Due Date Calculator

About Due Date Calculator

Welcome to our Due Date Calculator, a comprehensive pregnancy tool that helps you estimate when your baby will arrive. Whether you know your last menstrual period, conception date, or have ultrasound measurements, our calculator provides an accurate estimated due date along with a complete pregnancy timeline, trimester milestones, and week-by-week baby development information.

How is the Due Date Calculated?

The due date calculation uses Naegele's Rule, developed by German obstetrician Franz Naegele in 1812 and still widely used today. The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes:

• A regular 28-day menstrual cycle
• Ovulation occurring around day 14 of the cycle
• Conception happening within 24 hours of ovulation

If your cycle is longer or shorter than 28 days, our calculator adjusts the calculation accordingly by modifying the estimated ovulation date.

Understanding Your Pregnancy Timeline

Pregnancy is divided into three trimesters, each with distinct developmental milestones:

Calculation Methods Explained

Last Menstrual Period (LMP)

The most common method used by healthcare providers. Enter the first day of your last period, and we calculate your due date by adding 280 days. This method works best if you have regular cycles and remember your LMP date accurately.

Conception Date

If you know when conception occurred (through ovulation tracking, IVF, or other methods), this provides a more precise calculation. We add 266 days (38 weeks) to your conception date.

Ultrasound Date

Early ultrasounds (especially before 20 weeks) can accurately measure fetal development. If your ultrasound gave you a gestational age, enter the ultrasound date along with the weeks and days to calculate your due date.

What If My Cycle Isn't 28 Days?

Our calculator adjusts for different cycle lengths. The key factor is when ovulation occurs:

Gestational Age vs. Fetal Age

These terms can be confusing:

• Gestational Age: Measured from the first day of your LMP. This is what doctors typically use and what our calculator shows. At 40 weeks gestational age, you are ready for delivery.
• Fetal Age (Embryonic Age): Measured from conception, which occurs about 2 weeks after LMP. So fetal age is always about 2 weeks less than gestational age.

When to See Your Healthcare Provider

While this calculator provides a good estimate, your healthcare provider may adjust your due date based on:

• Early ultrasound measurements (most accurate between 8-12 weeks)
• Physical examination findings
• Your medical history
• Irregular menstrual cycles

Frequently Asked Questions

How is my due date calculated?

Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's Rule. Alternatively, if you know your conception date, we add 266 days (38 weeks) to that date. The due date is an estimate - only about 5% of babies are born on their exact due date.

What is the difference between gestational age and fetal age?

Gestational age is measured from the first day of your last menstrual period and is what doctors typically use. Fetal age (also called embryonic age) is measured from conception, which occurs about 2 weeks after LMP in a typical 28-day cycle. So gestational age is usually about 2 weeks more than fetal age.

How accurate is a due date calculator?

Due date calculators provide an estimated due date. Only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the estimated due date. An early ultrasound (before 20 weeks) can provide a more accurate due date by measuring the baby directly.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1-13, when major organs form. The second trimester covers weeks 14-27, when the baby grows rapidly and you may feel movement. The third trimester is weeks 28-40, when the baby gains weight and prepares for birth. Full term is considered 37-42 weeks.

What if my menstrual cycle is not 28 days?

If your cycle is longer or shorter than 28 days, your ovulation and conception likely occurred at a different time. Our calculator allows you to adjust for cycle length. For cycles longer than 28 days, ovulation occurs later, so conception happens later. The reverse is true for shorter cycles.

When is a baby considered full term?

A pregnancy is considered full term at 37 weeks. Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42+ weeks. Babies born at full term generally have better health outcomes than those born earlier.

Related Resources

• Pregnancy Calendar - Create a detailed week-by-week pregnancy calendar
• Conception Date Calculator - Estimate when conception occurred
• ACOG - How Your Fetus Grows During Pregnancy

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