Customized birth weight percentiles are weight-for-gestational-age percentiles that account for the influence of maternal characteristics on fetal growth. Although intuitively appealing, the incremental value they provide in the identification of intrauterine growth restriction (IUGR) over conventional birth weight percentiles is controversial. The objective of this study was to assess the value of customized birth weight percentiles in a simulated cohort of 100,000 infants aged 37 weeks whose IUGR status was known. A cohort of infants with a range of healthy birth weights was first simulated on the basis of the distributions of maternal/fetal characteristics observed in births at the Royal Victoria Hospital in Montreal, Canada, between 2000 and 2006. The occurrence of IUGR was re-created by reducing the observed birth weights of a small percentage of these infants. The value of customized percentiles was assessed by calculating true and false positive rates. Customizing birth weight percentiles for maternal characteristics added very little information to the identification of IUGR beyond that obtained from conventional weight-for-gestational-age percentiles (true positive rates of 61.8% and 61.1%, respectively, and false positive rates of 7.9% and 8.5%, respectively). For the process of customization to be worthwhile, maternal characteristics in the customization model were shown through simulation to require an unrealistically strong association with birth weight.
Conventional measures of gestational weight gain (GWG), such as average rate of weight gain, are likely to be correlated with gestational duration. Such a correlation could introduce bias to epidemiological studies of GWG and adverse perinatal outcomes because many perinatal outcomes are also correlated with gestational duration. This study aimed to quantify the extent to which currently used GWG measures may bias the apparent relationship between maternal weight gain and risk of preterm birth. For each woman in a provincial perinatal database registry (British Columbia, Canada, 2000-2009), a total GWG was simulated such that it was uncorrelated with risk of preterm birth. The simulation was based on serial antenatal GWG measurements from a sample of term pregnancies. Simulated GWGs were classified using three approaches: total weight gain (kg), average rate of weight gain (kg/week) or adequacy of GWG in relation to Institute of Medicine recommendations. Their association with preterm birth =32 weeks was explored using logistic regression. All measures of GWG induced an apparent association between GWG and preterm birth =32 weeks even when, by design, none existed. Odds ratios in the lowest fifths of each GWG measure compared with the middle fifths ranged from 4.4 [95% confidence interval (CI) 3.6, 5.4] (total weight gain) to 1.6 [95% CI 1.3, 2.0] (Institute of Medicine adequacy ratio). Conventional measures of GWG introduce serious bias to the study of maternal weight gain and preterm birth. A new measure of GWG that is uncorrelated with gestational duration is needed.
After decades of decline, stillbirth rates have increased in several industrialized countries in recent years. We examined data from the province of British Columbia, Canada, in an attempt to explain this unexpected phenomenon.
We carried out a retrospective population-based cohort study of all births in British Columbia from 2000 to 2010. Outcomes of interest included overall stillbirth rates, birth weight-and gestational age-specific stillbirth rates, rates of spontaneous stillbirths (excluding pregnancy terminations that satisfied the definition of stillbirth [fetal death with a birth weight = 500 g or gestational age at delivery = 20 wk], hereafter referred to as "pregnancy terminations") and rates of congenital anomalies among live-born infants. We used logistic regression to adjust for changes in maternal age, parity, weight before pregnancy and multiple births.
Overall, stillbirth rates increased by 31% (95% confidence interval [CI] 13% to 50%), from 8.08 per 1000 total births in 2000 to 10.55 per 1000 in 2010. The rate of stillbirths with a birth weight of less than 500 g increased significantly (p(trend) = 0.03), whereas the rate of stillbirths with a birth weight of 1000 g or more decreased significantly (p(trend) = 0.009). The rate of spontaneous stillbirths decreased nonsignificantly by 16%, from 5.7 per 1000 total births in 2000 to 4.8 per 1000 in 2010. There was a significant decline of 30% (95% CI 6% to 47%) in the rate of spontaneous stillbirth with a birth weight of 1000 g or more between 2000 and 2010; adjustment for maternal factors did not appreciably change this temporal effect. The prevalence of congenital anomalies among live-born infants decreased significantly, from 5.21 per 100 live births during the first 3 years (2000-02) to 4.77 per 100 during the final 3 years (2008-10).
Increases in pregnancy terminations were responsible for the increases observed in stillbirth rates and were associated with declines in the prevalence of congenital anomalies among live-born infants.
In perinatal epidemiology, the basic unit of analysis has traditionally been the individual pregnancy. In this study, we sought to explore the idea of a 'reproductive life'-based approach to modelling the effects of reproductive exposures and outcomes, where the basic unit of analysis is a woman's entire reproductive experience. Our objective was to explore whether a first pregnancy risk factor, excess gestational weight gain, has a direct effect on the birthweight outcomes of a subsequent pregnancy, independent of the weight gain and other risk factors of the second pregnancy. A study population was created by linking the obstetric records of 1220 women who delivered their first and second offspring at a McGill University teaching hospital in Montreal, Canada. Multivariable linear and logistic regression analyses were used to model the effects of gestational weight gain above recommendation on the birthweight Z-score and risk of large-for-gestational age (LGA) subsequent offspring. After adjusting for the risk factors of the second pregnancy, an independent effect from the first pregnancy was seen on the birthweight Z-score, (effect size OR 0.17 [95% CI 0.05, 0.28] but not risk of LGA of the second pregnancy 1.30 [95% CI 0.89, 1.89]). We concluded that a pregnancy-centred approach to research that conceptualizes pregnancies as self-contained and interchangeable events may not always be appropriate, and propose that analytical methods for some perinatal research questions may need to consider a given pregnancy in the context of a woman's past reproductive experiences.
We present a model for longitudinal measures of fetal weight as a function of gestational age. We use a linear mixed model, with a Box-Cox transformation of fetal weight values, and restricted cubic splines, in order to flexibly but parsimoniously model median fetal weight. We systematically compare our model to other proposed approaches. All proposed methods are shown to yield similar median estimates, as evidenced by overlapping pointwise confidence bands, except after 40 completed weeks, where our method seems to produce estimates more consistent with observed data. Sex-based stratification affects the estimates of the random effects variance-covariance structure, without significantly changing sex-specific fitted median values. We illustrate the benefits of including sex-gestational age interaction terms in the model over stratification. The comparison leads to the conclusion that the selection of a model for fetal weight for gestational age can be based on the specific goals and configuration of a given study without affecting the precision or value of median estimates for most gestational ages of interest.
Cites: BJOG. 2008 Oct;115(11):1397-40418823489
Cites: Am J Obstet Gynecol. 2008 Jul;199(1):e18-9; author reply e19-2018455137
Cites: Am J Obstet Gynecol. 2010 Jun;202(6):522-820074690
The fetoplacental ratio has been used conventionally to study the contribution of the placenta to fetal growth restriction. However, this measure is problematic because a normal fetoplacental ratio can reflect birth weight and placental weight that are both normal, both low, or both high. The objective of this study was to examine the independent association between placental weight for gestational age and perinatal mortality or serious neonatal morbidity.
A sex- and gestational age-specific placental weight z score was calculated for a cohort of 87,600 singleton births at the Royal Victoria Hospital in Montreal, Canada, 1978-2007. The relationship between placental weight z score and adverse perinatal outcomes (stillbirth, neonatal death, 5-minute Apgar score lower than 7, seizures, or respiratory morbidity) was examined using logistic regression. Multivariable models examined whether the relationship was independent of birth weight and other pregnancy risk factors.
: After controlling for birth weight, fetuses with a low placental weight z score were at significantly increased risk of stillbirth (odds ratio [OR] 2.0, 95% confidence interval [CI] 1.4-2.6, percent population attributable risk 17.8%). In contrast, adverse neonatal outcomes were significantly more likely among those with high placental weight z scores (OR 1.4, 95% CI 1.2-1.7, percent population attributable risk 5% for any serious neonatal morbidity). Similar trends were observed after further adjusting for pregnancy risk factors.
Placental weight for gestational age is an independent risk factor for adverse perinatal outcomes, above and beyond the known association with birth weight. The mechanisms behind the opposing effects of placental weight z score on risk of stillbirth compared with adverse neonatal outcomes require further elucidation.
National and international clinical practice guidelines, based on the meta-analysis of randomized trials, recommend antenatal corticosteroid (ACS) prophylaxis for threatened preterm delivery. We carried out a study to determine the extent to which current clinical practice in British Columbia adheres to these guidelines with a focus on preterm deliveries at 33 to 34 weeks of gestation.
Data were obtained from the British Columbia Perinatal Database Registry, a comprehensive provincial registry containing detailed information on all births in the province. All preterm live births between 2000 and 2009 were included in the study. The rate of ACS administration was assessed in different gestational age groups. Determinants of ACS administration (such as maternal characteristics and obstetric factors) were also studied. The frequency of ACS prophylaxis was estimated using rates and exact 95% confidence intervals, and associations were assessed using odds ratios and 95% confidence intervals.
Among 35 862 preterm births in British Columbia, the rate of ACS administration was 56.0% in the 26- to 32-week group (95% CI 54.7% to 57.4%) and 19.4% in the 33- to 34-week group (95% CI 18.5% to 20.4%). Rates were reasonably consistent between 2000 and 2009 and by region of residence in British Columbia. Women with hypertension (OR 1.51; 95% CI 1.32 to 1.72), gestational diabetes (OR 1.21; 95% CI 1.05 t01.40), and iatrogenic deliveries (OR 1.34; 95% CI 1.22 to 1.47) were significantly more likely to receive ACS.
Despite explicit clinical guidelines, ACS usage in preterm deliveries at 33 to 34 weeks of gestation appears to be suboptimal.
Conditional fetal growth percentiles are percentiles that are calculated taking into account (conditional on) an infant's weight earlier in pregnancy. Although they have been proposed in the statistical literature as a more methodologically appropriate method of measuring fetal growth, their ability to predict adverse perinatal outcomes due to fetal growth restriction is unknown. Using a large, unselected clinical ultrasound database at the Royal Victoria Hospital in Montreal, Canada, we calculated conditional growth percentiles for infants' weight at birth, given their weight at the time of a routine 32- or 33-week ultrasound. The risk of adverse perinatal outcome (perinatal mortality, low Apgar, acidaemia, or seizures/organ failure due to asphyxia) among small-for-gestational-age infants (SGA) as established by conditional growth percentiles was calculated as well as the risk among infants classified as SGA by conventional weight-for-gestational-age percentiles. Regardless of the threshold used to define SGA (fifth, 10th, 15th, 20th), conditional percentiles did not appear to improve the identification of adverse perinatal outcomes compared with conventional weight-for-gestational-age charts. Further work is needed to confirm our results as well as to explore potential reasons for the lack of benefits from using a measure of growth instead of size to identify fetal growth restriction.
Pregnancy weight-gain z score charts have recently been proposed as a new tool for classifying gestational weight gain and establishing the link between weight gain and adverse maternal and infant outcomes. However, existing charts are few in number, were based on small sample sizes, and were not population based.
We created population-based pregnancy weight-gain-for-gestational-age z score charts for Swedish women who were stratified by early pregnancy body mass index (BMI).
Serial prenatal electronic medical records were obtained from women who were receiving obstetrical care in the Swedish counties of Gotland and Stockholm. The study population was restricted to nonanomalous, singleton, term pregnancies with no prepregnancy hypertension or diabetes. A multilevel linear regression was used to express the repeated weight-gain measurements as a function of gestational age in underweight, normal-weight, overweight, and obese class I-III women. Observed weight-gain ranges were contrasted with current Institute of Medicine (IOM) pregnancy weight-gain recommendations.
A total of 711,615 serial prenatal weight measurements from 141,767 pregnant women were included. The smoothed means, SDs, and selected percentiles (3rd, 10th, 50th, 90th, and 97th) of weight gain were estimated for each week of gestation. The total weight gain and rate of weight gain decreased with increasing prepregnancy BMI. In all BMI categories, the observed range of pregnancy weight gain was considerably broader than the range currently recommended by the IOM.
The presented population-based pregnancy weight-gain charts can be used to express maternal weight gain as gestational age-standardized z scores with early pregnancy BMI taken into consideration. The z scores can be used to obtain a better understanding of the relation between pregnancy weight gain and maternal and infant health complications.