1 Epidemiology Unit, Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, University of London, Keppel St, London WC1E 7HT, UK
2 Current affiliation: Department of Obstetrics and Gynaecology, St Michael's Hospital, Southwell Street, Bristol BS2 8EG, UK
3 Academic Department of Obstetrics and Gynaecology, Imperial College Faculty of Medicine, Chelsea and Westminster Hospital, London SW10 9NH, UK
4 Department of Epidemiology and Public Health, Imperial College Faculty of Medicine, Norfolk Place, London W2 1PG, UK
Correspondence: Dr Roshni R Patel, Level D, Department of Obstetrics and Gynaecology, St Michael's Hospital, Southwell Street, Bristol BS2 8EG, UK. E-mail: roshni.patel{at}bristol.ac.uk
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Abstract |
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Methods The cohort comprised 122 415 nulliparous women with singleton live fetuses at the time of spontaneous labour, giving birth in the former North West Thames Health Region, London, UK.
Results The median gestational age at delivery was 39 weeks in Blacks and Asians and 40 weeks in white Europeans. Black women with normal body mass index (BMI) (18.524.9 kg/m2) had increased odds of preterm delivery (odds ratio [OR] = 1.33, 95% CI: 1.15, 1.56, adjusted for deprivation and BMI) compared with white Europeans. The OR of preterm delivery was also increased in Asians compared with white Europeans (OR = 1.45, 95% CI: 1.33, 1.56, adjusted for single unsupported status and smoking). Meconium stained amniotic fluid, which is a sign of fetal maturity, was statistically significantly more frequent in preterm Black and Asian infants and term Black infants compared with white European infants.
Conclusions This research suggests that normal gestational length is shorter in Black and Asian women compared with white European women and that fetal maturation may occur earlier.
Accepted 14 May 2003
The estimated date of delivery (EDD) is calculated clinically early in pregnancy, reflecting its social and medical importance. It is calculated using the date of the last menstrual period (LMP) by adding 280 days to the date of the first day of the LMP, giving a point estimate of 40 weeks for gestational length (including the 2 weeks before conception occurs). This method (sometimes known as Naegele's rule, although his method of adding 7 days and subtracting 3 months from the date of the LMP can give a date up to 3 days different to the 280-day method, because of the variation in the length of different months) has to be relied upon in areas without ultrasound access for dating. Even where ultrasound is available, the principle persists of using menstrual dates to determine the EDD, provided the date suggested by ultrasound measurement does not differ by more than 7 days. This is because the use of ultrasound as a dating technique requires the assumption that all fetal measurements are average for the gestation at which they are made, when they may truly be large or small for gestational age at that time. These methods of calculating the EDD are applied regardless of individual medical or demographic characteristics, and do not account for the fact that different babies mature at different rates.
Few women deliver on their calculated EDD and 510% of women deliver preterm.1,2 Several factors are known to affect the duration of pregnancy, including parity, socio-demographic characteristics, medical complications, previous preterm delivery, cigarette smoking, and maternal age.2,3
Obstetric outcomes may differ amongst ethnic groups when managed in the same setting. A British study found significant differences in duration and outcomes of labour when comparing white, Asian, and black women.4 Shorter gestational length has been observed in certain ethnic groups.1,5 Two studies have estimated an average gestational length 5 days shorter in black pregnancy.6,7 One study noted that differences were more strongly associated with the mother's rather than the father's race.8 Racial differences have also been observed in the rates of preterm (3337 weeks) and very preterm (<33 weeks) birth in black compared with white women.9,10 A UK study explored the factors associated with preterm delivery in different ethnic groups and found that gestation was shorter in UK Africans and Afro-Caribbeans even after correction for socioeconomic risk factors.11
One hypothesis for shorter average gestational length amongst black infants is that earlier maturation of the feto-placental unit relates to the maternal pelvic size. A smaller pelvis benefits the mother in evolutionary terms in relation to posture and stability when running. However, a smaller pelvis is also associated with a higher incidence of both obstructed labour and maternal mortality. Indeed, Africans have been observed to have amongst the highest emergency caesarean section rates. In fetal terms it is advantageous for the fetus to have a large head because of the improved brain growth. Thus, this creates conflict in the maternal/fetal relationship. It therefore would be in the interest of the fetus to mature faster and deliver earlier to avoid the complications described.
It is well recognized that gross motor skills develop in black infants earlier than in their white counterparts.12 There is also evidence of earlier fetal maturation. The incidence of the fetal passage of meconium during labour is strongly related to gestational age, increasing from less than 5% at 34 weeks in white European women, to over 25% post EDD.13 Black infants are significantly more likely to pass meconium in utero at all gestational ages, indicating earlier maturation.14,15
Perinatal mortality rates also differ amongst ethnic groups. Black infants in the US experience overall higher mortality compared with white infants.16,17 In the UK, the highest perinatal mortality rates have also been seen in ethnic minorities.18,19 However, this oversimplifies the relationship between perinatal mortality and ethnicity. Black gestational age specific mortality has been observed to be lower than white infants amongst those born preterm.17,19 After 37 weeks, this pattern is reversed with higher perinatal mortality amongst black infants compared with white infants.16 These observations suggest black infants mature earlier compared with white infants hence their survival advantage if born preterm. By contrast, black infants born after 40 weeks gestation may be susceptible to complications of post maturity at earlier gestations than white infants.
The aim of this study was to compare gestational length amongst three ethnic groups in nulliparous women with singleton pregnancies and spontaneous labour.
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Materials and Methods |
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The white European women were regarded as the reference (control) group. Black African and black Caribbean women were combined into one group, hereafter called Black. The second group comprises women from India, Pakistan, and Bangladesh, hereafter called Asian.
The main outcome measure was gestation at delivery (term birth). This is recorded in whole weeks on the SMMIS database and is derived from the EDD, which is calculated using a combination of LMP (available for 95% women), clinical examination, and ultrasound scan (available for 96% women). Term was defined as 37 completed weeks of pregnancy and preterm between 24 and 37 weeks. Length of neonatal stay in intensive care was compared as a binary variable of <1 day or
1 day.
Potential confounders were identified before analysis. These were marital status, single unsupported mother status, Carstairs deprivation score, maternal age at delivery, cigarette smoking, maternal height, body mass index (BMI) at booking, gestation at booking, history of diabetes mellitus, history of hypertension, any other ante-natal booking complications, and year of delivery.
Analysis strategy
The two ethnic groups (Black and Asian) under consideration were analysed separately and compared with the reference (white European) group. The distributions of ethnic groups in the cohort were calculated and differences in baseline characteristics were compared with the reference group using 2 significance tests.20 The odds ratio (OR) and 95% CI for the association between the ethnic groups and the outcome of interest was calculated. Two-sided likelihood-based significance tests were deemed statistically significant if the associated P-value was
5%.21,22 The effect of the identified potential confounders on the unadjusted OR between ethnic group and gestational age at birth was analysed. Information from this analysis was used to develop a log linear logistic regression model. Covariates were added into the model according to their confounding effect and retained if the crude OR changed by over 10% after adjustment. Interaction terms were managed similarly. Statistical significance was assessed using the likelihood ratio test (LRT). Stata 7.0 software (StataCorp, Texas) was used for analysis.
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Results |
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Conclusions |
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SMMIS covers 80% of the population within a geographical area as not all hospitals in the region participate in the SMMIS collaboration. The effect of their exclusion on the results can not be quantified, making the introduction of selection bias possible. In the UK, few women have private obstetric care and this enhances the representativeness of this cohort to the general population. SMMIS is useful for studies considering ethnicity as it is based in urban areas, which reflect the ethnic diversity of the UK. By maintaining a uniform system of data collection SMMIS provides high quality data for research, which has been validated by other studies.23,24
Classification of racial or ethnic groupings is problematic and reflected by the inconsistent approach of other studies addressing such issues. SMMIS ethnicity classification is based on Department of Health guidelines, which require self-reporting. The missing group formed 3.3% of the total. This may be missing due to non-collection of data or alternatively represent individuals unwilling to assign themselves to an ethnic group. If this is confined to a particular ethnic group this could bias results; however, given the small size of this group, it is unlikely to have a significant influence.
Calculation of gestational age in the hospitals contributing to SMMIS is by a combination of methods as described previously. Inaccuracies are inherently associated with estimation of EDD using LMP alone, but are reduced by using additional ultrasound scan information.25 EDD is liable to further error the later the first ultrasound scan. Data were unavailable for this study regarding timing of first scan and could be a source of bias in estimating EDD if ethnic groups book for antenatal care at different times. A recent study in England and Wales found that women from ethnic minorities initiated antenatal care later than white British women.26
The results of this research are consistent with other studies and indicate that Black ethnicity is associated with decreased gestational length.1,6,10,11 There is a paucity of data exploring this research question with British Asian women. One study found minimal effect of maternal characteristics on length of gestation but it was carried out in an area with less than the UK average of ethnic minorities and did not define what was meant by the term Asian.27 Aveyard et al.11 found an increased risk of preterm delivery for Afro-Caribbean women but not for African women. In the former group, they found that half of the excess risk was associated with marital status and deprivation. Henderson & Kay5 found a decrease in Negro compared with white pregnancy duration but only included women of low socioeconomic class and used LMP for dating.
The finding that Black infants had increased odds of 1.5 of meconium stained amniotic fluid is comparable to the few published studies in this area.15 It provides epidemiological evidence concerning fetal maturity to support the hypothesis of earlier maturation. Sedaghatian et al.14 found that meconium stained amniotic fluid varied with ethnicity and was highest in East African blacks. However, the ethnic grouping in this study is flawed as it is not based on standard classification, with African and Asian groups subdivided largely on the basis of skin colour.
If there is a difference in gestational length by ethnic group this could potentially influence a few areas of clinical practice, although further data regarding infant morbidity and mortality by ethnicity would be needed first. For example, steroid injections are administered to mothers before 34 weeks to decrease the risk of respiratory distress syndrome, if delivery is needed. There is evidence to support the safety of steroids although repeated doses may increase adverse maternal outcomes.28 If Black infants mature earlier, it may be possible to halt steroid treatment earlier without compromising survival.
In ethnic groups with shorter gestation it may be appropriate to utilize different definitions of term, for instance reducing cut-off points by one week. For example, women who deliver preterm are regarded as high risk in subsequent pregnancies, which in turn influences clinical management and may limit patient choice. Black women who deliver at 36 weeks may not warrant inclusion in this category. Elective caesarean section is typically performed at 39 weeks gestation. If this standard is applied to all ethnic groups Black and Asian women may be at higher risk of requiring emergency caesarean section with the extra complications this confers. Thus, consideration could be given to slightly earlier delivery in some groups. There are advantageous social implications of increased accuracy of EDD prediction such as facilitating better maternity leave planning and child care arrangements.
The presented results suggest that gestational length may vary by ethnicity. However, it is possible that these results have arisen from confounding by other variables, such as environmental factors. The number of available variables in the dataset permits limited further exploration of this point, although there is little evidence that adjustment for socioeconomic deprivation, BMI, maternal height, cigarette smoking, or marital status has any major effect on our results.
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Summary |
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KEY MESSAGES
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References |
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