Best Practice & Research Clinical Obstetrics & Gynaecology
4Risk factors for small for gestational age infants
Section snippets
Definitions of SGA
Small for gestational age (SGA) can be defined by a number of criteria but currently the most common definition is a birth weight <10th population centile. Customised birth weight centiles provide a better estimate of infants with high morbidity and mortality, who are more likely growth restricted as well as SGA.1, 2, 3 This definition of SGA is increasingly being used in practice. Almost all studies to date, investigating risk factors for SGA, have used population centiles to define SGA. The
Age
Women ≥35 years have been found in several studies to have an elevated risk of SGA.4, 5, 6, 7, 8 Others have found advanced maternal age was either not a risk factor for SGA or the effect of maternal age was no longer significant after adjustment for associated factors.9, 10
Ethnicity
Assessment of the influence of ethnicity on SGA is complex because of differences in nutrition, environment, age, parity, maternal height, weight and socioeconomic status and cigarette smoking. Most studies have addressed
Cigarette smoking
Many studies have shown that cigarette smoking has a dose-dependent and causative relationship with SGA and is the single most important risk factor for SGA in developed countries.*11, 12, 21 A recent publication from the SCOPE study reported that women who stopped smoking by 15 weeks' gestation had a rate of SGA equivalent to that of non-smokers.22 The majority of stopped smokers in this study had quit by 12 weeks, so it is recommended that women should be supported to stop smoking as early as
Caffeine
In 2001, the Committee on Toxicity of Chemicals in Food in the UK concluded that although caffeine intake of >300 mg daily might be associated with low birth weight, the evidence was inconclusive. A recent study from the CARE Study Group quantified caffeine intake from all sources during pregnancy and defined SGA as <10th customised centile.30 Increased SGA occurred in women who consumed 200–299 mg caffeine daily, risk ratio 1.5 (95% CI: 1.1–2.1) and >300 mg daily 1.4 (95% CI 1.0–2.0). This effect
Chronic hypertension
Population-based studies from several countries have shown that chronic hypertension, one of the most common medical conditions in pregnancy, is associated with increased SGA.53, 54, 55 SGA is more common with superimposed pre-eclampsia, 48% versus 21% in chronic hypertension with superimposed pre-eclampsia compared with chronic hypertension alone, (RR: 2.30 (95% CI: 1.85–2.84)).56 Sibai et al. reported that women with mild chronic hypertension did not have increased SGA unless they developed
Hyperemesis
Two large recent population-based surveys investigated pregnancy outcomes in women admitted with hyperemesis.86, 87 In the first Californian study, 0.4% of women were admitted with hyperemesis.86 Their babies had lower mean birth weight (3255 g vs. 3380 g, P < 0.0001) and increased SGA (29.2% vs. 20.8%, P < 0.0001). The second study from Nova Scotia analysed pregnancy outcomes in women admitted with hyperemesis according to maternal weight gain.87 Those who gained 7 kg or more did not have increased
Maternal psychosocial stress
The literature regarding maternal stress, social support and depression during pregnancy and risk of SGA also shows conflicting results. In a recent Dutch study, high levels of perceived stress at 14 weeks' gestation were associated with increased SGA but, after adjustment, the effect was no longer significant (aOR: 1.16 (95% CI: 0.92–1.47)).99 Pryor et al., in a case-control study with 836 term SGA infants, also did not find an association between perceived stress and SGA.100 In contrast,
Inter-pregnancy interval
A recent meta-analysis of 67 studies of inter-pregnancy interval and pregnancy outcome found that compared with an inter-pregnancy interval of 18–23 months (optimum birth outcomes) those with an inter-pregnancy interval of <6 months had higher rates of SGA (aOR: 1.26 (95% CI: 1.18–1.33)).105 Increased rates of SGA were also found for inter-pregnancy intervals 6–11 months (aOR: 1.18 (95% CI: 1.14–1.23)), 12–17 months (aOR: 1.08 (95% CI: 1.06–1.11)) and >60 months (aOR: 1.29 (95% CI: 1.20–1.39)).
Previous miscarriage(s)
Changing paternity
Two studies have reported the relationship between changing paternity and risk of later SGA.4, 112 Krulewitch et al. found that women who did not have an SGA infant in a previous pregnancy had increased SGA with a new partner (aOR: 1.19 (95% CI: 1.1–1.3)), adjusted for maternal and paternal age, race and occupational level.111 Kleijer et al. also reported that women with a different partner in the index pregnancy had an increased SGA (OR: 1.9 (95% CI: 1.3–2.9)) but did not report details of
Foetal size
Bukowski et al., in a large cohort of accurately dated IVF pregnancies, found that the risk of SGA decreased with increasing crown–rump length on the first trimester scan.116 These data were similar to findings from a large cohort of spontaneously conceived pregnancies where foetuses with first-trimester crown–rump length 2–6 days less than expected for gestation had a threefold increase in SGA.117 Several second-trimester ultrasound studies have also reported that foetuses with lower estimated
Conflict of interest
Neither author has any conflict of interest.
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