What is an OB wheel, and how do clinicians use it?

An OB wheel — also called an obstetric wheel, pregnancy wheel, or gestational wheel — is a small, circular slide-rule that dates a pregnancy. You line up one known date on the outer ring, and the inner ring reads off the rest: the estimated due date, how many weeks and days along the pregnancy is today, and the dates of key milestones. For generations it has lived in the coat pocket of almost every obstetrician, midwife, and labour-and-delivery nurse.

The physical wheel is elegant precisely because it hides the arithmetic. Underneath, it is doing nothing more mysterious than counting calendar days. Our online OB wheel does the same counting, but without the rounding errors a cardboard wheel introduces, and it handles leap years and uneven month lengths exactly.

What the wheel actually calculates

Every OB wheel is built around a single assumption: a typical pregnancy lasts about 280 days, or 40 weeks, measured from the first day of the last menstrual period (LMP). Set the wheel's pointer to the LMP and the due-date marker lands 280 days later. That method is known as Naegele's rule, and it is still the starting point for menstrual dating in guidelines from the American College of Obstetricians and Gynecologists (ACOG) and the NHS.

From the same alignment, the wheel also shows gestational age: the number of completed weeks and days between the LMP and today. This is the number clinicians care about most, because so many decisions — when to offer certain screening tests, when a baby is considered term, when to act on a pregnancy that has gone past its date — are keyed to gestational age rather than to the due date itself.

Why clinicians still reach for a wheel

Speed. In a busy clinic, a provider needs a dating answer in seconds, often one-handed, sometimes mid-conversation with a patient. A wheel gives that answer without a phone, a login, or a spreadsheet. The trade-off is precision: a physical wheel can be a day or two off because of how the printed scale rounds, and it can't easily re-date a pregnancy from an ultrasound or an IVF transfer.

That's exactly where a well-built online calculator earns its place. It keeps the one-glance speed of the wheel while adding exact date math and extra modes the cardboard version can't do.

The limits of the last-period method

Menstrual dating assumes ovulation on day 14 of a regular 28-day cycle. Real cycles vary, and many people don't know their LMP precisely. For that reason, a first-trimester ultrasound is generally the most accurate way to date a pregnancy, and professional guidance recommends dating by an early scan when a reliable one is available. A good wheel lets you switch to ultrasound dating when you have it — see how accurate is your due date for the details.

OB wheel vs. app vs. website

The paper wheel, a phone app, and a web calculator all answer the same question. The difference is convenience and trust. An app needs installing and updating; a website loads instantly and, if it's built well, keeps your data on your own device. Our version runs entirely in your browser — nothing you type is sent anywhere. If you're weighing the options, see OB wheel app vs. online calculator.

Using the wheel responsibly

Whatever the format, a dating wheel produces an estimate. Methods routinely disagree by a few days, and the estimated due date is a reference point, not a guarantee. It's a starting place for a conversation with a clinician, not a replacement for one.