Hum. Reprod. Advance Access originally published online on July 7, 2006
Human Reproduction 2006 21(10):2638-2639; doi:10.1093/humrep/del246
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Associate editor’s commentary on the article ‘Birth weight of singletons after assisted reproduction is higher after single than double embryo transfer’ by De Sutter et al.
How should we report on perinatal outcome?
Department of Obstetrics and Gynaecology, Academisch Ziekenhuis Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands
E-mail: jlan{at}sgyn.azm.nl
 |
Abstract |
|---|
 Top Abstract AcknowledgementReferences |
|---|
Authors should report on neonatal outcome after ART in a well-defined and uniform way, to enable readers to compare the outcome of different studies. For reporting on birthweight, SD scores (z-scores) are to be preferred, customized for the most important physiological variables affecting birthweight, e.g. gestational age, gender of the child and whether it is a singleton or part of a multiple. It may be suggested that ESHRE should support the construction of European customized birthweight tables and make available a software for calculating the z-scores in cohorts.
Key words: birth weight/gestational age/neonatal outcome
Publications on assisted reproductive techniques (ARTs) tend to occur according to an established pattern: the very first papers report on the technique, followed by papers on implementation and success rates and finally papers appear dealing with quality and safety. In 2004–2005, there were a series of debates in Human Reproduction on how best to define success rates in ART, and singleton live birth rate per started cycle was agreed to be the best outcome measure. Until now there have been no guidelines on how to report on quality and safety, and there is no agreement on how they should be defined and what the preferred outcome measure should be. Thus far, studies on safety and quality have compared birthweights of children born after ART, but this has not been done in a standardized way: some studies have compared proportions of children born with normal birthweight, low birthweight and very low birthweight, respectively (Pinborg et al., 2004
), matched for fetal gender, maternal age and parity (Ombelet et al., 2006) and others have used birthweight SD scores (Wennerholm et al., 2006) or mean weights and multivariate models for confounding factors (De Sutter et al., 2006). These differences in outcome measures make it impossible to compare outcomes between different studies, and the absolute findings (e.g. singleton newborns after double-embryo transfer weigh on average 120 g less than singletons after elective single-embryo transfer) (De Sutter et al., 2006) are hard to interpret in terms of risk.
Birthweight is a measure commonly used for the assessment of perinatal outcome, because it is associated with morbidity and mortality in both the short and the long term. Birthweight, however, is determined by many factors, not all related to risk. There are pathological factors affecting growth (e.g. maternal vascular disease and fetal congenital malformations), but birthweight is subject to physiological variation as well, because of gestational age at birth, gender of the child, whether the child is a singleton or part of a multiple and maternal parity and ethnicity. For example, boys tend to be heavier than girls at every gestational age, and children born to parous mothers have higher birthweights. This emphasizes the need for customized birthweight standards (Gardosi et al., 1995): for meaningful conclusions on birthweight in a particular group of newborns, their birthweight should be evaluated against the birthweight in a reference population customized for the most relevant physiological factors affecting birthweight (i.e. gestational age, gender and being a singleton or part of a multiple).
Various methods have been suggested for obtaining customized birthweights and are as follows:
- To eliminate the influence of gestational age on birthweight, categorization into small, large and appropriate for gestational age is used. This categorization, however, reduces the power to detect small associations between exposures and fetal weight, unless there is a change in the relationship exactly at the (arbitrarily chosen) threshold point. In addition, as most children born are appropriate for gestational age, this categorization makes it difficult to find differences within this group (Oken et al., 2003).
- Another approach has been to include gestational age in a multiple regression model along with other potential predictors of birthweight. Regression analysis assumes a linear relationship between birthweight and gestational age, but this may not always be the case (Oken et al., 2003).
- Growth charts using percentiles are frequently used, based on the position that the neonate’s birthweight occupies on a gestational age-specific birthweight distribution. Reference percentiles for newborns should be stratified for singletons and multiples separately by gender and, ideally, for maternal race and parity as well. Estimation of percentile position is imprecise, however, at the extremes of the distribution. Additionally, percentile positions represent an ordinal rather than a ratio scale, and the possibilities for valid statistical manipulation are limited (Blair et al., 2005).
- The World Health Organization’s (WHO’s) international reference growth chart does not use percentiles but SD scores (SDS) (also known as z-scores). This approach has been chosen because in developing countries, where the WHO reference is mainly used, many birthweights lie below the 1st percentile, which makes precise classification based on percentiles problematic. For calculating z-scores, tables can be used in which the median (50th percentile) and 1 SD are given for the customized reference population (Oken et al., 2003). An individual’s z-score can be calculated using the following formula: (weight of individual – median weight of reference population)/1 SD in the reference population, in which +1 SD is applied if the individual’s weight is above the 50th percentile of the reference population and –1 SD if the individual’s weight is below the 50th percentile. For the comparison of different populations (e.g. singletons born after elective single-embryo transfer and double-embryo transfer), z-scores can be used, which are easy to handle from a statistical point of view. Furthermore, absolute differences in terms of SD are more informative than a weight difference in grams.
To allow for comparison of different studies on neonatal outcome, it should be agreed how to define and standardize outcome. For reporting on birthweight, SDS (or z-scores) are to be preferred, customized for the most important physiological variables affecting birthweight, e.g. gestational age, gender of the child and whether the child is a singleton or part of a multiple. Tables I and II give examples of customized birthweight data in a cohort of Dutch newborns. European data are not available at the moment, and for the time being it may be suggested that authors reporting on neonatal outcome use their country-specific data sets as a reference. Alternatively, it may be suggested that European Society of Human Reproduction and Embryology (ESHRE) should support the development of uniform reporting of ART outcome by stimulating the construction of European customized birthweight tables and by making available a software for calculating the z-scores in cohorts. The ultimate goal would be to facilitate authors to report on safety and quality of ART in a well-defined and uniform way and to enable readers to compare the outcome of different studies.
|
|
|
Acknowledgement |
|---|
|
TopAbstractAcknowledgementReferences |
|---|
The author acknowledges Dr W.J.M. Gerver, pediatric endocrinologist, for providing the birthweight data of Dutch newborns.
|
References |
|---|
|
TopAbstractAcknowledgementReferences |
|---|
Blair EM, Liu Y, De Klerk NH, Lawrence DM. (2005) Optimal fetal growth for the Caucasian singleton and assessment of appropriateness of fetal growth: an analysis of a total population perinatal data base. BMC Pediatr 5:13.[CrossRef][Medline]
De Sutter P, Delbaere I, Gerris J, Verstraelen H, Goetgeluk S, Van der Elst J, Temmerman M, Dhont M. (2006) Birth weight of singletons after assisted reproduction is higher after single than after double embryo transfer. Hum Reprod [19 June, Epub ahead of print]:.
Gardosi J, Mongelli M, Wilcox M, Chang A. (1995) An adjustable fetal weight standard. Ultrasound Obstet Gynecol 6:168–174.[CrossRef][Web of Science][Medline]
Gerver WJM and De Bruin K. (2001) Paediatric Morphometrics, A Reference Manual(Universitaire Pers Maastricht, Maastricht.).
Oken E, Kleinman KP, Rich-Edwards J, Gillman MW. (2003) A nearly continuous measure of birth weight for gestational age using a United States national reference. BMC Pediatr 3:6.[CrossRef][Medline]
Ombelet W, Martens G, De Sutter P, Gerris J, Bosmans E, Ruyssinck G, Defoort P, Molenberghs G, Gyselaers W. (2006) Perinatal outcome of 12 021 singleton and 3108 twin births after non-IVF-assisted reproduction: a cohort study. Hum Reprod 21:1025–1032.
Pinborg A, Loft A, Rasmussen S, Schmidt L, Langhoff-Roos J, Greisen G, Nyboe Andersen A. (2004) Neonatal outcome in a Danish national cohort of 3438 IVF/ICSI and 10 362 non-IVF/ICSI twins born between 1995 and 2000. Hum Reprod 19:435–441.
Wennerholm UB, Bonduelle M, Sutcliffe A, Bergh C, Niklasson A, Tarlatzis B, Mau Kai C, Peters C, Victorin Cederqvist A, Loft A. (2006) Paternal sperm concentration and growth and cognitive development in children born with a gestational age more than 32 weeks after assisted reproductive therapy. Hum Reprod 21:1514–1520.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  G. Griesinger, E.M. Kolibianakis, K. Diedrich, and M. Ludwig Ovarian stimulation for IVF has no quantitative association with birthweight: a registry study Hum. Reprod., November 1, 2008; 23(11): 2549 - 2554. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
I. Delbaere, S. Vansteelandt, D. De Bacquer, H. Verstraelen, J. Gerris, P. De Sutter, and M. Temmerman Should we adjust for gestational age when analysing birth weights? The use of z-scores revisited Hum. Reprod., August 1, 2007; 22(8): 2080 - 2083. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||