Statistical analysis
Maternal TG concentrations and early-pregnancy BMI are presented as
medians and quartile 1 and quartile 3 categorical variables as
frequencies with proportions. Multilevel linear regression models were
employed to assess associations between maternal baseline
characteristics, early-pregnancy BMI, and maternal TG concentrations
among the maternal early-pregnancy blood pressure groups. Logistic
regression was used to estimate odds ratios with 95% CI for risk of
preeclampsia in women with stage 1 and stage 2 hypertension, and
elevated blood pressure compared with normal blood pressure for women
with or without risk factors.
Multivariable analyses were adjusted for potential confounders. Maternal
age (≤ 24, 25-29, 30-34, or ≥ 35 years), parity (nulliparous, parous),
education (≤ 12, 13-17, or ≥ 18 years), birthplace of the mother
(residents or immigrants) and pre-pregnancy BMI (≤ 18.4, 18.5-24.9, or ≥
25 kg/m2) were included as covariates and to assess
the risk factors for the association between maternal blood pressure in
early pregnancy and the risks for preeclampsia. For the analysis of the
impact of maternal obesity and high TG concentrations on the association
of maternal blood pressure and the risks of preeclampsia, women were
stratified based on the quartile values of BMI and TG concentrations.
The absolute risk difference compared to normal blood pressure was
calculated as the difference between the risk of preeclampsia among
women with one or two risk factors. To reduce the bias caused by fewer
samples, women were stratified into BMI ≥ 25 and < 25
kg/m2 subgroups; and TG ≥ 90th and
< 90th subgroups. The confidence interval
was determined using standard statistical packages. (20) A confidence
interval that contained a zero meant that there was no significant
difference between the event and the control in terms of risk.
All statistical analyses were performed using SPSS package ver. 16.0
(SPSS Inc., Chicago, IL, USA). A two-tailed P -value <
0.05 was used as the threshold for statistical significance.