Objective
The purpose of this study was to report the frequency of fetal growth restriction (FGR) based on indication for late preterm birth (LPTB).
Study Design
Singleton live born pregnancies that were delivered from 34-36 weeks 6 days of gestation over a 1-year period at a tertiary care medical center were studied. Indications for delivery were categorized as spontaneous (spontaneous preterm birth or premature rupture of membranes), medically indicated, or elective. A customized birthweight percentile was calculated for each pregnancy; the rate of FGR was compared based on indication for LPTB.
Results
There were 482 LPTBs that met all criteria. Customized birthweight percentiles (median; interquartile range) were different among groups (spontaneous, 45.5%; 20.8–73.5%; medically indicated, 26.9%; 4.1–63.6%; elective, 45.9%; 22.2–78.3%; P = .001). The rate of FGR was also different among groups (spontaneous, 13%; medically indicated, 32%; elective, 21%; P = .001).
Conclusion
With the use of customized birthweight standards, we found that FGR complicated approximately one-third of all cases of medically indicated LPTB.
Late preterm births (LPTB) accounted for 8.8% of all births in the United States in 2008. Compared with neonates born at term, LPTB infants have a higher rate of morbidity and mortality. McIntire and Leveno showed neonatal mortality rates of 1.1, 1.5, and 0.5 per 1000 live births at 34, 35, and 36 weeks gestation, respectively, vs 0.2 per 1000 live births at 39 weeks’ gestation ( P < .001). Healthcare use resources are also increased during the first year of life of these infants.
Customized growth curves with the use of maternal body mass index, parity, ethnic origin, and fetal gender have been used to determine a predicted birthweight for a pregnancy in optimal conditions. This method allows for a more sensitive and specific manner to identify abnormal fetal growth. The clinical impact and scientific value of a “customized” measurement for fetal growth assessment, compared with the use of population standards, has been reviewed recently by Zhang et al. Customized fetal growth curves and birthweight percentiles have been created for different populations (Gestation Related Optimal Weight; www.gestation.net ) and recently has become available for the US population.
One of the key areas for study that was proposed by the National Institute of Child Health and Human Welfare Workshop on LPTB was to better understand the pathophysiologic condition related to the development of LPTB. Previous studies have reported a higher rate of fetal growth restriction (FGR) that has been diagnosed by customized standards in preterm birth (PTB) at <37 weeks’ gestation and increased risk for neonatal morbidity after PTB, but this association has not been studied specifically in the LPTB population. Our objective was to report the frequency of FGR that was defined by customized birthweight percentiles in our LPTB population and to compare the rate of FGR based on the primary indication for LPTB.
Materials and Methods
This was an historic cohort study of women who delivered at a single tertiary hospital over a 1-year period (June 1, 2008, to May 31, 2009). A computerized perinatal database was used to identify women who delivered between 34 weeks 0 days and 36 weeks 6 days at Children’s Memorial Hermann Hospital–Texas Medical Center in Houston. Women with multiple gestation, antepartum stillbirth, or fetal structural/chromosomal abnormalities were excluded. Study personnel reviewed maternal and neonatal hospital charts of all eligible women to obtain pertinent clinical and outcome data.
Prespecified definitions were used to categorize indication for LPTB delivery. Delivery indications were classified as spontaneous, medically indicated, or elective. Spontaneous delivery included those with premature rupture of membranes or spontaneous preterm labor. Medically indicated delivery included medical indications (renal disease, cardiac disease, diabetes mellitus, other medical conditions) and obstetric indications (placenta previa, abruption, preeclampsia) for delivery and was subdivided into those with a severe or unstable medical/obstetric condition (eg, severe preeclampsia, hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome, nonreassuring fetal testing, placenta previa and/or accreta with vaginal bleeding, and unstable/worsening medical conditions) or stable but high-risk condition (mild preeclampsia, intrauterine growth restriction with reassuring fetal testing, previous classic cesarean delivery, oligohydramnios with reassuring fetal testing, and mild/stable medical conditions). Elective delivery was defined as those cases for which the obstetrician documented an elective indication or if there was no evidence of documentation of a medical or obstetric indication in the maternal hospital chart. A composite outcome for development of neonatal respiratory compromise was defined as a need for mechanical ventilation, continuous positive airway pressure with FiO 2 ≥40% for at least 2 hours (continuously), or FiO 2 ≥40% (nasal cannula or hood) for at least 4 hours (continuously) within the first 72 hours of life.
Customized birthweights were calculated for each pregnancy with the use of the Gardosi customized growth model, which generates an optimal birthweight for an individual pregnancy by taking into account maternal and fetal characteristics: maternal weight and height, ethnicity/race, parity, fetal/neonatal gender, and gestational age at delivery. A customized birthweight percentile was then calculated for each pregnancy (Bulk Centile Calculator, version 6.4, US Edition; downloaded from www.gestation.net ). FGR was defined as birthweight of <10th percentile. We also determined whether the birthweight was <10th percentile using population-based standards and then compared this with the results from customized standards.
SPSS statistical software (PASW Stats version 18.0; SPSS Inc, Chicago, IL) was used for data analysis. χ 2 , Wilcoxon rank-sum, 1-way analysis of variance, and Kruskal-Wallis tests were performed where appropriate. The rate of FGR was compared among groups based on the indication for LPTB. Multivariate logistic regression analysis was also performed to assess the relation between FGR (totalizing customized standards) and neonatal respiratory morbidity. The dependent variable in the regression analysis was the composite respiratory morbidity, and the explanatory variables were gestational age at delivery, race, indication for LPTB, cesarean delivery without labor, treatment with antenatal corticosteroids, and FGR. A probability value of < .05 was considered statistically significant. This study was approved by both university and hospital institutional review boards.
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