Statistical analysis
Baseline characteristics of study population were described
for
preterm birth and term birth separately. Data were expressed as mean ±
standard deviation (SD) for normally distributed variables or median
(interquartile range, IQR) for skewed variable or frequency (percentage)
for categorical variable. To test for difference between preterm birth
and term birth groups, continuous variables were compared byt -test or Wilcoxon test and categorical variables were compared
by Chi -square test. ApoB, ApoA-1 and ApoB/ApoA-1 were categorized
into tertiles (ApoA-1: < 1.98, 1.98-2.18 or ≥ 2.18 g/L; ApoB:
< 0.88, 0.88-1.06 or ≥ 1.06 g/L; ApoB/ApoA-1: <
0.42, 0.42-0.52 or ≥ 0.52) and the lowest tertile was used as the
reference in the analysis. Logistic regression was used to estimate the
odds ratio (ORs) and 95% confidence intervals (CIs) for the
associations between ApoB, ApoA-1, ApoB/ApoA-1 ratio and the risk of
PTB. ApoB, ApoA-1and their ratio were included in two ways: categorical
or continuous variables. All multivariate models were adjusted for
potential confounders including maternal age at delivery (continuous),
pre-pregnancy BMI (continuous), parity (primiparous or multiparous),
delivery mode (vaginal delivery or cesarean delivery), GDM (yes or no)
and infant sex (male or female). Since smoking is rare among Chinese
pregnant population and in our study only four women admitted smoking
during pregnancy, maternal smoking status were not adjusted. Restricted
cubic spline analysis with four knots (5th, 35th, 65th and 95th
percentiles) were conducted to further explore non-linear dose-response
relationship between mid-pregnancy apolipoprotein profile and risk of
preterm birth.
Subgroup analyses were conducted according to maternal pre-pregnancy BMI
(≤ 24 kg/m2, >24
kg/m2), maternal age at delivery (< 35
years, ≥ 35 years) and parity (primiparous,
multiparous). The cut-off of
pre-pregnancy BMI was decided according to the recommendation of the
Working Group on Obesity in China27.
The
presence
of pregnancy complications may affect the association between
apolipoprotein and preterm birth. Therefore, we performed a sensitivity
analysis among a subgroup of 5,080 mother-newborn pairs after excluding
mothers with GDM.
All statistical analyses were performed by R software (version 3.5.3)
and SAS (version 9.4; SAS Institute Inc.). P < 0.05 was
considered statistically significant.