Preterm Weight-for-Gestational-Age Percentile Calculator (Olsen)

How Preterm Weight-for-Gestational-Age Percentiles Work

This calculator uses the Olsen intrauterine growth curves to assess weight-for-gestational-age for premature infants born between 23 and 41 weeks of gestation. Based on Olsen et al. (2010), these curves were developed from a large, contemporary U.S. dataset of neonatal birth weights.

The calculator uses the LMS method (Lambda-Mu-Sigma) with gestational-age-specific parameters. Lambda (L) accounts for skewness, Mu (M) is the median weight at each gestational age, and Sigma (S) is the coefficient of variation.

Understanding Percentiles for Preterm Infants

A percentile indicates where a preterm infant’s weight falls compared to other infants born at the same gestational age. An infant at the 50th percentile weighs the same as the median for that gestational age.

• Below the 10th percentile is classified as Small for Gestational Age (SGA)
• 10th to 90th percentile is Appropriate for Gestational Age (AGA)
• Above the 90th percentile is Large for Gestational Age (LGA)

Example Calculation

A boy born at 32 weeks gestational age weighing 1,829 g (1.829 kg) would be at approximately the 50th percentile, since the Olsen median weight for boys at 32 weeks is 1.829 kg.

Frequently Asked Questions

What gestational age range does this calculator cover?

This calculator covers gestational ages from 23 to 41 weeks, which encompasses the full range of viable preterm births through full-term delivery.

How is this different from the WHO growth charts?

WHO growth charts are designed for postnatal growth of full-term infants. The Olsen charts are specifically designed for intrauterine growth assessment of preterm infants, using birth weight data matched to gestational age.

Should I enter weight in grams or kilograms?

You can use grams, kilograms, or pounds. Grams are most commonly used in neonatal settings. The calculator converts automatically between units.