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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

No. This calculator is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare provider for accurate pregnancy dating and prenatal care.

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.

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.

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.