Perinatal outcome in SGA births defined by customised versus population-based birthweight standards

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Abstract

Objective To determine whether customised birthweight standard improves the definition of small for gestational age and its association with adverse pregnancy outcomes such as stillbirth, neonatal death, or low Apgar score.

Design Population based cohort study.

Population Births in Sweden between 1992-95 (n=326,377).

Methods Risks of stillbirth, neonatal death, and Apgar score under four at five minutes were calculated for the lowest 10% birthweights according to population-based and customised standards, and were compared with the data from the group with birthweights over this limit. Population attributable risks for stillbirth using various birthweight centile cutoffs were calculated for the two standards.

Outcome measures Odds ratios and 95% confidence intervals for stillbirth, neonatal death and Apgar score under four at five minutes, and population attributable risks for stillbirth at different birthweight centiles.

Results Risks of stillbirth, neonatal death, and Apgar score under four at five minutes and population attributable risks of stillbirth were consistently higher if ‘small for gestational age’ was classified by a customised rather than by the population-based birthweight standard. Compared with infants who were not small for gestational age by both standards, the odds ratio for stillbirth was 6.1 (95% CI 5.0-7.5) for small for gestational age by customised standard only, whereas it was 1.2 (95 % CI 0.8-1.9) for small for gestational age by population standard only.

Conclusions Compared with the population-based birthweight standard, a customised birthweight standard increases identification of fetuses at risk of stillbirth, neonatal death and Apgar score under 4 at 5 minutes, probably due to improved identification of fetal growth restriction.

Introduction

Intrauterine growth restriction is associated with increased risks of stillbirth, neonatal death and other adverse outcomes1. Since fetal growth is difficult to measure, small for gestational age (SGA) is often used as a proxy for intrauterine growth restriction. However, SGA is a heterogeneous category, including not only growth restricted infants, but also infants with chromosomal abnormalities2 and small, healthy infants. Population-based birthweight standards define SGA as the lowest 10th or 5th centile, or as two standard deviations below the mean birthweight for gestational age. The individualised or customised growth standard sets an optimal fetal growth rate for each pregnancy, based on maternal anthropometrics, parity and ethnic background, thus providing an attempt to separate intrauterine growth restriction from the small, healthy infant. Previous studies have shown that this method improves prediction of infants at risk of poor neonatal outcomes such as the need for operative delivery for fetal distress, admission to the neonatal intensive care unit, or artificial ventilation3. Using a large nationwide dataset, we constructed an individually adjustable or ‘customised’ and a fixed population based birthweight standard. These standards were used to compare risks of stillbirth, neonatal death and Apgar score below 4 at 5 minutes in infants classified as SGA by either birthweight standard.

Section snippets

Methods

The Swedish Birth Register, held by the National Board of Health and Welfare, contains data on more than 99% of all births in Sweden4. Information is collected prospectively, and includes demographic data, reproductive history, and complications during pregnancy, delivery and the neonatal period. Copies of the standardised individual antenatal, obstetric, and paediatric records are forwarded to the Birth Registry, where the information is stored. All births and deaths are validated every year

Results

Table 1 displays maternal characteristics and risks of SGA by population based and customised standards (the lowest 10% of birthweights according to each classification). Teenage mothers had a 50% increase in risk of SGA by population standards, but no increase in risk by customised standards. Mothers who were 30 years or more had no increased risk of SGA by the population standard, but an increased risk of SGA by the customised standard. Primiparity was associated with an almost doubled risk

Discussion

This study compares two methods of identifying small for gestational age births: a population based standard, adjusted only for gestational length and fetal gender and similar to a variety of national standards in common use; and a customised birthweight standard, based on the prediction of optimal growth in each individual pregnancy. Our conclusion is that the latter has an improved capacity to identify adverse effects related to fetal growth restriction, such as stillbirth, neonatal death,

Acknowledgements

This study was supported by grants from Förenade Liv, Mutual Group Life Insurance Company, Stockholm, Sweden.

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