Explainer
Which growth chart a preterm baby is read against
The neonatal charts run to 64 weeks post-menstrual — about 6 months corrected — while the ordinary charts already apply from the due date. For that half year two charts both fit, and can disagree.
Three charts, three different ages
The question is not which chart is best. It is which one the measurement in front of you belongs to. They are indexed differently, so the same baby on the same day has a different percentile on each — and none of them is wrong.
| Stage | Age on the axis | The chart |
|---|---|---|
| At birth | Gestational age at birth, 24–43 weeks | INTERGROWTH-21st Newborn Size Standards — the small, appropriate or large for gestational age call |
| Neonatal unit and after | Post-menstrual age, 27–64 weeks | Fenton 2013, or INTERGROWTH-21st’s preterm postnatal standard |
| From the due date | Corrected age, to 24 months | The ordinary charts — WHO, or your country’s own |
What post-menstrual age means
Gestational age at birth plus the weeks lived since. A baby born at 28 weeks who is 6 weeks old is at 34 weeks post-menstrual. It is the axis the neonatal charts use because, before the due date, weeks of maturity matter more than weeks since delivery.
These charts run to 64 weeks — about 6 months corrected. That overlaps the period the ordinary charts already cover, so for several months two charts both apply and can disagree. Units differ in which they keep using; the number matters less than plotting the same baby on the same chart each time.
After the due date, the ordinary charts — at corrected age
From term-equivalent onward, a preterm baby is read against the same tables as everyone else, but at corrected age: actual age minus the time born early. A baby born at 37 weeks or later is at term and needs no correction.
Skipping the correction is the most common false alarm there is. A boy born at 32 weeks who is exactly on the median by corrected age reads as the 0.2th percentile at three months if he is looked up by date of birth. Nothing is wrong with him; the age used to read the table is wrong.
How long to correct is banded — until 12 corrected months for a birth at 32–36 weeks, until 24 for a birth before 32 — and the banding comes from RCPCH UK-WHO plotting guidance rather than from the growth standards themselves.
Which one to use today
While a baby is still read against post-menstrual age, the chart to use is the one the neonatal unit is already plotting on — Fenton or INTERGROWTH-21st, whichever that is. The useful thing to bring away from an appointment is not the percentile but which chart it came from, because the next reading only means something against the same one.
From the due date onward, the ordinary charts apply, read at corrected age. The corrected age calculator shows both ages at once, which is what belongs in the record.
Common questions
- Is the Fenton chart the same as INTERGROWTH-21st?
- No, though they cover much the same window and are read the same way — against post-menstrual age, meaning gestational age at birth plus weeks lived. Fenton 2013 is a meta-analysis of size at birth extended into the postnatal period; INTERGROWTH-21st’s preterm postnatal standard was built by following preterm babies from 27 to 64 weeks. Units differ in which they use; both give a percentile, and the two do not always agree.
- Why did my baby’s percentile change when the chart changed?
- Because the reference changed, not the baby. Each chart describes a different group of children on a different axis, so the same measurement lands in a different place. A step at a chart change is a change of ruler. It is worth writing down which chart a number came from, so a later reading is compared against the same one.
- When do the ordinary growth charts start applying?
- From term-equivalent — the due date — read at corrected age rather than age since birth. Correction continues to 12 corrected months for a birth at 32–36 weeks and to 24 for a birth before 32 weeks, and after that the tables are read at actual age like any other child.