1 Department of Obstetrics & Gynaecology, The University of Hong Kong and 2 Tsan Yuk Hospital, Hong Kong, PRC
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Abstract |
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Key words: birthweight/body mass index/impaired glucose tolerance
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Introduction |
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On the other hand, maternal weight excess and obesity are associated with increased incidence of GDM (Gross et al., 1980; Duthie et al., 1988
; Tilton et al., 1989
; Naeye, 1990
; Cnattingius et al., 1998
). As well, women with even mild GDM in the form of impaired glucose tolerance (IGT) by the World Health Organization criteria (WHO, 1980) tended to be heavier with increased BMI (Al-Shawaf et al., 1988
; Nord et al., 1995
; Lao and Lee, 1998
). In view of the inter-relationship between adverse outcome, maternal obesity, and GDM (Engelgau et al., 1995
; Galtier-Dereure et al., 1995
), it is possible that some of the adverse outcomes associated with maternal weight excess were in fact related to the presence of GDM. Conversely, it is also possible that some of the complications attributed to GDM, especially for the milder form of IGT, were actually related to maternal weight excess. This is because it has been reported that good glycaemic control did not normalize birthweight percentiles, but maternal weight at delivery was the only significant predictor of birthweight percentile (Jacobson and Cousins, 1989
). Thus IGT diagnosed for the first time in pregnancy might only be a feature of maternal weight excess but not a pathological condition per se, as the clinical significance of IGT has also been disputed (Li et al., 1987
; Nasrat et al., 1994
).
In our hospital, the incidence of GDM, mostly in the milder form of IGT, has been on the increase over the last decade. It is not clear whether such a phenomenon was merely a reflection of the increasing obesity among women of reproductive age in our society because of the improvement in socio-economic status. There is no data on the effect of maternal IGT on the pregnancy outcome in Asian women with a high BMI. In order to address this question, a retrospective case-control study was performed on subjects with a pre-pregnant BMI > 26 kg/m2, who were delivered in our hospital over a 3 year period, to determine whether the presence of IGT impacts on the pregnancy outcome.
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Materials and methods |
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Women diagnosed with IGT (OGTT 2 h value 8.0 mmol/l) or diabetes mellitus (2 h value
11.0 mmol/l) are categorized as having GDM, as recommended recently (Alberti and Zimmet, 1998
). They are referred to a dietitian and put on diet control (30 kcal/kg), and then assessed with pre- and 2 h post-prandial blood sugar profile. Insulin therapy will be started for inadequate control (fasting plasma glucose >5.9 mmol/l and/or post-prandial glucose >7 mmol/l) if dietary readjustment fails to normalize the blood sugar profile. The details of the antenatal management of these mothers have been described before (Lao and Lee, 1998
). All the patients who had the OGTT arranged at the clinic are registered.
In this study, firstly the serial numbers and names of the women who underwent the OGTT over a 3 year period, who were carrying singleton pregnancies, and who were delivered in our hospital, were obtained from the registry. During this period, a total of 3436 women with singleton pregnancies had undergone one or more antenatal OGTT. Among them were 1482 women diagnosed with IGT and delivered in our hospital. There were also 127 women diagnosed with DM who were not included in this analysis. Another 259 women had defaulted and the details on their outcome were not available. As the population served by our hospital is largely southern Chinese, who are generally of shorter stature, the calculation of the BMI would be an appropriate means of relating weight to height. The criterion of >26 kg/m2 as the definition of high BMI was chosen, following the Institute of Medicine (IOM) recommendation (IOM, 1990) and which has been used in studies examining the effect of maternal obesity and weight gain on pregnancy outcome (Johnson et al., 1992; Scholl et al., 1995
; Edwards et al., 1996
; Crane et al., 1997
).
The case notes of the women whose pre-pregnancy BMI were >26 kg/m2 and whose diagnosis was IGT or normal glucose tolerance were then retrieved for review. There were 394 women (12.4% of the 3177 women who delivered in our hospital) with a pre-pregnancy BMI of >26 kg/m2 who had either a normal OGTT result (34.0%) or IGT (66.0%). Each subject with IGT and her respective control were matched for the pre-pregnancy BMI (to within 0.1 kg/m2). The purpose of using controls matched for the exact BMI was to eliminate as far as possible the presence, or to minimize the effects, of other biological variables which might be associated with GDM and which could have influenced pregnancy outcome. This was because these variables might not have been easily identified for adjustment by statistical methods.
Because of the much smaller number of women with normal OGTT results, only 128 women with IGT could be matched with 128 women with normal OGTT results. These cases formed the study and control groups respectively for the final analysis. All the women with IGT were treated with diet restriction only. The two groups were compared for maternal demographic and anthropometric parameters, obstetric complications, mode of delivery and perinatal outcome. Comparison of continuous random variables were made using Student's t-test, and percentages were compared using Fisher's exact test. The statistical analysis was performed with a commercial statistical package (Statistics Package for Social Sciences).
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Results |
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Discussion |
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While the clinical significance of IGT may have been disputed (Li et al., 1987; Nasrat et al., 1994
), this condition was nevertheless associated with such outcomes as LGA or macrosomic infants, Caesarean delivery, and complications like hypertension (Al-Shawaf et al., 1988
; Nord et al., 1995
; Lao and Lee, 1998
). However, in view of the fact that these women were also heavier with higher BMI (Al-Shawaf et al., 1988
; Nord et al., 1995
; Lao and Lee, 1998
), maternal weight excess could have been the underlying factor for these outcomes. Indeed, one study found that even though obese women with GDM gained less weight in pregnancy than normal or lean women with GDM, the infants in the former group were still heavier than the latter two groups (Algert et al., 1985
). Furthermore, even moderately obese mothers when compared with age and parity matched normal weight controls had increased incidence of gestational hypertension, Caesarean section, LGA, GDM and infant morbidity (Tilton et al., 1989
). Excess weight in the mother could therefore have been the more important factor, especially in the case of mild GDM.
In order to clarify the impact, if any, of IGT on the outcome of pregnancy in the overweight women, it is necessary to control for maternal weight or BMI. This is because the occurrence of outcomes such as diabetes, hypertension, and perinatal mortality, increase progressively with increasing maternal BMI (Naeye, 1990; Cnattingius et al., 1998
). Furthermore, infant size was correlated with maternal BMI but was not influenced by glycaemic control, and it was proposed that the increased fetal growth in GDM may be related more to the metabolic abnormalities associated with obesity than to those associated with well controlled GDM (Jacobson and Cousins, 1989
). Such an effect could have played a more important role in women with shorter stature, such as the southern Chinese, whose weight could well be within normal by Western standards but whose BMI was nevertheless increased.
In this study, it is apparent that the majority of our women with a high BMI had impaired glucose tolerance. However, by controlling for the maternal BMI, any difference in the maternal characteristics is likely to be the predisposing factors for, and that in pregnancy outcome is likely to be related to, maternal IGT. Thus this study has shown that while there was no difference in maternal weight, weight gain, or BMI during the course of pregnancy, the IGT group was older with more women at 35 years of age or more. As well, the IGT women had a higher haemoglobin concentration at booking, which might have suggested a better nutritional status, or a higher iron store as reflected in their ferritin concentration (Lao and Tam, 1997). Their fasting glucose concentration was also higher, and despite the similar incidence of multiparae, the incidence of a past history of GDM was significantly higher too. These findings suggested that despite a similar BMI, the women with IGT had had some form of underlying metabolic disturbance that predisposed them to the development of IGT during pregnancy.
Except for the increased incidence of urinary infection, there was no difference in pregnancy complications or mode of delivery in the IGT group. Nevertheless, the birthweight ratio was increased together with the incidence of LGA and macrosomic infants in the IGT group, even though the mean gestational age and birthweight appeared similar. This phenomenon was probably related to the combined effects of a slightly shorter gestational age and higher birthweight in the IGT group. Thus an effect of maternal IGT on the size of the infants was demonstrated in spite of maternal diet restriction. Although it was possible that the IGT group was not compliant with their diet regime, their pre-delivery weight and BMI, as well as the absolute and percentage weight gain in pregnancy, were no different from the values in the control group. The increased incidence of LGA and macrosomic infants in the IGT group could not therefore be attributed to obesity before pregnancy or excess weight gain during pregnancy. This finding is in agreement with the observed effect of GDM on the risk of infant macrosomia that was independent of other risk factors (Weeks et al., 1994). Furthermore, the IGT group had increased incidence of treatment for neonatal jaundice. This was probably a reflection of the presence of neonatal polycythaemia due to maternal diabetes (Mimouni et al., 1986
), although the cord blood haemoglobin or haematocrit results were not available to support this hypothesis.
The results of this study suggest that the increased risk of some of the outcomes attributed to GDM, such as Caesarean delivery and hypertension, are probably related to associated maternal weight excess or obesity, because women who develop GDM tend to be heavier and more obese. Once corrected for maternal BMI, maternal IGT appears to have minimal influence on the pregnancy complications, and none on the mode of delivery. On the other hand, even mild glucose intolerance in the form of IGT does affect the infant size, despite the normalization of maternal weight gain with diet restriction. It is possible that apart from maternal weight and obesity, other factors that predispose the pregnant women to the development of IGT play important roles in determining the infant outcome. Furthermore studies are warranted to elucidate the factors associated with excess fetal growth in mild glucose intolerance.
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Notes |
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References |
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Submitted on January 24, 2000; accepted on May 24, 2000.