OB Wheel (Pregnancy Calculator)
Welcome users!
This site is designed for mobile devices, but can be viewed on any size screen.
Use the menu in the upper right corner to navigate the site.
Click the Quick Help button on some pages for a guided tour!
Recent updates are listed in the Changelog.
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Contents
- About this app
- Key Features
- How to use
- Detailed instructions
- TimeMachine -- best feature!
About this app
This is a Progressive Web App version of my Android app, "OB Wheel".Works best on Google Chrome.
For Google Chrome on Android, the webapp can be installed on your mobile device like a normal app by going to "Add to Desktop" in your menu. You may be automatically prompted to install the app by the browser. If installation does not work correctly, you can just bookmark this page and use it as-is.
As far as I can find, this is the most versatile pregnancy calculator around. It will instantly calculate gestational age or due date from any other reference point in the pregnancy. This app calculates exactly 280 days (40 weeks) from the first day of a woman's last menstrual period (LMP) and the estimated due date (EDD), and accordingly for any date in between. It automatically adjusts for leap year if needed.
Please note that the circular physical pregnancy wheels used are inherently inaccurate. Different pregnancy wheels may vary by as much as 3 or 4 days. Please keep this in mind when comparing the values obtained here to those given in clinics/hospitals.
Key Features
Shareable calculations
Use theResponsive Web Design
Visit the page on different browsers or screen sizes and the layout will rearrange itself for optimal interaction.Calculate dates of pregnancy (from and to)
- Last menstrual period (LMP)
- Estimated due date (EDD)
- Estimated gestational age (EGA)
- Calculate gestational age from Ultrasound measurements
- Calculate gestational age from IVF embryo implantation date
- TimeMachine can calculate "How many weeks will she be on ___ date?" and "When will she be __ weeks gestation?"
How to Use
- Check out the how-to video on YouTube!
(slightly outdated)
- Touch/click the light blue bar on the right side of the rows to enter a desired date. App automatically calculates the rest.
- Touch & drag your finger horizontally on the row labels to increase/decrease the days/weeks.
- To calculate based on today's gestational age, touch/click the number input boxes on the EGA row. App will automatically start calculating as you enter the information.
- Calculate using ultrasound dating criteria: Touch/click the "Ultrasound" label to enter the date of ultrasound. Touch/click the number input boxes to enter the gestational age at the time of ultrasound. App will automatically calculate.
Detailed Instructions
Date selection method:
Everything you see is a button! Touch any of the light blue boxes to bring up the date selection calendar. You can calculate the dates and the gestational age using any date along the pregnancy as a reference point.
For example, selecting EDD will allow you to enter an Estimated Due Date, and the app will calculate the rest of the dates according to the date you entered. Selecting Conception will allow you to enter a date for the approximate date of conception, and the other dates will be calculated relative to this date. The calculated data will appear in the light blue text boxes in the right hand column.
You can also touch/click & drag along the boxes to smoothly scroll through the dates. Try it out! Abbreviations: LMP = Last Menstrual Period (use first day of period); EGA = Estimated Gestational Age; EDD = Estimated Due Date

You can also touch/click & drag along the boxes to smoothly scroll through the dates. Try it out! Abbreviations: LMP = Last Menstrual Period (use first day of period); EGA = Estimated Gestational Age; EDD = Estimated Due Date
Calendar Date picker:

- Touch/click any square to select that date
- Touch/click anywhere outside the floating calendar to cancel
Month selection:

- The double arrow buttons change the year
- The single arrow buttons change the month
- Touch the month name to bring up the month selection grid. Month can be selected directly

Direct date entry:

- Touch the calendar heading ("Estimated Due Date" in the example image) to bring up a text input box.
- Date can be entered in almost any format.
Sliding date selection
- touch/click and drag on the light blue date input boxes to scroll through dates
- [IMG]
Estimated Gestational Age (EGA):

- Touch EGA weeks input area to input weeks
- The cursor will automatically move to days input box if weeks value has 2 digits
- Days input box takes only one digit
- The dates will be calculated accordingly.
Buttons:
- Reset
- Delivery date
- Save
- Copy/share
- Patient list
TimeMachine
This is the feature that sets this app apart from all others!
It answers the questions: What would the gestational age be at ___ date? and When will the patient be __ weeks gestational age?
It answers the questions: What would the gestational age be at ___ date? and When will the patient be __ weeks gestational age?

- Touch the "TimeMachine" text to set the date; this will calculate the gestational age for any given date.
- Touch the weeks/days input boxes to enter a gestational age; this will calculate the date for a given gestational age.
- Using TimeMachine does not affect any of the other rows or the base data.
- TimeMachine calculation and directionality will remain even if primary calculation is changed
- eg - Select TM date to determine gestational age at that date. Change main calculation by entering new LMP/EDD/EGA/etc. TM row will still show the previously selected date with the corresponding new GA.
- If GA is entered for the TimeMachine, changing the main calculation will retain the TM GA and show the resulting TM date calculation.
- TimeMachine will reset if the calculator is manually reset, or a mode tab is selected (US/IVF/CRL)
Ultrasound dating:

In-vitro fertilization (IVF) dating:
- enter embryo transfer date
- cursor auto advances to embryo age input - one digit only

Crown-Rump Lenth (CRL) calculator
- enter date of CRL measurement (ultrasound)
- enter crown-rump measurement
- formulas are only validated up to 84 mm
- touch preferred formula to use that gestational age
- reference link for the selected formula will appear below the list


- Sahota et al: \(26.643 + 7.822 \sqrt{crl}\)
- Robinson & Fleming: \(23.73 + 8.052 * \sqrt{1.037 crl}\)
[Formula obtained from systematic review article by Napolitano et al] - Verburg et al: \(7 e^{1.4653 + 0.001737 crl + 0.2313 \log{crl} }\)
(original formula gave GA in weeks) - INTERGROWTH-21: \(40.9041 + 3.21585 \sqrt{crl} + 0.348956 crl\) https://intergrowth21.tghn.org
- Drumm et al: \({{0.374 + \sqrt{0.374^2 + 0.048 crl}} \over 0.024} \)
- Napolitano et al: Systematic review article, lists various formulas
Notes:
- enter free text to be saved as "patient information"
Cycle length
- drag slider to adjust menstrual cycle length
- (if accurate cycle length is not known, use the default 28 days)
- touch "Cycle Length" label to reset to default