What is an OB wheel calculator?

An OB wheel calculator (obstetric wheel or pregnancy wheel) is a medical tool used to estimate a pregnant woman's due date, gestational age, and other key pregnancy milestones. It is based on the same calculation methods used by obstetricians and midwives in clinical practice.

How does an OB wheel calculator work?

The calculator primarily uses Naegele's Rule, which estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). It also supports ultrasound dating, IVF transfer date calculations, and reverse calculations from a known due date.

What is Naegele's Rule and how is it used to calculate due dates?

Naegele's Rule is a standard formula used in obstetrics: Estimated Due Date = LMP + 280 days. It assumes a 28-day menstrual cycle with ovulation occurring on day 14. For cycles of different lengths, an adjustment is made by adding or subtracting the difference from 28 days.

How accurate is an OB wheel calculator?

OB wheel calculators provide a medically accepted estimate, but only about 5% of babies are born on their exact estimated due date. Full-term delivery typically occurs within a range of 37 to 42 weeks. Accuracy depends on the reliability of the input data, especially the LMP date.

How do I calculate my due date from my last menstrual period (LMP)?

Select the LMP Method tab, enter the first day of your last menstrual period, and optionally adjust your average cycle length. The calculator will apply Naegele's Rule to estimate your due date, gestational age, and trimester dates.

What if I don't remember the exact date of my LMP?

If you are unsure of your exact LMP date, consult your healthcare provider. An early pregnancy ultrasound (dating scan) can provide an accurate estimate of gestational age. You can use our Ultrasound tab to calculate based on ultrasound findings.

How does an ultrasound estimate gestational age?

During early pregnancy, ultrasound measures the crown-rump length (CRL) of the embryo to determine gestational age. First-trimester ultrasounds (before 14 weeks) are the most accurate for dating, with a margin of error of approximately 5 to 7 days.

What is a dating ultrasound?

A dating ultrasound is an early pregnancy scan specifically performed to determine gestational age and estimate the due date. It is most accurate when performed between 8 and 12 weeks of pregnancy.

When should I get my first prenatal ultrasound?

Most healthcare providers recommend a first ultrasound between 8 and 12 weeks of pregnancy. This dating scan helps confirm the pregnancy, determine gestational age, and check for the fetal heartbeat.

Can I use this calculator for IVF pregnancies?

Yes. Select the IVF Transfer tab and enter the date of your embryo transfer and whether it was a Day 3 or Day 5 transfer. IVF due dates are calculated using the formula: Transfer Date + (266 - Embryo Age) days, making them typically the most precise.

What is the difference between a Day 3 and Day 5 embryo transfer?

A Day 3 transfer uses an embryo that has been developing for 3 days after fertilization (cleavage stage). A Day 5 transfer uses a blastocyst that has developed for 5 days. Day 5 transfers are more common and the calculation accounts for the embryo's age at transfer.

How is the conception date calculated?

The estimated conception date is calculated as LMP plus the cycle length minus 14 days. For a standard 28-day cycle, this places conception approximately on day 14, which corresponds to the expected time of ovulation.

What is the difference between gestational age and fetal age?

Gestational age is measured from the first day of the last menstrual period (LMP) and includes approximately 2 weeks before conception actually occurs. Fetal age (embryonic age) is measured from the actual date of conception. Gestational age is always about 2 weeks more than fetal age.

How many weeks is a full-term pregnancy?

A full-term pregnancy is 39 to 40 weeks according to ACOG (American College of Obstetricians and Gynecologists). Early term is 37 to 38 weeks, late term is 41 weeks, and post-term is 42 weeks or later.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1 through 12, the second trimester spans weeks 13 through 27, and the third trimester spans weeks 28 through 40 (delivery). Each trimester is marked by specific developmental milestones.

When does each trimester start and end?

The first trimester runs from the LMP to the end of week 12. The second trimester starts at week 13 and ends at week 27. The third trimester begins at week 28 and continues until delivery, which typically occurs around week 40.

What if I have irregular menstrual cycles?

If you have irregular cycles, LMP-based calculations may be less accurate. Enter your best estimate of average cycle length, or use the Ultrasound method for a more reliable due date. Consult your healthcare provider for the most accurate dating.

How does cycle length affect my due date calculation?

Naegele's Rule assumes a 28-day cycle. If your cycle is longer or shorter, the calculator adjusts the due date accordingly. For example, a 35-day cycle would add 7 extra days to the standard calculation, resulting in a later due date.

What percentage of babies are born on their exact due date?

Only about 4 to 5 percent of babies are born on their exact estimated due date. Most healthy babies are born within a window of 2 weeks before or after the EDD.

What is preterm birth?

Preterm birth is delivery before 37 weeks of gestation. Babies born preterm may require additional medical care in a neonatal intensive care unit (NICU). Risk factors include multiple pregnancies, infections, and certain medical conditions.

What is post-term pregnancy?

A post-term pregnancy extends beyond 42 weeks of gestation. Healthcare providers typically recommend induction of labor or increased monitoring after 41 weeks due to potential risks to both mother and baby.

How do hospitals and doctors determine expected delivery dates?

Doctors use a combination of LMP date, early ultrasound measurements, and clinical examination to determine the expected delivery date. When there is a discrepancy between LMP and ultrasound dating, the ultrasound estimate is generally preferred if performed in the first trimester.

Is an online OB wheel calculator as accurate as a doctor's assessment?

An online OB wheel calculator uses the same mathematical formulas as clinical tools. However, a doctor's assessment includes additional factors such as ultrasound measurements, physical examination, and medical history that an online tool cannot evaluate. Always consult your healthcare provider.

Can I use this calculator if I am not sure about my dates?

You can use this calculator with your best estimate, but the results will only be as accurate as the information you provide. If you are uncertain about your dates, we recommend consulting your healthcare provider for an ultrasound-based assessment.

What is the history of the obstetric wheel in medicine?

The obstetric wheel is based on Naegele's Rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. The physical cardboard wheel with two rotating disks became a standard clinical tool in the 20th century and has since been digitized into online calculators.

How is this calculator different from other pregnancy calculators online?

This calculator offers four calculation methods (LMP, Ultrasound, IVF, and Known Due Date), provides comprehensive results including gestational age, trimester dates, pregnancy progress, and week-by-week developmental information. It is free, requires no registration, works on all devices, and follows the same medical standards used in clinical practice.

Due Date Calculator – Estimate Your Baby’s Arrival Date

Instantly calculate your estimated due date (EDD) using the standard Naegele's Rule formula trusted by obstetricians, midwives, and healthcare professionals worldwide. Enter your dates below for accurate results in seconds.

First Day of Your Last Menstrual Period (LMP) Select the first day of your most recent menstrual period.

Average Menstrual Cycle Length (days) Most cycles are 28 days. Normal range is 21–35 days.

How the Due Date Calculator Works

Our Due Date Calculator uses Naegele's Rule, the gold standard formula in obstetrics, to estimate when your baby will arrive. The calculation adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), then adjusts for your unique cycle length. This is the same method used by obstetricians in clinical practice and is recommended by the American College of Obstetricians and Gynecologists (ACOG) as the primary tool for establishing an estimated due date when early ultrasound is not available.

Three Methods for Maximum Accuracy

LMP Method:

The most widely used approach. Enter the first day of your last period and your average cycle length. The calculator applies Naegele's Rule with an automatic cycle length adjustment for non-28-day cycles.

Ultrasound Dating:

Considered the most accurate dating method. Based on crown rump length (CRL) measurements taken during your first trimester ultrasound. Most reliable between 8 and 13 weeks of gestation.

IVF Transfer:

The most precise calculation available because the exact fertilization date is known. Enter your embryo transfer date and whether it was a Day 3 or Day 5 transfer.

Benefits of Using Our Due Date Calculator

Instant results:

No page reloads. Your estimated due date, gestational age, trimester dates, and pregnancy progress appear immediately.

Cycle length adjustment:

Unlike basic calculators that assume a 28 day cycle, ours adjusts for cycle lengths between 20 and 45 days.

Complete pregnancy timeline:

See your conception date, current trimester, trimester start/end dates, and a visual progress bar, all from a single calculation.

Week by week information:

After calculating, view developmental milestones and size comparisons for your current pregnancy week.

Privacy first design:

All calculations happen in your browser. We never collect, transmit, or store your personal health information.

Copy, print, share, or download:

Save your results in the format that works best for you. Take them to your next prenatal appointment.

Accuracy of Due Date Calculations

According to research published in the Journal of Obstetric, Gynecologic & Neonatal Nursing, Naegele's Rule provides a clinically useful estimate, but it is important to understand its limitations:

Only 4 to 5% of babies are born on their exact estimated due date.

About 80% of babies are born within 2 weeks (before or after) of their EDD.

First-trimester ultrasound dating is accurate to within ±5 to 7 days and is considered the most reliable method for establishing an EDD.

ACOG recommends that if ultrasound dating differs from LMP dating by more than 5 days (before 9 weeks) or more than 7 days (between 9 to 14 weeks), the ultrasound date should be used as the official EDD.

Understanding Your Results

Estimated Due Date (EDD)

Your EDD represents the date at which you will have completed 40 weeks (280 days) of gestation from the first day of your LMP. This is the central target around which your healthcare provider plans prenatal care, screening tests, and delivery preparations.

Gestational Age

Displayed in weeks and days (e.g., "32 weeks and 4 days"), this tells you exactly how far along your pregnancy is today. Gestational age is essential for timing prenatal tests, monitoring fetal growth, and determining whether a birth is considered preterm, full term, or post-term.

Conception Date

The estimated date of fertilization is calculated based on your cycle length and the assumption that ovulation occurs approximately 14 days before the next expected period. For IVF pregnancies, this is known precisely.

Frequently Asked Questions

How accurate is the Naegele's Rule due date calculation?

Naegele's Rule is the standard clinical formula used worldwide. However, only about 4 to 5% of babies are born on their exact EDD. The estimate is most accurate when combined with early ultrasound dating. Think of the due date as the center of a 4 week window (2 weeks before and after) during which delivery is most likely.

Can my due date change during pregnancy?

Yes. If an early ultrasound (before 14 weeks) shows a gestational age that differs significantly from the LMP based estimate (more than 5 to 7 days depending on gestational age), your healthcare provider will likely revise your official due date to match the ultrasound.

What if I have an irregular menstrual cycle?

If your cycles are irregular, enter your best estimate of average cycle length. Keep in mind that LMP based calculations are less accurate with irregular cycles. In this case, early ultrasound dating is strongly recommended for a more reliable EDD.

How does cycle length affect my due date?

Naegele's Rule assumes a standard 28 day cycle with ovulation on day 14. If your cycle is longer (e.g., 35 days), ovulation likely occurred later, so your due date is pushed back by 7 days. Our calculator automatically applies this adjustment, adding or subtracting the difference from 28 days.

Which due date method is the most accurate?

IVF dating is the most precise because the exact fertilization date is known. First trimester ultrasound is the most accurate clinical method (±5 to 7 days). LMP based dating is the most common but depends on accurate recall of the last period and regular cycle length.

What percentage of babies are born on their due date?

Only about 4 to 5% of babies are born on their exact estimated due date. Most healthy full term babies are born within a window of 37 to 42 weeks of gestation.

When is a pregnancy considered full term?

According to ACOG, Early term is 37w0d to 38w6d, Full term is 39w0d to 40w6d, Late term is 41w0d to 41w6d, and Post term is 42w0d and beyond.

What is the difference between an estimated due date and a delivery date?

The EDD is a mathematical estimate based on standard obstetric formulas. The actual delivery date depends on many factors, including fetal development, maternal health, and the natural course of labor. Your healthcare provider will monitor your pregnancy and discuss delivery timing with you.

Should I use LMP or ultrasound for my due date?

ACOG recommends using the LMP date initially, but adjusting to the ultrasound date if there is a significant discrepancy. First trimester ultrasound is generally considered more reliable than LMP recall, especially for women with irregular cycles.

How early can a due date be calculated?

You can calculate your estimated due date as soon as you know the first day of your last menstrual period. For clinical confirmation, most providers recommend a dating ultrasound between 8 and 12 weeks of pregnancy.

Does this calculator store my personal health data?

No. All calculations are performed entirely in your web browser using JavaScript. No dates, results, or personal information are ever transmitted to our servers or stored in any database. Your privacy is fully protected.

Is this calculator a substitute for seeing my doctor?

No. This calculator provides a medically based mathematical estimate, but it is not a substitute for professional prenatal care. Always consult with your healthcare provider for a clinical assessment, ultrasound confirmation, and personalized pregnancy guidance.

Medical Disclaimer: This calculator is for educational and informational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician, obstetrician, midwife, or other qualified healthcare provider for accurate pregnancy dating and personalized prenatal care. No personal health data is collected, transmitted, or stored by this tool.