Interaction of body mass index and hemoglobin concentration on blood pressure among pregnant women in Guangxi, China

BACKGROUND Body mass index (BMI) and hemoglobin (Hb) are positively associated with hypertensive disorders among pregnant women. The aim of this study was to estimate a potential interaction between high BMI and high Hb concentrations on systolic blood pressure (SBP) and diastolic blood pressure (DBP) in pregnancy. METHODS We recruited 4497 single-birth women aged 18-43 years who received routine antenatal care at three hospitals of Guigang, Guangxi, China, from December 2007 to January 2011. Of 4497 participants, 3472 women were in the first trimester, with following up, 2986 women and 2261 women were left in the second and third trimester, respectively. Clinical data were derived from medical records of each woman. We used multivariable linear regression, by trimesters of pregnancy, to evaluate the associations of high BMI and high Hb concentrations with SBP and DBP according to cross-sectional design. RESULTS In multivariable analyses, BMI was positively associated with SBP throughout all trimesters, but the corresponding association for Hb concentrations only in the first trimester, whereas both BMI and Hb concentrations were positively associated with DBP in the first and third trimesters. After full adjustment for confounding, the average differences in SBP and DBP comparing women with high BMI and high Hb to those with non-high BMI and non-high Hb were 2.9 mmHg (95% CI: 0.8 to 5.0 mmHg) and 3.9 mmHg (95% CI: 1.5 to 6.3 mmHg) in the first trimester, 2.6 mmHg (95% CI: 0.4 to 4.8 mmHg) and 1.5 mmHg (95% CI: -1.3 to 4.3 mmHg) in the second trimester, and 4.8 mmHg (95% CI: 2.3 to 7.4 mmHg) and 5.7 mmHg (95% CI: 3.2 to 8.3 mmHg) in the third trimester, respectively. With respect to the interaction, significant combined effects between high BMI and high Hb were confirmed on SBP (P = 0.02) and DBP (P = 0.004) in the third trimester, and the amount of interaction on SBP and DBP were 2.0 mmHg (95% CI: 0.1 to 3.9 mmHg) and 2.3 mmHg (95% CI: 0.4 to 4.3 mmHg), respectively. CONCLUSION Our findings suggest that high BMI and high Hb concentrations may have a synergistic effect on blood pressure in late stage of pregnancy.