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.