Q & A


See the help page for detailed instructions.

Who created this app

The OBWheel app was created by Gary C Wang, MD, board-certified ObGyn physician.

The Why of OB Wheel

I created the app in medical school during my ObGyn rotation. I became frustrated with the available pregnancy calculator apps after trying nearly every Android app and web app available. All the apps I encountered were very cumbersome to use, with very poor user interface design. Incidentally I also came to discover that the circular physical pregnancy wheels were inaccurate and highly prone to errors due to imprecise alignment. Different pregnancy wheels could vary by as much as 3 or 4 days.

Problems with other apps

Many of the apps required first a "mode" selection before any input is made. The user is asked to choose from LMP, EDD, or gestational age calculation types. Once the calculation mode is selected the user is usually taken to another page to make the actual input.
In most apps the methods of date selection depended on the user's operating system, or in the case of web apps the user's browser. Often the date was input by selecting the year, month, and day from separate drop down lists.
Once the date selection is made, most apps also required the user to click a "submit" or "calculate" button to finally get the result. The result would usually appear on a different page, or at least replace the visible content (which amounts to the same thing).
Some web apps are written in a way that requires the date selection to be sent to the server. The calculation is performed on the server side after which the results are returned along with a whole new web page. This process takes more time and uses bandwidth unnecessarily.
While all of these little details may seem inconsequential, they add up. The multi-tier process creates visual disruptions that increase user effort, which simply is not good design.

Creating OB Wheel

I had previously created a few other simple Android apps, so I decided to try my hand at something a little more complex. So I created OB Wheel for Android.
After publishing the Android app 2010-03-12, I received numerous requests for an iOS version. Although I used to be a solid Apple fan, I had transitioned over to the far geekier Linux and no longer had any active apple or iOS devices. Because of Apple's exclusivity, in order to publish apps in the iTunes/App store I would need a Mac running the most recent MacOS, an iOS device, plus pay the Apple Developer Program membership fee of US$99 per year.
In contrast, app development for Android is much more affordable and open. The developing environment runs on Mac, Windows, or Linux. Developer registration is a one time fee of US$25. App developers can also choose to publish/sell their apps on third-party (non-Google) app stores.
I kept the app as an Android app because starting up development in iOS would have been expensive in terms of both time and money, for minimal return. I also thought (and still think) that the annual developer's membership fee is a little ridiculous and potentia lly prohibitive for some beginning developers.
Some time ago (in 2018) I decided to convert the app into a web app to make it available to more people. From a developer standpoint, web apps are much easier and faster to deploy.

How is this app better than others?

  • Instant calculation - no finicky flipping through multiple screens or pages, or having to wait for results to return from the server
  • Time zone independent (see FetalMedicine.org for an example of this problem)
  • Accounts for leap year - calculator measures 280 calendar days
  • Calculate by CRL
  • Calculate by embryo transfer (IVF) date
  • Calculate by gestational age (reverse calculation)
  • TimeMachine function (see the section in the help page)
  • Input date will never be "out of range" (see MDcalc.com for an example of this problem.)

How do I know this is accurate?

Nearly all date-based pregnancy calculators use an interval of 280 days from LMP to EDD. To check if a pregnancy calculator is giving the correct result, you can use a date interval calculator/counter such as this Days Interval calculator. The number of days from LMP to EDD should be 280 days, assuming a 28 day cycle length.