comparison of risk factors for small-for-gestational-age and preterm in a portuguese cohort of...

Matern Child Health J (2007) 11:417–424DOI 10.1007/s10995-007-0195-2

ORIGINAL PAPER

Comparison of Risk Factors for Small-for-Gestational-Ageand Preterm in a Portuguese Cohort of NewbornsTeresa Rodrigues · Henrique Barros

Received: 19 August 2006 / Accepted: 6 February 2007 / Published online: 7 March 2007C© Springer Science + Business Media, LLC 2007

Abstract Objective: To identify risk factors for small-for-gestational-age and preterm in a Portuguese cohort of new-borns. Study design: Socio-demographic, anthropometric,behavioural and obstetrical characteristics were evaluatedin 4.193 women consecutively delivered. Term small-for-gestational-age (n = 342) and non-small-for-gestational-age preterm (n = 148) were compared to non-small-for-gestational-age term births (n = 3538). Adjusted odds ra-tios and etiologic fractions were calculated. Results: Lowheight, low weight when entering pregnancy and low weightgain were significantly associated with small-for-gestational-age, but not preterm. These were the factors with thehighest etiologic fraction for small-for-gestational-age. Anincreased risk of small-for-gestational-age was found forwomen who smoked during pregnancy (OR = 2.39; 95%CI: 1.66–3.46) and began antenatal care after pregnancyfirst trimester (OR = 1.86; 95% CI: 1.32–2.62). Previ-ous abortion was associated with small-for-gestational-age(OR = 1.72; 95% CI: 1.16–2.55) and previous preterm withpreterm (OR = 3.20; 95% CI: 1.26–8.14). Conclusions:Low anthropometrics, smoking and late antenatal care wererisk factors for small-for-gestational-age, but not preterm.Maternal anthropometrics were the factors with the high-est impact on small-for-gestational-age. No factor showed agreat contribution to preterm birth.

Keywords Small-for-gestational-age . Preterm . Riskfactors

T. Rodrigues (�) · H. BarrosDepartment of Hygiene and Epidemiology, University of PortoMedical School,Al. Prof. Hernani Monteiro,4200-319 Porto, Portugale-mail: [email protected]

Introduction

A large set of socio-demographic, anthropometrical, be-havioural and obstetrical characteristics have been relatedto the occurrence of low birth weight, preterm birth andsmall-for-gestational-age newborns, either assessing the ef-fect of different risk factors on a single outcome [1–3] orcomparing the effect of one factor on different outcomes[4, 5]. However, few studies investigated the role of severalfactors simultaneously on preterm and small-for-gestational-age in the same population [6–10]. Studies that comparedrisk factors for preterm and small-for-gestational-age in thesame cohort found low level of social class, single mar-ital status and young maternal age were major determi-nants of preterm delivery, while smoking and maternal nu-tritional status mainly resulted in small-for-gestational-agebabies.

Risk factors for preterm and small-for-gestational-age dis-closed in different studies should not be directly comparedsince they may reflect differences in population character-istics or discrepancies in the strategy to control for con-founders.

The identification of specific risk factors for small-for-gestational-age or preterm might contribute to reveal thepathways leading to low birth weight, preterm and intrauter-ine growth restriction. Such knowledge finally promotesmore adequate driven preventive and therapeutic interven-tions.

The objective of this study was to find discrepantrisk factors for small-for-gestational-age and pretermbirth, contrasting socio-demographic, anthropometrical, be-havioural and obstetrical characteristics of mothers of small-for-gestational-age term and non-small-for-gestational-agepreterm with those of non-small-for-gestational-age term in-fants.

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418 Matern Child Health J (2007) 11:417–424

Material and methods

We evaluated maternal-infant pairs identified as part of across-sectional survey of 4193 women consecutively deliv-ered of singleton live newborns, at two general hospitals (oneLevel II and the corresponding Level III referral hospital)from the northern region of Portugal. All women were inter-viewed 48 to 96 hours postpartum, by a resident or certifiedobstetrician, using a structured questionnaire designed to ob-tain information on maternal social and demographic charac-teristics (age, marital status, education), obstetrical history(parity, previous abortion, low birth weight and preterm),behavioural factors (employment status and smoking duringpregnancy) and any diagnosis of chronic disease. Data onmaternal anthropometrical characteristics (height, weight atthe beginning of pregnancy and weight gain) and antenatalcare (gestational age at first antenatal visit and number of an-tenatal visits) was abstracted from antenatal routine recordsand confirmed with the women during hospital interview.

Gestational age and newborn characteristics (singletonversus multiple, gender, birth weight and major congeni-tal malformations) were extracted from obstetrical chartsshortly after birth. Gestational age was based on last men-strual period if the discrepancy with ultrasound ascertainedgestational age was less than one week, otherwise ultra-sounds were preferred.

Weight gain during pregnancy was estimated by subtract-ing pre-pregnancy weight from the last measured weightbefore delivery. Maternal weight gain rate was calculatedby dividing weight gain by gestational age at last weightmeasurement minus two weeks, because gestational age es-timate was based on last menstrual period. Low rate of gainwas defined as less than 0.27 kg per week, which representsa total maternal weight gain of less than 10 kg at 40 weeksof gestation.

Small-for-gestational-age was defined as birth weight be-low the 10th percentile for gestational age and gender, asdefined by Thomson et al. [11]. Preterm was defined asbirth before 37 completed weeks of gestation. For analy-sis, we compared term small-for-gestational-age (n = 342)and non-small-for-gestational-age preterm (n = 148) witha group of non-small-for-gestational-age term newborns(n = 3538). Non-small-for-gestational-age preterm infantswere restricted to moderate preterm (gestational age ≥ 32and < 37 weeks) since only ten very preterm cases werefound in the sample. Additionally, small-for-gestational-age babies (n = 383) were compared with non-small-for-gestational-age (n = 3700) and preterm (n = 201) withterm (n = 3911) infants. Newborns with major congenitalmalformations and those with missing information on keyvariables were excluded.

Adjusted odds ratios for the occurrence of term small-for-gestational-age and non-small-for-gestational-age preterm,

as well as for small-for-gestational-age and preterm as awhole, and respective 95% confidence intervals were esti-mated for each exposure variable, using logistic regressionmodels. Multivariable models for term small-for-gestational-age and non-small-for-gestational-age preterm contained thesame set of covariates to allow a more direct comparison ofrisk factors of interest. Variables included in the models werematernal age, marital status, education, maternal height, pre-pregnancy weight and weekly weight gain during pregnancy,number of pregnancies, smoking during pregnancy, antenatalcare, chronic diseases and work during pregnancy. The effectof previous obstetrical outcomes was assessed only amongplurigravid women, hence parity and other pregnancies out-comes were not included in the models. Statistical analysiswas performed in Stata r© (version 7.0). Using data on the dis-tribution of risk factors among small-for-gestational-age andpreterm births, and adjusted odds ratios using all small-for-gestational-age and preterm cases, etiologic fractions werecalculated according to Bruzzi et al. [12].

Results

Table 1 shows the distribution of socio-demographic, anthro-pometric, behavioural and obstetrical risk factors for non-small-for-gestational-age term, term small-for-gestational-age and non-small-for-gestational-age preterm groups.Mothers of term small-for-gestational-age babies were sig-nificantly more likely single, shorter than 155 cm, weighingless than 50 kg at the beginning of pregnancy, gained lessthan 0.27 kg per gestation week, smokers during pregnancy,attended less than three antenatal visits, had no antenatalcare during the first trimester of pregnancy and presented achronic disease. Maternal age, school education, parity oremployment status during pregnancy, were not significantlyassociated with term small-for-gestational-age birth. A termsmall-for-gestational-age baby was significantly more com-mon after a previous abortion or low birth weight baby.

The crude risk for delivering a non-small-for-gestational-age preterm was significantly increased among singlewomen, women attending less than three antenatal visits andthose presenting a chronic disease.

Table 2 shows adjusted odds ratios of term small-for-gestational-age and non-small-for-gestational-age pretermfor each factor. Short stature, low weight at the beginningof pregnancy and low rate of weight gain during pregnancysignificantly increased the risk of term small-for-gestational-age. A trend of decreasing odds ratios of term small-for-gestational-age with maternal weight at the beginning ofpregnancy has been shown. No maternal anthropometricalfactor was significantly related to non-small-for-gestational-age preterm birth. Tobacco smoking during pregnancy wasthe factor most strongly related to term small-for-gestational-

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Matern Child Health J (2007) 11:417–424 419

Table 1 Distribution of maternal socio-demographic, anthropomet-rical, behavioural and obstetrical factors according to birth outcome:non-small-for-gestational-age term (Non-SGA Term), term small-for-

gestational-age (Term SGA) and non-small-for-gestational-age preterm(Non-SGA Preterm)

Non-SGA Term (n = 3538) Term SGA (n = 342) Non-SGA Preterm (n = 148)Maternal characteristics n % n % n %

Maternal age (years)< 20 180 5.1 17 5.0 12 8.120–34 3132 88.5 296 86.5 126 85.1≥ 35 226 6.4 29 8.5 10 6.8

Marital statusMarried/cohabiting 3488 98.3 325 95.0 140 94.6Single 62 1.7 17 5.0 8 5.4

School education (years)< 4 88 3.0 17 5.7 6 4.84–8 2176 74.0 222 74.3 87 69.09–12 501 17.0 41 13.7 24 19.013–16 127 4.3 13 4.3 5 4.0≥ 17 49 1.7 6 2.0 4 3.2

Maternal height (cm)< 155 745 21.1 105 31.0 36 24.5155–163 2038 57.8 175 51.6 88 59.9≥ 164 745 21.1 59 17.4 23 15.6

Weight at the beginning of pregnancy (kg)< 50 483 13.9 74 22.6 31 21.550–59 1607 46.1 167 50.9 57 39.660–69 973 27.9 67 20.4 43 29.970–79 316 9.1 14 4.3 8 5.5≥ 80 104 3.0 6 1.8 5 3.5

Weight gain rate (kg/wk)< 0.27 1048 30.2 140 43.1 45 31.5≥ 0.27 2421 69.8 185 56.9 98 68.5

Number of gestations1 1838 51.7 163 47.7 79 53.4> 1 1714 48.3 179 52.3 69 46.6

Tobacco smoking during pregnancyYes 293 8.3 55 16.3 18 12.3No 3250 91.7 283 83.7 128 87.7

Job during pregnancyYes 3019 85.1 281 82.4 123 83.1No 527 14.9 60 17.6 25 16.9

Number of antenatal visits< 3 46 1.3 16 4.7 10 6.8≥ 3 3504 98.7 326 95.3 138 93.2

Beginning of antenatal care (weeks of gestation)< 14 3080 86.8 256 74.9 120 81.1≥ 14 470 13.2 86 25.1 28 18.9

Chronic diseasesYes 456 15.6 62 21.8 27 22.9No 2476 84.4 223 78.2 91 77.1

Previous abortionYes 442 25.8 65 36.3 18 26.1No 1270 74.2 114 63.7 51 73.9

Previous preterm deliveryYes 75 4.4 12 6.7 6 8.7No 1632 95.6 166 93.3 63 91.3

Previous low birthweightYes 86 5.0 18 10.2 4 5.8No 1622 95.0 159 89.8 65 94.2

SGA: small-for-gestational-age; Non-SGA: Non-small-for-gestational-age.

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Table 2 Adjusted odds ratiosfor termsmall-for-gestational-age (termSGA) andnon-small-for-gestational-agepreterm (Non-SGA preterm)

Term SGA Non-SGA pretermMaternal characteristics OR (95% C.I.) OR (95% C.I.)

Maternal age (years)∗

< 20 0.81 (0.43–1.50) 1.09 (0.50–2.37)20–34 1.00 1.00≥ 35 1.17 (0.71–1.90) 1.31 (0.63–2.73)

Marital status∗

Married/cohabiting 1.00 1.00Single 1.72 (0.82–3.63) 2.27 (0.88–5.83)

School education∗ (years)< 4 1.21 (0.57–2.56) 1.77 (0.67–4.66)4–8 1.00 1.009–12 0.81 (0.56–1.18) 1.24 (0.76–2.04)13–16 1.11 (0.60–2.05) 1.07 (0.41–2.78)≥ 17 1.05 (0.40–2.76) 2.51 (0.86–7.32)

Maternal height (cm)∗

< 155 1.55 (1.14–2.10) 1.13 (0.70–1.83)155–163 1.00 1.00≥ 164 1.10 (0.76–1.59) 0.74 (0.43–1.27)

Weight at the beginning of pregnancy (kg)∗

< 50 1.28 (0.91–1.82) 1.56 (0.90–2.68)50–59 1.00 1.0060–69 0.59 (0.42–0.84) 1.46 (0.91–2.34)70–79 0.30 (0.15–0.60) 1.09 (0.49–2.41)≥ 80 0.41 (0.16–1.06) 1.54 (0.52–4.58)

Weight gain rate (kg/wk)∗

< 0.27 1.90 (1.45–2.50) 1.06 (0.69–1.61)≥ 0.27 1.00 1.00

Number of gestations∗

1 1.00 1.00> 1 1.14 (0.86–1.52) 0.66 (0.44–1.01)

Tobacco smoking during pregnancy∗

Yes 2.39 (1.66–3.46) 1.40 (0.77–2.53)No 1.00 1.00

Job during pregnancy∗

Yes 1.00 1.00No 1.16 (0.81–1.64) 1.43 (0.87–2.34)

Number of antenatal visits∗

< 3 0.82 (0.32–2.11) 3.64 (1.29–10.26)≥ 3 1.00 1.00

Beginning of antenatal care (weeks of gestation)∗

< 14 1.00 1.00≥ 14 1.86 (1.32–2.62) 0.88 (0.48–1.64)

Chronic diseases∗

Yes 1.72 (1.25–2.37) 1.66 (1.05–2.63)No 1.00 1.00

Previous abortion∗∗

Yes 1.72 (1.16–2.55) 1.06 (0.54–2.11)No 1.00 1.00

Previous preterm delivery∗∗

Yes 1.42 (0.67–3.01) 3.20 (1.26–8.14)No 1.00 1.00

Previous low birthweight∗∗

Yes 1.72 (0.90–3.30) 1.59 (0.54–4.74)No 1.00 1.00

SGA: small-for-gestational-age;Non-SGA:Non-small-for-gestational-age.∗Odds ratios estimated in amodel including maternal age,marital status, education,maternal height, pre-pregnancyweight and weekly weight gainduring pregnancy, number ofpregnancies, smoking duringpregnancy, antenatal care,chronic diseases and workduring pregnancy.∗∗Analysis restricted toplurigravid women. Odds ratiosare adjusted for all othervariables except parity and otherprevious obstetrical outcomes.

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Table 3 Factors significantly associated with small-for-gestational-age in multivariable analysis and corresponding etiologic fractions (%).Odds ratios are adjusted for all variables presented in the table plus age,marital status, school education, parity, number of antenatal visits andemployment status

Small-for-gestational-ageMaternal characteristics Adj. OR (95% C.I.) EF(%)

Maternal height (cm)< 155 1.56 (1.16–2.08)155–163 1.00≥ 164 1.07 (0.75–1.52) 12.4

Weight at the beginning of pregnancy (kg)< 50 1.27 (0.91–1.77)50–59 1.0060–69 0.57 (0.41–0.80)70–79 0.39 (0.21–0.71)≥ 80 0.37 (0.15–0.95) 55.9

Weight gain rate (kg/wk)< 0.27 1.82 (1.40–2.36)≥ 0.27 1.00 19.6

Tobacco smoking during pregnancyYes 2.35 (1.65–3.33)No 1.00 9.2

Beginning of antenatal care (weeks of gestation)< 14 1.00≥ 14 2.01 (1.46–2.77) 13.7

Previous abortion∗

No 1.00Yes 1.72 (1.19–2.49) 15.2

Chronic diseasesYes 1.72 (1.27–2.33)No 1.00 9.5

OR: odds ratio.

EF: etiologic fraction.∗analysis restricted to plurigravid women.

age birth (adjOR = 2.39; 95% C.I.: 1.66–3.46). No signifi-cant effect of smoking during pregnancy has been noted forpreterm. Women with no antenatal care during pregnancyfirst trimester have shown a significant increase in the riskof term small-for-gestational-age (OR = 1.86; 95% C.I.:1.32–2.62) but not non-small-for-gestational-age preterm,whereas women with less than three antenatal visits dur-ing the whole pregnancy presented a significantly higherrisk of non-small-for-gestational-age preterm (OR = 3.64;95% C.I.: 1.29–10.26), but not term small-for-gestational-age. The risk of both term small-for-gestational-age andnon-small-for-gestational-age preterm was significantly in-creased among mothers presenting a chronic disease.

Risk factors disclosed as significantly associated withoverall small-for-gestational-age (Table 3) and preterm (Ta-ble 4) were similar to those found for term small-for-gestational-age and non-small-for-gestational-age preterm(Table 2), respectively; except the history of previous pretermthat was not significantly associated when overall preterm

Table 4 Factors associated with preterm in multivariable analysisand corresponding etiologic fractions (%). Odds ratios are adjustedfor all variables presented in the table plus age, marital status, schooleducation, parity, number of antenatal visits and employment status

Preterm birthMaternal characteristics Adj. OR (95% C.I.) EF (%)

Number of antenatal visits< 3 2.80 (1.14–6.90)≥ 3 1.00 4.8

Previous preterm delivery∗

Yes 2.17 (0.60–2.10)No 1.00 4.2

Chronic diseasesYes 1.80 (1.22–2.66)No 1.00 11.2

OR: odds ratio.

EF: etiologic fraction.∗analysis restricted to plurigravid women.

were considered (OR = 2.17; 95% C.I.: 0.60–2.10), but waswhen preterm was restricted to non-small-for-gestational-age preterm (OR = 3.20; 95% C.I.: 1.26–8.14).

In our population, maternal weight at the beginningof pregnancy accounted for the largest etiologic fractionfor small-for-gestational-age (55.9%, Table 3). It was fol-lowed by a weekly rate of weight gain less than 0.27 kg(19.6%), entering antenatal care after the first trimester ofpregnancy (13.7%), low maternal height (12.4%), chronicdiseases during pregnancy (9.5%), smoking during preg-nancy (9.2%), and among plurigravid women previous abor-tion (15.2%). Regarding preterm (Table 4), the only inde-pendent factors contributing to the etiologic fraction werechronic diseases (11.2%), less than three antenatal visits dur-ing pregnancy (4.8%), and previous preterm among parouswomen (4.2%).

Comment

In Portugal, the prevalence of low birth weight increasedfrom 4.6% in 1980 to 7.6% in 2004, and very low birthweight from 0.5% to 0.9% in the same period [13], attainingrates quite higher than in most developed European countries[14].

For most countries, the prevalence of preterm birth hasbeen reported steady or increasing [15]. However, in Portu-gal, a decreasing trend was evidenced in Health Statistics,from 12.0% in 1990 to 6.7% in 2004 [13]. Among us, thesetrends in low birth weight and preterm birth may reflectan undisclosed increasing rate of small-for-gestational agenewborns.

Low birth weight is the result of a shortened gestationor intrauterine growth restriction. To understand variations

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across time and region, and to intervene, it is important toidentify specific factors operating for intrauterine growthrestriction and preterm birth.

In large epidemiological studies, it is very difficult toassess intrauterine growth, small-for-gestational-age beingusually used as a proxy. However, small-for-gestational-agedefinition is based on population statistical data rather thannewborn clinical examination, thereby it does not discrim-inate growth restricted infants from infants constitutionallysmall.

Accuracy of small-for-gestational-age classificationlargely depends on the method used to ascertain gestationalage. Most large epidemiological studies relied on last men-strual period-based gestational age [10] or did not state theproportion of births whose gestational age was confirmed byfetal ultrasounds [6, 7]. Quality of information on gestationduration is one of the strengths of this study, since virtuallyevery pregnant woman had at least two fetal ultrasounds per-formed before 22 weeks gestation, according to the nationalobstetrical recommendations.

Few studies examined and compared a same set of riskvariables for small-for-gestational-age and preterm in thesame population. Differences in risk factors for small-for-gestational-age and preterm are not consistent across studies;however restrictions on the selected outcomes may have re-sulted in those findings [1–5]. Some studies did not excludesmall-for-gestational-age from the preterm group, allowingfactors mainly related to fetal growth to appear associatedwith preterm, as growth restricted foetuses are more proneto preterm birth.

Few characteristics have been shown to present an inde-pendent effect on preterm or low birth weight [16, 17]. Mostfactors, such as social class, marital status, antenatal careor maternal job, have no established direct effect on birthoutcome. Due to important differences between populationsand changes over time in medical care and in the social con-text of pregnancy, determinants of poor pregnancy outcomesneed to be re-assessed regularly. However, the comparison, inthe same population, of risk factors for intrauterine growthrestriction and preterm does not guarantee an independenteffect. They may just remain indicators of increased risk foreach of these birth outcomes. Nevertheless, these discrepan-cies highlight the need and the way for further research onoutcome specific risk factors.

This study looked at whether the association and impact ofsocio-demographic, anthropometrical, behavioural and ob-stetrical risk factors differed between small-for-gestational-age and preterm births. Maternal characteristics for this studywere selected on the basis of previously reported risk fac-tors. As we considered only term small-for-gestational-ageand non-small-for-gestational-age preterm babies, excludingsmall-for-gestational-age preterm, we expected to contrastthe effect of studied risk factors on distinct underlying pro-

cesses, intrauterine growth and pregnancy duration. In ourstudy, small-for-gestational-age preterm group was not eval-uated separately since small numbers (41 newborns) did notallow a reliable estimate for the combined effect of pretermand growth abnormalities. This outcome has been reported tobe mainly related to hypertension disorders during pregnancyand to result in higher neonatal and post-neonatal mortal-ity [18]. Intrauterine growth restriction among very pretermnewborns also increases the risk of cognitive disorders laterin childhood [19]. Small-for-gestational-age term newbornsmay constitute a group with lower morbidity and mortality,but since they correspond to a much larger proportion of allnewborns they may represent a potential important publichealth burden. We also restricted non-small-for-gestational-age preterm to moderate preterm since our sample only com-prised ten very preterm newborns, and the criteria we used forclassification of small-for-gestational-age were not reliableat low gestational age.

We identified several factors associated with small-for-gestational-age but only few antenatal visits, maternalchronic diseases and a previous preterm delivery were inde-pendently related to preterm birth.

Maternal anthropometrics, including short stature, lowweight at the beginning of pregnancy and low weight gainrate were the strongest risk factors for small-for-gestational-age. These factors were not significantly related with pretermbirth. Other authors found that low weight gain (less than0.27 kg/wk) was associated with spontaneous preterm birth(adjusted OR = 2.5; 95%CI:2.0–3.1), but this effect wasmodified by maternal race or ethnicity and history of previ-ous preterm birth [20]. Although, some authors stated thatlow prepregnancy body mass index is one of the strongestpredictors of preterm and fetal growth retardation [21], ina recent cohort study it was found only a 14% increasein the risk of moderate preterm birth among underweightwomen ( < 20 kg/m2) when compared with normal weight(20–24.9 kg/m2) [22]. Maternal stature and weight at the be-ginning of pregnancy are largely influenced by social and nu-tritional circumstances from infancy until adult life, whereasweight gain during pregnancy depends largely on caloricintake during pregnancy.

Smoking habits during pregnancy only presented a sig-nificant independent effect on small-for-gestational-age, andthe risk at least doubled for smokers. These results agreewith previous reports, like those from Horta et al. who re-ported smoking was associated with an odds ratio for in-trauterine growth restriction of 2.07 [23]. Moreover, mater-nal smoking has been more frequently reported to inducegrowth restriction than preterm birth. The interpretation ofthe results regarding the association between smoking dur-ing pregnancy and preterm birth is still controversial [3].Some studies reported that the association may just reflectan association between smoking and intrauterine growth

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restriction and other fetal complications that lead to iatro-genic or eventually spontaneous preterm. Supporting the pre-vious hypothesis, Zeitlin et al also found a stronger associa-tion between smoking and small-for-gestational-age pretermthan with other preterm births [24]. Recently, maternal smok-ing has been reported as increasing the risk of very pretermbirth caused by preterm labor, preterm rupture of membranesand late pregnancy bleedings but not by hypertensive disor-ders [25, 26]. Others have showed a relationship betweenheavy smoking and preterm birth mostly for women withlow obstetric risk [27]. The contribution of maternal smok-ing to preterm birth also seems to differ among ethnic groupsof the same country [28, 29].

A previous abortion was related with term small-for-gestational-age, whereas previous preterm was associatedwith non-small-for-gestational-age preterm birth. Previ-ous preterm was not significantly associated with pretermwhen small-for-gestational-age preterm infants were not ex-cluded, probably reflecting a higher proportion of small-for-gestational-age among overall preterm births (41 out of 189)than in other populations [10].

Maternal chronic diseases were the only common in-dependent factor that we found related to both small-for-gestational-age and preterm. We did not specify chronic dis-eases and non-differential misclassification may have oc-curred, but the main conclusion for this variable is that itsassociation with both small-for-gestational-age and pretermwas quite similar.

In this population, maternal age, married status, school ed-ucation, parity and employment status during pregnancy didnot appear as independent risk factors for any birth outcome.In univariate analysis, single marital status was associatedwith both small-for-gestational-age and preterm, but aftercontrolling for covariates the effect was considerably atten-uated and statistically non-significant. Although insufficientstatistical power after adjustment for multiple covariates maypartly explain these findings, another reason social factors donot show an independent effect on small-for-gestational-ageor preterm may be free access to medical care, social supportand protective legislation of labor provided to all Portuguesepregnant women. The role of socio-demographic factors onbirth outcome may change considerably over time and re-gions, and this probably occurs because these factors are notdirect causes of intrauterine growth restriction or pretermbirth.

After controlling for socio-demographic and anthropo-metric factors, no antenatal care in pregnancy first trimesterwas found to be a risk factor only for small-for-gestational-age, suggesting that improving early access to antenatal caremay have greater influence on intrauterine growth restric-tion than preterm birth prevention. Less than three antena-tal visits were disclosed as a risk factor only for pretermbirth. This association remained statistically significant in

both strata of early and late antenatal care initiation (datanot shown). Although this association might be explainedby reverse causation, other analysis conducted by us in thesame population revealed an association between quantita-tively inadequate antenatal care (an index dependent on thebeginning of antenatal care and duration of pregnancy) andpreterm birth [30]. Contrary to our findings, Lang et al. didnot find any association between no antenatal care in preg-nancy first trimester and small-for-gestational-age or pretermbirth [10].

Our results support earlier findings that small-for-gestational-age and preterm have different determinants. Theretrospective nature of this study does not allow conclusionsregarding causal relationships, but it contributes to the iden-tification of several preventable risk factors for intrauter-ine growth restriction and preterm birth in our population.Among us, the most important risk factors for small-for-gestational-age as a public health issue were unfavourablematernal anthropometrics followed by no antenatal care inpregnancy first trimester and smoking during pregnancy. Al-though Kramer [31], in 1987, has pointed out that cigarettesmoking was the leading risk factor for intrauterine growthrestriction in developed countries, in our study a smallerproportion of small-for-gestational-age may be attributed tosmoking because of a relatively lower prevalence of smokersduring pregnancy. For preterm, we found that less than threeantenatal visits was the modifiable factor with more impacton this outcome.

Entering pregnancy in better nutritional conditions, ad-equate weight gain during pregnancy, early beginning ofantenatal care and not smoking during pregnancy may beregarded as preventive measures leading to the decline ofsmall-for-gestational-age. According to our results, attend-ing an adequate number of antenatal visits seems to be theonly relevant intervention to prevent preterm birth amongPortuguese women.

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