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.

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.

Can my due date change during pregnancy?

Yes. If an early ultrasound shows a gestational age significantly different from the LMP-based estimate (more than 7 days), your healthcare provider may revise your due date. First-trimester ultrasound is considered the most reliable for dating.

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.

Gestational Age Calculator – How Many Weeks Pregnant Are You?

Determine exactly how many weeks and days pregnant you are right now. Tracking your precise gestational age is essential for timing prenatal screenings, monitoring fetal development, and planning for delivery.

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.

Understanding Gestational Age

Gestational age is the standard medical measurement for how far along a pregnancy has progressed. It is counted in weeks and days from the first day of the mother's last menstrual period (LMP), not from the date of conception. This means the first two weeks of pregnancy occur before fertilization actually takes place. This dating convention is used universally in obstetric medicine because the LMP date is generally easier to identify than the exact moment of conception. All prenatal care guidelines, growth charts, and screening test schedules are based on gestational age.

Why Gestational Age Matters

Prenatal Testing Windows:

Many critical screenings have strict gestational-age windows. For example, the nuchal translucency scan must be performed between 11 weeks 0 days and 13 weeks 6 days, while the anatomy scan is best performed between 18 and 22 weeks.

Fetal Growth Monitoring:

Healthcare providers compare your baby's measurements against standardized growth curves indexed by gestational age. This helps identify potential growth restriction or macrosomia.

Delivery Classification:

Whether a birth is classified as preterm, early term, full term, late term, or post term depends entirely on the gestational age at delivery. This classification affects neonatal care decisions.

Medication Dosing:

Certain medications given during pregnancy (such as corticosteroids for lung maturity) are timed based on precise gestational age.

Benefits of Tracking Your Gestational Age

Know your exact position:

See weeks, days, and your overall progress through the 40 week timeline at a glance.

Stay on schedule:

Never miss a time sensitive prenatal test by knowing your exact gestational week.

Understand your baby's development:

Each week brings specific milestones. Our calculator shows you what's happening right now.

Communicate clearly:

When you tell your provider you are 28 weeks and 3 days, you are speaking the same clinical language used in your medical records.

Accuracy and Clinical Standards

The accuracy of gestational age dating depends on the method used:

LMP based dating:

Accurate to within ±2 weeks when the patient has regular 28 day cycles and a reliable recall of the LMP date.

First-trimester ultrasound (before 14 weeks):

Accurate to within ±5 to 7 days. This is the gold standard for clinical dating.

Second-trimester ultrasound (14–28 weeks):

Accurate to within ±10 to 14 days.

IVF dating:

The most precise method because the exact date of embryo transfer is known.

ACOG recommends that the best obstetric estimate of gestational age should be determined using the combination of LMP date and the earliest available ultrasound.

Frequently Asked Questions

What is the difference between gestational age and fetal age?

Gestational age is measured from the first day of the LMP and includes approximately 2 weeks before conception. Fetal age (embryonic age) is measured from the actual date of fertilization. Gestational age is always about 2 weeks greater than fetal age. Medical records and prenatal care are based on gestational age.

How is gestational age written in medical records?

Gestational age is typically written as weeks and days. For example, 28w4d means 28 weeks and 4 days. In some contexts, it is expressed as a fraction: 28 4/7 weeks.

Am I in my 12th week or my 13th week of pregnancy?

If your gestational age is 12 weeks and 3 days, you have completed 12 full weeks and are currently living through your 13th week. Think of it like birthdays: when you are 12 weeks old, you are in your 13th week of life.

Why does pregnancy math seem so confusing?

Because gestational age starts from the first day of your last period, the first 2 weeks of your pregnancy occur before conception. When you are 4 weeks pregnant by gestational age, your embryo is actually only about 2 weeks old. This is a universal convention in medicine.

Can gestational age be determined without knowing the LMP?

Yes. An early ultrasound can measure the embryo's crown-rump length (CRL) and estimate gestational age independently of the LMP date. This is especially useful for women who cannot recall their LMP or have irregular cycles.

What prenatal tests are timed by gestational age?

Key tests include First trimester combined screening (11 to 14 weeks), cell free DNA screening (after 10 weeks), anatomy scan (18 to 22 weeks), glucose tolerance test (24 to 28 weeks), Group B Streptococcus screening (36 to 37 weeks), and antenatal corticosteroids if preterm delivery is expected (24 to 34 weeks).

How is gestational age different from pregnancy months?

Pregnancy is medically tracked in weeks, not months, because months vary in length (28 to 31 days). Roughly weeks 1 to 4 are equal to month 1, weeks 5 to 8 are equal to month 2, and so on through weeks 37 to 40 are equal to month 9. However, weeks provide much more precision for clinical care.

What is considered preterm, full term, and post term?

Per ACOG definitions Preterm is before 37 weeks, Early term is 37w0d to 38w6d, Full term is 39w0d to 40w6d, Late term is 41w0d to 41w6d, and post term is 42 weeks and beyond. These classifications affect care decisions around delivery timing.

How often should gestational age be reassessed?

Once an EDD is established using the best available data (ideally first trimester ultrasound), it should not be changed unless a significant discrepancy is found. Recalculation in the second or third trimester is less reliable and generally not recommended.

Does this calculator work for twin pregnancies?

This calculator estimates gestational age based on standard singleton pregnancy formulas. Twin and higher order multiple pregnancies often deliver earlier (average 36 to 37 weeks for twins). Consult your maternal fetal medicine specialist for twin-specific dating and delivery planning.

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.