Materials and Methods
Details of the population-based cohort studied here were presented previously.8 In short, the cohort was 1049 pregnant nulliparas who participated between January 2, 1996, and August 14, 1996, in the Down screening program at 16 weeks' gestation. Clinical data including blood pressure (BP) values and measurement of urine protein at each trimester were collected from the medical records at the maternity care units and the Helsinki University Hospitals, responsible for maternity care and deliveries in this area. After excluding women with twin pregnancies, spontaneous abortions, insulin-dependent diabetes, and those with missing or incomplete data, 917 women remained eligible for the study. Of them, 94 remained normotensive, 34 developed preeclampsia, and 54 developed gestational hypertension. Chronic hypertension was present in 35, and superimposed preeclampsia was diagnosed in five. In the present study, all 34 women with preeclampsia were included, and as controls for each we chose two subsequent women who attended the Down screening program and remained normotensive. Fourteen of the 68 controls and seven of 34 women in the preeclampsia group had class A diabetes.
Preeclampsia was diagnosed if BP higher than 140/90 mmHg was confirmed by two measurements (in the sitting position) at 6 or more hours apart or a sustained rise of 30 mmHg in systolic and 15 mmHg in diastolic BP, and proteinuria of 300 mg or more per day after 20 weeks' gestation was found in previously normotensive women. The criteria for gestational hypertension were similar to those for preeclampsia except proteinuria. Superimposed preeclampsia was diagnosed if preeclampsia occurred in women who already had chronic hypertension (BP at least 140/90 mmHg). Assay for total homocysteine (including homocysteine-cysteine, mixed disulfides, and protein-bound homocysteine) was performed using high-performance liquid chromatography and fluorescence detection as described in detail previously.6
Student t test and Fisher exact test were used in statistical comparisons and P < .05 was considered statistically significant. Before commencing the study, we calculated the sample size as follows: To detect a 1.25-μmol/L difference in the homocysteine level with a significance level of .05 and with a standard deviation (SD) of 1.75 (based on our previous experience), a minimum of 30 cases and 30 controls would be needed for a power level of greater than .80.
Demographic data on women in the preeclampsia and control groups are given in Table 1. Figure 1 shows that the distribution of serum homocysteine levels was similar when preeclampsia and control groups were compared. At 16 weeks' gestation the mean values ± SD [95% confidence interval (CI)] were 6.99 ± 1.56 (6.4, 7.5) μmol/L and 6.91 ± 1.76 (6.45, 7.34) μmol/L, respectively, without any significant difference between groups (P = 0.83). The upper 25th percentile limit of homocysteine values in the control group was 7.7 μmol/L. The odds ratio (OR) of subsequent preeclampsia with a value greater than 7.7 μmol/L was 1.59, CI (0.57, 4.43), P = not significant.
In the control group, homocysteine values in the subgroup of 14 women with class A diabetes and in the remaining 54 controls did not show any significant difference (6.63 ± 0.29 and 6.97 ± 0.24 μmol/L, respectively). In the preeclampsia group, homocysteine values did not differ between the subgroup of seven women with class A diabetes and the 27 remaining preeclamptic women (6.6 ± 1.25 and 7.0 ± 1.64 μmol/L, respectively).
In the present study, nonfasting levels of homocysteine were studied and therefore we were not able to exclude a possible effect of dietary factors on homocysteine values.9 Such factors probably did not differ in the present control and preeclampsia groups. The present homocysteine values in healthy pregnant women at 16 weeks' gestation were slightly higher than those recently reported at normal late pregnancy.6 This difference can be explained by a decrease of homocysteine concentrations with advancing gestation.10,11 A similar decrease of homocysteine levels does not occur, however, in women who develop preeclampsia.6 Several factors may increase homocysteine levels in women with preeclampsia. Metabolism in the kidney is the major route by which homocysteine is cleared from plasma12 and this route of elimination may be affected by preeclamptic changes in the kidney. Furthermore, fetoplacental extraction of homocysteine from the maternal circulation may be diminished in preeclampsia13 or there may exist a reduction of vitamin B12, which is essential for the formation of methionine from homocysteine.
Women with class A diabetes have an increased risk of developing preeclampsia.14 In pregnant women with preeclampsia, plasma homocysteine levels were found to be negatively related to insulin sensitivity,6 but a possible relationship between homocysteine metabolism and insulin resistance is not known. In the present study, we did not find any significant change in midtrimester homocysteine levels to be associated with class A diabetes whether women remained normotensive or developed preeclampsia. However, the small size of the subgroups limited the statistical power of these comparisons.
In the present population-based cohort study, we were not able to demonstrate any difference in serum homocysteine levels at 16 weeks' gestation between women who later developed preeclampsia and those who remained normotensive. The elevation of plasma homocysteine level found by other authors in the last trimester in preeclamptic pregnancy could not be observed in the second trimester before the appearance of clinical preeclampsia in the present study. Thus, the present findings failed to confirm any significant changes in homocysteine metabolism preceding the development of preeclampsia. In agreement with the previous findings of Sorensen et al,7 we found a tendency of increase in the risk of developing preeclampsia in women with the highest 25th percentile of values. However, no significant difference in the midtrimester homocysteine levels was found in either study between women who subsequently developed preeclampsia and those who remained normotensive. Thus, homocysteine level does not appear to be a reliable predictive test for clinical use.
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