How to read a pregnancy wheel (the circle chart), step by step

A pregnancy wheel — sometimes called a pregnancy circle chart — looks intimidating the first time you hold one, with two rings of dates and a scatter of little markers. It's simpler than it looks. The whole device does one thing: it lines up a calendar with a pregnancy timeline so you can read one from the other.

The two rings

The outer ring is a calendar: every date of the year, printed once around the circle. The inner ring is a pregnancy timeline, marked in weeks of gestation from 0 up to about 42. There's usually an arrow or window labelled “LMP” or “first day of last period,” and a separate marker labelled “EDD” or “40 weeks.”

Step by step

1. Find the LMP. Rotate the inner ring until its “LMP” arrow points to the first day of the last menstrual period on the outer calendar ring.
2. Read the due date. Without moving anything, look at the “40 weeks” or “EDD” marker on the inner ring. The calendar date it now points to is the estimated due date.
3. Read today's gestational age. Find today's date on the outer ring. The week mark on the inner ring beside it is the current gestational age — for example, “24 weeks.” The small ticks between week numbers are individual days.
4. Read milestones. The wheel usually prints the same date for landmarks such as the end of the first trimester and the start of term; each sits at its week mark on the inner ring.

What the markers mean

Beyond the LMP and EDD, most wheels flag a handful of clinically useful weeks: the start of the second trimester around 14 weeks, viability around 24 weeks, and the term window from 37 weeks. If you'd like those explained, see pregnancy trimesters and milestones.

Common mistakes when reading the wheel

• Using the wrong start date. Dating begins on the first day of the last period, not the day it ended, and not the estimated day of conception.
• Ignoring cycle length. A paper wheel assumes a 28-day cycle. If yours is longer or shorter, the reading can be off by several days.
• Reading the wrong ring. It's easy to read a nearby calendar date as a week mark. Follow the tick you want straight across.
• Trusting it over a scan. If an early ultrasound gives a different date, the scan usually wins.

Why an online circle chart is easier to read

A digital wheel removes the two biggest sources of error: mis-aligning the rings and mis-reading a tick. You type one date and the calculator returns the due date, the exact gestational age in weeks and days, and the milestone dates as plain calendar entries. It also adjusts for cycle length automatically and can re-date from an ultrasound or IVF transfer, which a printed wheel can't. If you're curious how the numbers are produced, the how it's calculated page shows every formula.