OB Wheel Calculator

Estimate Your Due Date with the OB Wheel Calculator

Calculate your pregnancy timeline instantly. Using the clinically trusted Naegele's Rule, our digital obstetric wheel estimates your Expected Due Date (EDD), gestational age, trimester milestones, and key pregnancy phases.

Interactive Obstetric Wheel

Calculate Your Due Date

Enter the first day of your last menstrual period (LMP) and average cycle length to map out your pregnancy milestones.

days

Medical Disclaimer: This obstetric wheel calculator provides estimates based on standard clinical calculations. It is intended for educational and informational purposes only and does not substitute professional medical diagnosis, advice, or treatment. Always align with a licensed healthcare provider regarding your unique pregnancy details.

How OB Wheel Calculator Works

Three simple steps to estimate your pregnancy due date accurately.

1

Enter Your LMP Date

Input the first day of your last menstrual period into our calculator.

2

Automatic Calculation

Our tool uses Naegele's Rule (LMP + 280 days) to calculate your estimated due date.

3

View Your Results

Get your EDD, gestational age, trimester dates, and key pregnancy milestones instantly.

Pregnancy Weeks to Months Converter

Understand how pregnancy weeks translate into months and trimesters. Clinicians date by weeks, while families track by months.

Month 1

1st Trimester

Weeks 1–4

Development: Conception & Implantation

Month 2

1st Trimester

Weeks 5–8

Development: Embryonic Stage & Heartbeat

Month 3

1st Trimester

Weeks 9–13

Development: Fetal Transition & Early Checkups

Month 4

2nd Trimester

Weeks 14–17

Development: Golden Trimester Starts

Month 5

2nd Trimester

Weeks 18–21

Development: Anatomy Scan & Quickening

Month 6

2nd Trimester

Weeks 22–27

Development: Fetal Growth & Viability Marker

Month 7

3rd Trimester

Weeks 28–31

Development: Final Stretch Begins

Month 8

3rd Trimester

Weeks 32–35

Development: Rapid Brain & Lung Maturation

Month 9

3rd Trimester

Weeks 36–40

Development: Full Term Status & Delivery

Braxton Hicks vs. True Labor Signs

Learn how to differentiate practice contractions from the actual onset of active labor.

Practice Contractions

Braxton Hicks Signs

  • Irregular Timing Contractions occur randomly, are spaced far apart, and do not get closer together.
  • Weakens with Movement Contractions often slow down or completely stop when you change positions, walk, or drink water.
  • Frontal Discomfort Tightness is primarily felt in the front of the abdomen or pelvis and does not radiate.
Active Delivery

True Labor Signs

  • Regular, Progressive Intervals Contractions occur at regular intervals, last 30–70 seconds, and consistently get closer together.
  • Intensifies Regardless of Rest Contractions continue or increase in strength even if you move, walk, rest, or hydrate.
  • Back-to-Front Pain & Water Break Pain begins in the lower back and radiates around to the front. May accompany amniotic fluid leakage.

Pregnancy Milestones & Gestational Age Timeline

Trace key developmental markers and clinical milestones throughout your forty-week pregnancy roadmap.

Week 4

Positive Pregnancy Test

1st Trimester

hCG levels detectable; embryo implantation is complete.

Week 6

Heartbeat Detectable

1st Trimester

Fetal heartbeat becomes visible on early transvaginal ultrasound scans.

Week 8

First Prenatal Visit

1st Trimester

Dating ultrasound scan, initial screening panel, and prenatal checkups.

Week 10

Fetal Movements Begin

1st Trimester

Reflexive fetal movements begin; these are still too early to feel.

Week 12

End of First Trimester

1st Trimester

Nuchal translucency scan option; miscarriage risk drops significantly.

Week 13

Second Trimester Begins

2nd Trimester

Maternal energy levels often return; early morning sickness begins to clear.

Week 16

Gender May Be Discerned

2nd Trimester

Anatomical traits begin to display; early quickening sensations occur.

Week 20

Mid-Pregnancy Anatomy Scan

2nd Trimester

Detailed structural ultrasound scan checking vital organs and growth progress.

Week 24

Clinical Fetal Viability

2nd Trimester

Fetus reaches medical viability milestones requiring intensive NICU support.

Week 28

Third Trimester Begins

3rd Trimester

Glucose tolerance screening; daily fetal movement checkups are initiated.

Week 36

Fetal Descending (Lightening)

3rd Trimester

Baby descends lower into the pelvis, helping ease maternal respiration.

Week 37

Early Term Milestone

3rd Trimester

Fetal system counts as early term; lung pathways are fully operational.

Week 39–40

Full Term Delivery Window

3rd Trimester

Pregnancy reaches full term status; labor onset can occur at any time.

Prenatal Screening & Scan Timeline

Plan your pregnancy clinic schedule. Review key tests and scans recommended at each gestational age.

Weeks 8–10

First Prenatal Checkup

Ultrasound dating checkup, clinical history analysis, initial blood work panel, and GYN screening.

Routine Booking Visit
Weeks 11–13

First Trimester Screening

Nuchal Translucency (NT) ultrasound scan combined with NIPT blood screen to check chromosome health.

Genetic Screen Window
Weeks 18–22

Mid-Pregnancy Anatomy Scan

Detailed anatomical structure ultrasound scanning all major organs, growth rates, and placental location.

Essential Growth Check
Weeks 24–28

Glucose Challenge Test

Gestational diabetes screening checklist along with maternal anemia screening panel checks.

Metabolic Assessment
Weeks 35–37

Group B Strep (GBS) Screening

Swab check routine to verify presence of GBS bacteria to prepare delivery room safety guidelines.

Pre-Delivery Safety Check

Pregnancy Wheel vs Due Date Calculator

While both tools estimate your pregnancy timeline, their mechanisms, accuracy, and ease of use differ significantly.

Traditional Pregnancy Wheel

A physical, circular slide rule traditionally made of paperboard or plastic. Clinicians rotate the wheel manually to line up the Last Menstrual Period (LMP) date with the calendar.

  • ✕ Requires manual rotation, introducing human alignment errors.
  • ✕ Assumes a rigid 28-day cycle and cannot adjust for irregular periods.
  • ✕ Fails to account for leap years, shifting the estimation by 1–2 days.
Manual Office Tool

Digital Due Date Calculator

An advanced, software-based implementation of the obstetric wheel. It automates Naegele's rule formulas instantly via input forms to yield precise calculations.

  • ✓ Calculates dates instantly and mathematically with zero alignment slip.
  • ✓ Fully customisable cycle lengths ranging from 20 to 45 days.
  • ✓ Accounts for leap years and calendar variations automatically.
Precise Digital Solution

Methods for Calculating Estimated Due Date

Medical professionals employ several clinical methods to establish or refine your pregnancy timeline.

LMP

Naegele's Rule

The standard calculation formula: adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a regular 28-day cycle.

Standard Standard
28+

Parikh's Rule

Used for irregular or non-28 day cycles. It adjusts by adding or subtracting the difference: LMP + 280 Days + (Cycle Length - 28 Days).

Irregular Cycle Adjuster
USG

Ultrasound Dating

Dating via crown-rump length (CRL) during a first-trimester scan (before 13 weeks). It has a tight clinical accuracy margin of ±5-7 days.

Clinical Dating Gold Standard
IVF

Conception / IVF

Calculated based on the exact day of ovulation or the transfer date of a day-3 or day-5 embryo, offering highly precise scheduling milestones.

Assisted Reproduction

IVF Due Date Calculator & Guide

Dating an IVF pregnancy differs from natural conception. Instead of your last period, the timeline is anchored to embryo age and transfer date.

How IVF Dating Works

In natural conception, ovulation is assumed to happen on Day 14 of the cycle. In IVF, the exact age of the embryo (typically 3 or 5 days old) and the precise transfer date are known. This allows for highly precise due date tracking:

3D

Day-3 Embryo Transfer (Cleavage Stage)

Due date is calculated as: **Transfer Date + 263 days**.

5D

Day-5 Embryo Transfer (Blastocyst Stage)

Due date is calculated as: **Transfer Date + 261 days**.

*Clinical Note: In both cases, the fetal age is matched exactly, establishing Day 14 as ovulation.

IVF Dating Calculator

Input your embryo details below to instantly calculate your estimated due date (EDD) and current milestone.

Glossary of Obstetric Terms

De-mystify clinical charts. Review common abbreviations used on your pregnancy ultrasound reports and checks.

EDD

Estimated Due Date

The estimated date when spontaneous onset of labor is expected (historically LMP + 280 days).

LMP

Last Menstrual Period

The first day of your last menstrual period; used as the default starting benchmark for clinical gestational dating.

GA

Gestational Age

The age of the pregnancy measured in weeks and days from the first day of the last menstrual period (LMP).

CRL

Crown-Rump Length

The ultrasound measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump).

hCG

human Chorionic Gonadotropin

A hormone produced by the placenta after implantation. Its detection is the basis of most home pregnancy tests.

FHR

Fetal Heart Rate

The number of heartbeats per minute of the fetus. Typically ranges from 110 to 160 beats per minute in late pregnancy.

Frequently Asked Questions

Find answers to the most common questions about pregnancy due date calculation.

What is an OB Wheel Calculator?

An OB Wheel Calculator (obstetric wheel) is a tool used by healthcare providers and expectant mothers to estimate the Expected Due Date (EDD) based on the first day of the Last Menstrual Period (LMP). Our digital version automates the calculation using Naegele's Rule.

How does the OB Wheel Calculator work?

The calculator uses Naegele's Rule: it adds 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). This is the same method used by most OB-GYNs and midwives worldwide.

How accurate is the OB Wheel Calculator?

The OB Wheel Calculator provides an estimate based on a standard 28-day cycle. Only about 5% of babies are born on their exact due date. Your doctor may adjust the EDD based on ultrasound measurements, especially in the first trimester.

What is Naegele's Rule?

Naegele's Rule is the standard formula for calculating the estimated due date: EDD = LMP + 280 days. Alternatively, subtract 3 months from LMP, add 7 days, and add 1 year. It assumes a 28-day menstrual cycle with ovulation on day 14.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your LMP and is typically 2 weeks longer than fetal age. Fetal age (embryonic age) is counted from the estimated date of conception. Medical professionals use gestational age as the standard.

Can I use the OB Wheel Calculator if I have irregular periods?

The calculator works best with a regular 28-day cycle. For irregular periods, your healthcare provider may use ultrasound dating or alternative formulas like Parikh's Rule for a more accurate estimate.

When should I see a doctor to confirm my due date?

Schedule your first prenatal visit between 8-10 weeks of pregnancy. A first-trimester ultrasound (before 13 weeks) provides the most accurate dating, with a margin of error of only ±5-7 days.

Is this OB Wheel Calculator a substitute for medical advice?

No. This calculator is for informational and educational purposes only. Always consult with your healthcare provider for personalized medical advice regarding your pregnancy.