Interpretation
Elevated ApoB level or ApoB/ApoA-1
ratio represent a more atherogenic lipid profile and was known to be
related to several chronic diseases including metabolic syndrome16, insulin resistance 17 and
cardiovascular disease 15 among non-pregnant
population. In addition, it has been reported that ApoB/ApoA-1 ratio can
predict cardiovascular disease better than any other cholesterol indices28. Previous studies have also investigated the
relationship between maternal apolipoprotein profile and pregnancy
complications. Several studies have demonstrated that ApoB and
ApoB/ApoA-1 ratio during pregnancy was positively related to the risk of
preeclampsia18-20. Recently, an animal research found
that intravenous infusion of apoA1 could lower pregnancy-induced insulin
resistance in rats30. But Ravi et al. reported that
serum apoA1 level during mid-pregnancy was not associated
with insulin resistance or the risk
of GDM in pregnant women31. However, studies regarding
the association between apolipoprotein profile and adverse birth outcome
is still scarce so far. Till now, only three studies have explored the
association between maternal ApoA-1 and PTB. A nested case-control study
conducted in America revealed that ApoA-1 concentration during early
pregnancy was positively correlated with the risk of spontaneous preterm
birth24. However, another case-control study conducted
in Australia found that maternal early pregnancy ApoA-1 level was
negatively related to the risk of spontaneous preterm
birth.23. In addition, Abiaka et
al.25 reported that serum ApoA-1 level during labor
was significantly lower in preterm delivery mothers. In the present
study, we found no association between maternal ApoA-1 and PTB. The
discrepancies in the findings among former studies and ours might be
explained partly by the difference in trimester for ApoA-1 testing and
difference in racial background for the study participants.
The results of subgroup analyses showed that the positive association
between maternal ApoB/ApoA-1 ratio during mid-pregnancy and risk of PTB
was more pronounced among participants with pre-pregnancy BMI ≤ 24
kg/m2, age at delivery ≥ 35 years or those who were
multiparous, although there was no interaction between all of the
stratification variables and preterm birth. Prior studies have shown
that women with higher pre-pregnancy BMI were more likely to develop
pre-eclampsia and diabetes during pregnancy, which greatly increased the
risk of PTB32. Primiparity is also known to be a risk
factor for PTB33. Therefore, a logical explanation
might be that the
existence
of strong risk factors for PTB such as obesity might mask the effects of
ApoB/ApoA-1 on the occurrence of PTB. Maternal ageing is related to
increased levels of inflammation and oxidative
stress34 and have an adverse impact on the
placenta35. Therefore, pregnant women with an advanced
age are more vulnerable to the elevation of atherogenic lipoproteins
during pregnancy might explain the different results between these two
age groups.
There are several mechanisms might explain the increased risk of PTB
among women with higher serum ApoB level or ApoB/ApoA-1 ratio. Previous
studies have revealed a positive association between ApoB level and
oxidative stress36. Elevated level of oxidative stress
during pregnancy is related to higher risk of PTB37.
Furthermore, both experimental38, 39 and
epidemiological studies36, 40 have reported that
elevated ApoB level or ApoB/ApoA-1 ratio was associated
with endothelial damage. Chen et
al.41 reported that impaired endothelial function was
present in women with preterm delivery. In addition, previous study have
found that competitive inhibition of fibrinolysis by ApoB were
associated with hemorheology modification in
intrauterine growth restriction
cases and suggested that ApoB/ApoA-1 might be a good marker of early
detection of intrauterine growth restriction22.