Preterm Growth Charts (Olsen)

The Olsen et al. (2010) preterm growth charts are the U.S. standard for assessing intrauterine growth and immediate-postnatal growth in very-low-birth-weight (VLBW, <1500 g) and extremely-low-birth-weight (ELBW, <1000 g) infants. The reference is gestational-age-anchored rather than chronological-age-anchored — appropriate for the first weeks after birth in the NICU.

Each calculator returns a percentile and z-score against the Olsen reference for the entered gestational age. Use these in neonatal intensive care contexts where the standard term-infant references are not applicable.

When to use these calculators

Use the Olsen charts during the neonatal period for preterm infants. Once a preterm infant reaches term-equivalent age (40 weeks postmenstrual), transition to the standard WHO 0-24 month chart using corrected age.

Frequently Asked Questions

What is the difference between gestational age and chronological age?

Gestational age is measured from the mother's last menstrual period (or its estimate) — the standard fetal-age reference. Chronological age is measured from birth. For preterm infants the two differ by weeks; the Olsen chart uses gestational age.

When do I switch from Olsen to WHO?

At about term-equivalent age (40 weeks postmenstrual). After that, growth assessment uses the standard term-infant reference (WHO 0-24 months) plotted against CORRECTED age (chronological age minus prematurity weeks) until the child has caught up.