What is an OB wheel?
An obstetric (OB) wheel – also called a gestation wheel or pregnancy wheel – is a pair of printed cardboard discs that doctors and midwives have used for decades. Line up the arrow for the first day of the last period with the calendar date, and the wheel shows the due date and the gestational age on any day. Pharmaceutical companies often hand them out, and many gynaecologists in India still keep one on their desk.
How to use this pregnancy wheel
- Choose LMP, due date or ultrasound, enter the date and press Set the wheel.
- The coloured ring shows the three trimesters; the pink arc and marker show how far along the pregnancy is on the selected date.
- The outer labels show the calendar date on which each 4-week mark falls.
- Use Spin the wheel to convert any date into weeks + days, or any week into a calendar date – for example, to find when you’ll reach 28 weeks.
Why a digital wheel is more accurate
Paper wheels have to squeeze 365 days onto a circle a few centimetres across, and some older designs don’t account for leap years. Studies comparing paper wheels with computer calculations found that wheels can be off by several days. A digital version uses exact date arithmetic, so the answer is always correct to the day.
Frequently asked questions
What is a pregnancy wheel used for?
It quickly converts between calendar dates and weeks of pregnancy, and shows the due date from the LMP.
Is the OB wheel accurate?
The method is the same as Naegele’s rule. Paper wheels can be off by a few days because of printing and alignment; a digital wheel is exact.
Can I use the pregnancy wheel with an ultrasound date?
Yes. Choose the Ultrasound tab and enter the scan date and gestational age from the report.
How do I find what date I’ll be 30 weeks?
Set the wheel, then choose 30 weeks 0 days in the ‘Gestational age → calendar date’ box.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org