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.

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.

Pregnancy Week Calculator – Track Your Baby’s Growth Week by Week

Find out exactly which week of pregnancy you are in and explore week by week developmental milestones for your baby. Enter your dates to generate a personalized pregnancy timeline with trimester information and progress tracking.

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.

Tracking Your Pregnancy Week by Week

Pregnancy is divided into three trimesters spanning approximately 40 weeks from the first day of your last menstrual period. Each week brings remarkable developmental changes for your baby and new physiological experiences for you. Knowing your exact week helps you stay informed, prepare for upcoming milestones, and communicate effectively with your healthcare team.

The Three Trimesters

First Trimester (Weeks 1 to 12):

The most critical period for organ development. By week 12, all major organ systems have formed. The risk of miscarriage decreases significantly after this trimester. Key milestones include the heartbeat (week 6), limb formation (week 8), and measurable CRL on ultrasound (week 8 to 12).

Second Trimester (Weeks 13 to 27):

Often called the "golden period" because many early pregnancy symptoms ease. Your baby begins to move (quickening), and you may feel the first kicks between weeks 18 and 22. The anatomy scan at 18 to 22 weeks provides a detailed assessment of fetal structures.

Third Trimester (Weeks 28 to 40):

Rapid weight gain and brain development. Your baby's lungs mature significantly between 34 and 37 weeks. Prenatal visits become more frequent, and your provider will monitor fetal position, growth, and signs of labor readiness.

Benefits of Week by Week Tracking

Stay informed:

Know exactly what developmental stage your baby is at, from neural tube formation to lung surfactant production.

Prepare for milestones:

Understand when key prenatal tests are due and what to expect during each phase.

Manage expectations:

Each trimester has characteristic symptoms. Knowing what's normal for your current week reduces unnecessary anxiety.

Bond with your baby:

Learning that your baby is the size of a lemon, or that they can now hear your voice, makes the pregnancy experience more personal and real.

Visual progress tracking:

Our progress bar and trimester timeline give you a clear visual of how far you've come and how far you have to go.

Accuracy of Pregnancy Week Calculations

The accuracy of your pregnancy week depends on the dating method used. First trimester ultrasound is the most reliable clinical method, accurate to within ±5 to 7 days. LMP based dating assumes a regular cycle with ovulation on day 14. For IVF pregnancies, the week is known with near certainty. Our calculator mirrors the exact methodology used by healthcare providers, ensuring your week count matches what you see in your medical records.

Frequently Asked Questions

Why does my pregnancy app show a different week than my doctor?

Different apps may use different reference dates (LMP vs. ultrasound) or different rounding methods. Our calculator uses the same clinical standard as your provider: gestational age from LMP, displayed as completed weeks and days. If your doctor adjusted your due date based on an ultrasound, make sure to use the ultrasound tab for consistent results.

How do I count pregnancy weeks correctly?

Pregnancy weeks are counted from the first day of your last menstrual period. Week 1 starts on that day. When you have completed 4 full weeks, you are "4 weeks pregnant" and entering your 5th week. It works like age: after completing your first year of life, you turn 1.

What happens during the first 2 weeks of "pregnancy"?

During weeks 1 and 2, you are not actually pregnant yet. Your body is completing its menstrual cycle and preparing for ovulation. Conception typically occurs around the end of week 2 or the beginning of week 3 (day 14 of a 28 day cycle). This pre-conception period is included in gestational age by convention.

When will I feel my baby move?

Most first time mothers feel fetal movement (quickening) between 18 and 22 weeks. Experienced mothers may feel movement as early as 16 weeks. By 28 weeks, kick counting becomes a routine part of fetal monitoring. Contact your provider if you notice a significant decrease in movement.

Is it normal for my baby to measure ahead or behind?

Minor variations (within 1 to 2 weeks) in ultrasound measurements are completely normal and reflect natural biological variation. Significant discrepancies may lead your provider to revise your due date (in the first trimester) or investigate fetal growth patterns (in the second or third trimester).

What is the viability threshold for preterm babies?

Viability generally begins around 23 to 24 weeks, though outcomes vary significantly. At 24 weeks, survival rates in modern NICUs are approximately 40 to 70%. By 28 weeks, survival exceeds 90%. Every additional week in the womb significantly improves outcomes.

When should I start preparing for delivery?

Most healthcare providers recommend starting delivery preparations during the third trimester (after 28 weeks). This includes hospital bag packing, childbirth education classes, discussing birth preferences with your provider, and arranging parental leave. Having a hospital bag ready by 36 weeks is a common recommendation.

How many weeks is each month of pregnancy?

Approximately: Month 1 = Weeks 1 to 4, Month 2 = Weeks 5 to 8, Month 3 = Weeks 9 to 13, Month 4 = Weeks 14 to 17, Month 5 = Weeks 18 to 22, Month 6 = Weeks 23 to 27, Month 7 = Weeks 28 to 31, Month 8 = Weeks 32 to 35, Month 9 = Weeks 36 to 40. Medical professionals use weeks for precision.

What size is my baby this week?

After calculating your gestational age, our calculator displays a week specific developmental card showing your baby's current size comparison (e.g., "size of a lemon"), key developmental milestones, and helpful tips for that stage of pregnancy.

Why is pregnancy 40 weeks but only 9 months?

40 weeks equals 280 days, which is about 9 months and 7 days. The discrepancy arises because calendar months vary in length (28 to 31 days). Additionally, the first 2 weeks of gestational age occur before conception, making the actual pregnancy duration closer to 38 weeks (about 8 months and 3 weeks) from fertilization.

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.