due date is an estimate, not an exact prediction, with only about 4% of babies arriving on their precise due date. While LMP-based calculations provide a good starting point, early ultrasounds (between 8 and 12 weeks) offer the most accurate dating, often within a 5-7 day margin, especially for those with irregular cycles.

What are the trimesters of pregnancy?

Pregnancy is divided into three trimesters, each lasting approximately three months. The first trimester is from week 1 to 12, the second from week 13 to 27, and the third from week 28 to 40 (or delivery). Each trimester is characterized by distinct developmental milestones for the baby and specific physical and emotional changes for the pregnant person.

What is a 'full-term' pregnancy?

According to the American College of Obstetricians and Gynecologists (ACOG), a full-term pregnancy is defined as birth occurring between 39 weeks 0 days and 40 weeks 6 days of gestation. Babies born before 37 weeks are considered preterm, while those born between 37 weeks 0 days and 38 weeks 6 days are classified as early term. Post-term refers to births at 42 weeks 0 days or later.