Period of gestation (POG): what it means and how to calculate it

If you've seen POG written on an antenatal chart and wondered what it meant, it's simply another name for how far along a pregnancy is. POG stands for “period of gestation,” and it's used interchangeably with gestational age in many clinics, particularly across the UK, South Asia, and other regions that follow British obstetric convention.

POG and gestational age are the same thing

Both describe the time elapsed since the first day of the last menstrual period, expressed in completed weeks and days — for example, POG 28+3 means 28 weeks and 3 days. There's no calculation difference between POG and gestational age; they're two labels for one measurement. If you understand one, you understand the other. For the underlying concept, see gestational age vs. fetal age.

How POG is written

The usual shorthand is weeks + days. So:

• POG 12+0 — twelve weeks exactly.
• POG 37+5 — thirty-seven weeks and five days (early term).
• POG 40+0 — the estimated due date.

You'll also meet EDD (estimated date of delivery) and, in some notes, EDC (estimated date of confinement) — older wording for the same due date.

How to calculate POG

Calculating POG is exactly the gestational-age calculation: count the days from the last menstrual period to the date in question, then express them as weeks and days. Our calculator shows POG for any “as of” date you choose — set the reference date and read the weeks-and-days figure. You can also ask it the reverse question: when will the pregnancy reach a given POG, such as 36+0, using the built-in target look-up.

“POG calculator app” vs. a web tool

Many people search for a POG calculator app to avoid counting weeks by hand. A web calculator does the same job with nothing to install and, in our case, nothing sent anywhere — the maths runs in your browser. If you're comparing formats, see OB wheel app vs. online calculator.

A quick reference

Because POG drives clinical timing, a few values are worth knowing: the second trimester begins at POG 14+0, viability is around 24+0, term begins at 37+0, and post-term is 42+0 and beyond. These milestones are laid out in pregnancy trimesters and milestones. Whatever term your clinic uses — POG or gestational age — the number means the same thing, and the wheel will calculate it for you.