How to Calculate PMA: Postmenstrual Age Formula (NICU)

How to calculate PMA is addition, not a developmental score. Postmenstrual age is gestational age at birth plus chronological time since birth, written in completed weeks and leftover days. A baby born at 29 weeks + 2 days who is now 10 days old has a PMA of 30 weeks + 5 days.

If you only needed the definition — PMA vs gestational age vs corrected age — use PMA meaning in the NICU. This page is the arithmetic. Run the same dates through the gestational age / PMA calculator after you do it by hand once.

This is educational, not medical advice. Screening calendars and discharge dates come from the infant’s clinicians.

The formula

Write both clocks in weeks + days. There are 7 days in a week. Do not mix in months.

1. Gestational age at birth (GA) from the admission note or discharge summary — example: 27 weeks + 5 days.
2. Chronological age from date of birth to the date on the note — example: 3 weeks + 2 days (23 days of life).
3. Add the days. 5 + 2 = 7 days → that is 1 extra week and 0 leftover days.
4. Add the weeks. 27 + 3 + 1 = 31 weeks + 0 days.

PMA = (GA weeks + chronological weeks + carry from days) weeks + leftover days.

If you only have dates, count exact calendar days of life first, then convert: days ÷ 7 = weeks, remainder = days. Add that to GA the same way. Leap days matter if the interval crosses February 29 — the PMA calculator handles the calendar so you are not doing that on a napkin.

Worked example (different dates than the meaning post)

Born 3 March 2026 at 28 weeks + 1 day. The eye-exam note is dated 14 April 2026.

• Chronological age: 3 March → 14 April is 42 days = 6 weeks + 0 days.
• PMA: 28 weeks + 1 day + 6 weeks + 0 days = 34 weeks + 1 day.

If someone says “she’s 34 weeks,” they almost always mean PMA, not birthday age. Birthday age here is 6 weeks.

A second check: born 22 June 2026 at 31 weeks + 0 days. On 6 July 2026 the infant is 14 days old (2 weeks + 0 days). PMA is 33 weeks + 0 days. If the GA on the chart was actually 31 + 4, PMA would be 33 + 4 — a four-day dating error moves every later PMA by four days.

What not to add

Do not add PMA to corrected age. Corrected age subtracts the weeks born before 40 weeks from chronological age. It is a different clock for milestones after term. Mixing the two produces a number no protocol uses.

Do not use today’s date if the note is about yesterday’s exam. PMA is “age on that event date.”

Do not update gestational age. GA is fixed at birth. Only PMA (and chronological age) increases.

Do not convert PMA into years. The unit stays weeks until the team stops using it — usually around term, when the conversation shifts to chronological or corrected age. See the corrected age guide.

Days of life versus weeks + days

Some flowsheets print DOL 17 instead of 2 weeks + 3 days. Convert before you add:

• 17 days = 2 weeks + 3 days.
• Born 26 + 4; PMA = 26 + 4 + 2 weeks + 3 days = 29 weeks + 0 days after you carry the 7 days.

If the unit writes completed weeks only (“PMA 33”) they may be dropping leftover days. Ask whether the protocol uses completed weeks or weeks + days. Rounding rules are theirs, not a blog’s.

Calculator check

After the hand calc:

1. Open the gestational age / PMA calculator.
2. Enter the same due-date or GA method the chart used (LMP vs ultrasound dating). A one-week dating disagreement is a one-week PMA disagreement forever.
3. Enter date of birth and the event date.
4. If the screen disagrees with your napkin, you probably carried days wrong or used the report-printed date instead of the exam date.

Days of life alone can be checked on the chronological age calculator, then added to GA by hand. One-screen PMA is less error-prone when you are tired.

FAQ

How do you calculate PMA in the NICU?
Add chronological age (weeks + days since birth) to gestational age at birth (weeks + days). Carry every 7 days as another week.

Is PMA the same as gestational age?
No. Gestational age is assigned at birth and does not increase. PMA keeps increasing.

Can I use months?
Not for PMA. PMA is a pregnancy-week clock. Age in months is a different conversion — see age in months, days, and weeks.

When do I stop calculating PMA?
When the team has switched to chronological or corrected age, usually once the infant is at or past term. Ask what the current note is using.

Does this page replace the meaning explainer?
No. PMA meaning is the vocabulary. This URL is the steps.

Discharge, feeding, and screening dates belong to the neonatologist and current professional guidance, not to a general how-to.