Fetal growth and birth outcome are associated with perinatal survival and health, with the patterns of postnatal growth, and with differential risk for cognitive, metabolic and cardiovascular disease later in life.1 Birthweight and gestational age are the main perinatal indicators that support this relationships between fetal and postnatal stages, as both variables (i.e. reduced birthweight and/or preterm births) have been associated with epigenetic, hormonal and metabolic regulation mechanisms affecting health throughout life cycle.2 Birthweight for gestational age charts allow assessing the prevalence of small for gestational age (SGA) newborns, those born with a birthweight under 10th percentile for their gestational age. However, fetal growth charts prepared for one specific population are not appropriate for evaluating growth in different populations as clinically important differences in both the mean birthweight and percentile values may be found.3 These differences are due to biosocial characteristics of the population used as the reference and the study methodology. Thus, it is important to differentiate between reference growth curves and standard growth curves, as Rao and Tompkins4 remember: reference curves show the fetal growth of a particular population at a specific time, while standard curves show how a newborn should grow according to an ideal healthy growth, and hence are of prescriptive nature.

Different growth charts by gestational age and sex have been designed for Spanish newborns based on hospital data.5–9 Some of these charts are still widely used in the Spanish public health system for assessing birth outcome and infant growth although Spain has officially adopted the new WHO standards for term births and up to five years old children.10 Most recently, Ramos et al.11 and González-González et al.12 have proposed models for calculating optimal fetal and neonatal weight curves from population and hospital data respectively, and García-Muñoz Rodrigo et al.13 have published the first growth standards for very preterm Spanish newborns (22-28 weeks) using data from 62 hospitals. Most of these charts were prepared without differentiating parity and type of delivery. However, parity is a well-recognized predictor of infant birthweight, with infants born to primiparous women registering significantly lower birthweight and higher prevalence of intrauterine growth restriction (IUGR).14 At the same time, the increasing rates of induced deliveries and Cesarean sections (CS) before week 37 has been associated with the increased prevalence of preterm births with extremely low weight in developed countries.15,16

The profile of Spanish (national) mothers has undergone a significant change over the past decades, with a sustained increase in the percentage of mothers who start reproducing at later ages, as well as in the rates of obstetric interventions in general, and CS deliveries in particular.17 From 2008 onwards the economic crisis is strengthening these trends,18 specifically the growing predominance of primiparous mothers of ever-increasing age. According to the latest available data,19 54.3% of Spanish mothers are primiparous with an average age for first maternity of 31.06 years, while the rate of CS is 25.2% —a figure that is double that recommended by WHO20—, with higher rates in private hospitals and in public hospitals with a lower technological level.21 As proposed for other European countries,22 these trends in maternal profile and obstetric practices might be contributing towards trends of lowering mean birthweight and increasing prevalence of low birth weight (LBW: birthweight under 2,500 grams) described in Spain.17 In this context, to stablish and compare charts of birthweight by parity and type of delivery may contribute both to a fitter evaluation of the impact of these trends on gestational growth and birth outcome in Spain, and to a greater understanding of the causes underlying the sustained increase in the rate of CS in the country.

With these aims, using data from the Spanish Birth Statistics Bulletin for the period 2010-2014, the aim of this study is to prepare new reference charts of birthweight by gestational age in Spain, stratified by sex, parity and type of delivery.

Shown in Table 1 are the number of total births (N) and prevalence of CS delivery by gestational age, sex of the newborn, and parity. The highest rates of CS deliveries occurred between weeks 28 and 32, and then again after week 40, both in primiparous and multiparous mothers. The total rate of CS deliveries was significantly higher for primiparous than for multiparous mothers (28.8% and 21.8% respectively; χ2=8,896.015; d.f.=1; p<0.001, both sexes considered). The rate of CS was significantly higher among primiparous mothers both in preterm births (males: χ2=53.211, d.f.=1, p<0.001; and females: χ2=29.557; d.f.=1; p<0.001) and in term births (males: χ2=5,492.929; d.f.=1; p<0.001; and females: χ2=3,495.540, d.f.=1, p<0.001).

Tables I-VIII (and Figures I-VIII) of the online Appendix of this article present the percentile values and L, M and S parameters proposed for evaluating birthweight by gestational age, sex, parity and type of delivery in newborns of Spanish mothers. Based on these data, Figures 2 and 3 present the birthweight charts (10th, 50th and 90th percentiles) for newborns to primiparous and multiparous women according to sex and type of delivery. Independently of sex and type of delivery, the mean birthweight (50th percentile) of newborns to primiparous mothers was significantly lower than that of newborns to multiparous mothers (from gestational age 30 weeks in males and 34 weeks in females born by vaginal deliveries, respectively, and from 25 and 26 weeks, respectively, in those born by CS deliveries: p<0.036; Tables IX-XII of the online Appendix).

Figure 2.

Birthweight by gestational age 10th, 50th and 90th percentiles for males (a) and females (b) by vaginal delivery to primiparous (solid lines) and multiparous mothers (dotted lines). (Data from Spanish Birth Statistics Bulletin, single live births, Spanish mothers, 2010-2014).

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Figure 3.

Birthweight by gestational age 10th, 50th and 90th percentiles for males (a) and females (b) by Cesarean section delivery to primiparous (solid lines) and multiparous mothers (dotted lines). (Data from Spanish Birth Statistics Bulletin, single live births, Spanish mothers, 2010-2014).

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Likewise, Figures 4 and 5 show the birthweight charts of newborns delivered vaginally and by Cesarean section according to parity and sex. Independently of sex and parity, mean birthweight of preterm newborns delivered by CS was significantly lower than those delivered vaginally (from 24 to 37 weeks in males, and from 25 to 37 weeks in females to primiparous mothers; and from 27 to 36 weeks in males, and from 26 to 36 weeks in females born to multiparous women: p<0.048; Tables XIII-XVI of the online Appendix). Accordingly, the prevalence of SGA was also significantly higher among newborns born at early gestational ages by CS delivery compared with those delivered vaginally, both in primiparous (from 26 to 38 weeks of gestation in both sexes: p<0.08; Tables XVII and XIX of the online Appendix) as well as in multiparous mothers (from 28 to 37 weeks of gestation in both sexes: p<0.027; Tables XXI and XXIII of the online Appendix). Conversely, Figures 4 and 5 also show that mean birthweight of term newborns delivery by Cesarean section was greater than those born vaginally (from 38 to 42 weeks in both sexes and parities: p values <0.030; Tables XIII-XVI of the online Appendix). As expected, in these gestational ages prevalence of LGA was higher in newborns born by CS delivery, again both among primiparous (from 36 to 42 weeks in males, and from 37 to 42 weeks in females: p<0.035; Tables XVIII-XX of the online Appendix) and multiparous mothers (from 37 to 42 weeks in males, and from 38 to 42 weeks in females: p<0.007; Tables XXII-XXIV of the online Appendix).

Figure 4.

Birthweight by gestational age 10th, 50th and 90th percentiles for males (a) and females (b) to primiparous mothers by vaginal delivery (solid lines) and Cesarean section delivery (dotted lines). (Data from Spanish Birth Statistics Bulletin, single live births, Spanish mothers, 2010-2014).

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Figure 5.

Birthweight by gestational age 10th, 50th and 90th percentiles for males (a) and females (b) to multiparous mothers by vaginal delivery (solid lines) and Cesarean section delivery (dotted lines). (Data from Spanish Birth Statistics Bulletin, single live births, Spanish mothers, 2010-2014).

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This paper presents the first nationally representative reference tables and charts for birthweight by gestational age and sex according to parity and type of delivery for Spanish population. These tables allow calculating the exact percentile or z-score of the newborn depending on parity, type of birth and newborn sex due to the inclusion of L, M and S values. Our results support the suggestion by Hinkle et al.28 of establishing parity-specific birthweight references. Live births of both sexes and all gestational ages born to primiparous mothers showed significantly lower birthweight than those of multiparous mothers, as Liu et al.29 have recently confirmed for Chinese population. Primiparity determines a reduction in birthweight and is a well-known risk factor for greater incidence of LBW and IUGR, when other maternal characteristics are controlled, including type of delivery.30 The main biological mechanism proposed as an explanation of the effect of parity on fetal growth is a lesser vascular uteroplacental capacity in primiparous mothers, which reduces the supply of oxygen and nutrients to the fetus.31,32 There are also structural factors for primiparous mothers (such as a less intra-uterine space in the first pregnancy than in later pregnancies) which also tend to reduce birthweight.33

Results also confirm that newborns delivered by CS showed more variation of weight at birth, depending on gestational age, than those born vaginally, as previous research has shown.34,35 CS deliveries were associated with a substantial, significant reduction in birthweight among earlier gestational ages of both sexes and all maternal parities in such a way that the distribution of the 10th percentile is skewed towards lower birthweight among preterm live births delivered by CS compared with those born vaginally. As extreme low birthweight among preterm births is associated with increased risk of neonatal death,36 the rising rates of CS deliveries at early gestational ages in developed countries have been associated with improved perinatal survival,15,37 a positive trend supported by the dramatic technological breakthroughs in Perinatology and Neonatology of last decades. However, this obstetric trend does not explain the dramatic increase in CS deliveries in Spain, given that the most of this type of delivery took place among gestations of 37 weeks or more —over 90%, in both primiparous and multiparous mothers (as can be confirmed from Table 1). Certainly, the prevalence of LGA newborns was higher in newborns born by CS delivery compared with those born vaginally, both among primiparous and multiparous mothers, although not as heavily as the previously described increase in the prevalence of SGA among preterm live births, as figures 4 and 5 clearly show; likewise, although the mean birth weight is significantly higher among term newborns born by CS delivery compared with those born vaginally, the differences are very small (Tables I-VIII of the online Appendix). In summary, the high rates of CS deliveries registered in Spain cannot be explained by a high prevalence of dystocia by macrosomia or cephalopelvic disproportion, neither for primiparous nor for multiparous mothers. As Bernis et al.17 and Redondo et al.21 consider, research on non-clinical factors affecting delivery could clarify the ever-growing rate of CS in Spain, an issue that goes beyond the objectives of this paper. Meanwhile, the utilization of neonatal charts for birthweight taking into account type of delivery is highly recommended to evaluate the effect that the increase in the rate of CS deliveries during the last two decades in Spain would have on the assessment of fetal growth if type of delivery is not considered, particularly in order to avoid both underestimating the prevalence of SGA in preterm births and overestimating the prevalence of LGA in term births among those delivered vaginally.38,39

The evaluation of fetal growth and its secular trends are important for human biologists, epidemiologists, gynaecologists and public health professionals, as birth weight is related with perinatal survival and health, with the patterns of postnatal growth, and with differential risk for metabolic and cardiovascular disease later in life. Because the main environmental factors involved in its variability (maternal profile and protocols of delivery) have been rapidly changing during the last decades in Spain as in other developed countries, there is an increasing agreement that the temporal tendencies of fetal growth and birth outcome must be based on reference tables and charts for birthweight by gestational age and sex according to parity and type of delivery. Results support this consideration, giving important clues to address the causes of the increasing rate of CS deliveries in Spain.

Glòria Pérez.

The corresponding author on behalf of the other authors guarantee the accuracy, transparency and honesty of the data and information contained in the study, that no relevant information has been omitted and that all discrepancies between authors have been adequately resolved and described.

J.M. Terán and C. Varea designed the study and proposed a first draft. J.M. Terán managed and analysed data. C. Bernis, B. Bogin and A. González-González contributed reviewing the literature, editing all versions of the paper and providing relevant contributions. All authors approved the final version.

The authors acknowledge financial financial support from the School of Sport, Exercise & Health Sciences, Loughborough University (UK), to help defray the publication charges for this article.

None.