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

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Olsen Preterm Weight-for-Gestational-Age Growth Chart

Plot a preterm infant's birth weight against gestational age using the Olsen intrauterine growth curves (2010). Covers 23-41 weeks for classification as SGA, AGA, or LGA.

LMS Method: Z = ((X/M)^L - 1) / (L x S)

How It Works

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 LMS method with gestational-age-specific parameters computes Z-scores and percentiles, with classification as Small (SGA, below 10th), Appropriate (AGA, 10th-90th), or Large for Gestational Age (LGA, above 90th).

Example Problem

A boy born at 32 weeks gestational age weighs 1,829 g (1.829 kg). What is his weight-for-gestational-age percentile?

  1. Select 'Boy' for gender.
  2. Enter 32 weeks for gestational age.
  3. Enter 1829 g (or 1.829 kg) for weight.
  4. The calculator finds the Olsen LMS parameters for boys at 32 weeks.
  5. Result: approximately the 50th percentile, since the Olsen median weight for boys at 32 weeks is 1.829 kg. Classification: AGA (Appropriate for Gestational Age).

Key Concepts

The Olsen charts classify preterm infants by birth weight relative to gestational age: below the 10th percentile is Small for Gestational Age (SGA), 10th-90th is Appropriate for Gestational Age (AGA), and above the 90th is Large for Gestational Age (LGA). SGA classification has important implications for neonatal management, including hypoglycemia screening and feeding protocols. These charts are designed for intrauterine growth assessment, not postnatal growth tracking.

Applications

  • Birth weight assessment for preterm infants in the NICU
  • SGA/AGA/LGA classification at delivery
  • Neonatal risk assessment and management planning
  • Research on preterm birth outcomes
  • Clinical counseling for parents of preterm infants

Common Mistakes

  • Using standard WHO or CDC charts for preterm weight assessment — those are designed for postnatal full-term growth
  • Using these intrauterine charts for postnatal growth tracking — they assess birth weight only
  • Confusing gestational age with chronological age
  • Not accounting for the significant difference between grams and kilograms when entering data
  • Using SGA classification alone without clinical context — some constitutionally small infants are healthy

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.

Reference: Olsen IE, et al. New intrauterine growth curves based on United States data. Pediatrics. 2010;125(2):e214-e224.

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