Gestational, Postnatal, and Postmenstrual Age in PBPK Models

Why Infant Age Needs a Clear Time Origin

An infant’s age can refer to time before birth, time since birth, or a combination of the two. These are different quantities, and substituting one for another can place the same child at the wrong point on a developmental timeline. In a pediatric PBPK model, that distinction matters when age is used to represent the developmental state of drug-metabolizing processes.

A useful starting point is to ask what the age variable is intended to measure. Time since conceptional development began is not the same as time since delivery. A preterm infant may have spent several weeks developing outside the uterus while a term infant of the same postnatal age was still in utero for part of that period. Gestational age, postnatal age, and postmenstrual age make these time origins explicit.

For an infant who has been born, the relevant gestational age in the PMA calculation is the infant’s GA at birth, not an age that continues to increase independently after delivery. Before birth, GA describes the ongoing pregnancy. After birth, PNA advances from the birth date, while PMA combines the birth GA and elapsed PNA on one continuous scale.

Calculating the Three Ages Consistently

The safest arithmetic method is to convert each age to days, perform the addition or subtraction in days, and then convert the result back to weeks and days. One week contains 7 days. If a total of \(D\) days is written in week-and-day notation, the number of complete weeks is the whole-number quotient when \(D\) is divided by 7, and the remaining days are the remainder.

Equivalently, when both ages are expressed in weeks and days, add the weeks and add the days, then carry each complete group of 7 days into the week total. This is ordinary duration arithmetic, not decimal addition: 30+4 weeks plus 1+3 weeks is 31 weeks and 7 days, which is 32+0 weeks.

Which Age Scale Represents Developmental Capacity?

When the modeling question concerns a developmental process across infants born at different gestational ages, PMA is usually the most informative single age scale. It includes both the maturity reached before birth and the time elapsed since delivery. PNA alone records only the second component: two infants with equal PNA can have substantially different PMA if their GAs at birth differed.

For example, consider two hypothetical infants assessed after 12 weeks of PNA. One was born at 30+2 weeks and the other at 39+2 weeks. Their PMAs are 42+2 and 51+2 weeks, respectively. The same postnatal duration therefore does not imply the same total developmental age. Conversely, infants at the same PMA may have had different amounts of postnatal exposure, so PMA is not a complete description of their histories.

Using PMA as an age descriptor does not establish that every infant of a given PMA has identical drug-metabolizing capacity. PMA is an age scale, not a direct assay of enzyme abundance or activity. Individual biology, illness, treatments, and the distinction between intrauterine and extrauterine development may matter. Those influences should not be silently folded into the age definition.

Choosing and Checking the Age Variable

The age scale should follow the scientific question and the data available. The comparison below gives a practical guide.

A common alternative is to compare infants only by PNA. That is appropriate for questions explicitly about elapsed time since birth, but it can be misleading if interpreted as equivalent developmental maturity. Another alternative is to use GA at birth alone. That describes maturity at delivery but becomes increasingly incomplete as time passes after birth. PMA combines the two durations and is therefore generally preferable when a single age covariate is intended to represent developmental progression across preterm and term infants.

In some clinical settings, corrected age is also reported for infants born preterm. It adjusts chronological age relative to an expected term date for developmental follow-up. It is not a replacement name for PMA: the quantities have different reference points and uses. For PBPK work, report the age definition explicitly rather than relying on an ambiguous label such as “age” or “corrected age.”

Diagnostics, Limitations, and Common Errors

Age data should be checked before they are used as a model input. Confirm whether the source reports completed weeks, week-and-day notation, or decimal weeks; confirm the birth date and assessment date; and establish whether the reported GA is the best obstetric estimate or another recorded estimate. If age is given only in completed weeks, the missing days are not known exactly and should not be invented. Inconsistent conventions across data sources can create apparent discrepancies that are actually recording differences.

• Adding week notation as decimals. Treating 30+4 as 30.4 weeks is wrong. It means 30 weeks plus 4 days, or \(30+4/7\) weeks.
• Using PNA as if it were PMA. Equal PNA does not imply equal PMA when birth GAs differ. Add GA at birth before interpreting an age as a combined developmental timeline.
• Forgetting to carry days. A total such as 36 weeks and 11 days must be normalized to 37+4, not left as 36+11 or converted by decimal addition.
• Claiming PMA proves equal capacity. PMA aligns age, but it does not guarantee identical enzyme activity or remove effects of clinical history and developmental environment.
• Using the wrong interval. If the question is duration after birth or time after a dose, PNA or dose-relative time is directly relevant; PMA answers a different question.