In the present study, we comprehensively explored the associations between maternal fasting blood lipids during late pregnancy and adverse birth outcomes. Maternal lipid profiles were closely linked to adverse birth outcomes.
We found that maternal TG levels in late pregnancy were associated with increased risk of macrosomia and LGA, in line with previous cohort studies conducted at home and abroad[12, 16–18]where TG was found to be an independent risk factor for macrosomia and LGA. We found no association between TG levels and preterm birth, as also reported by other research. However, W Zheng, W Huang, L Zhang, Z Tian, T Wang, T Zhang, Z Zhang, W Zhang and G Li  reported that higher TG levels were associated with an increased risk of preterm birth. Participants were healthier in their study as women with chronic conditions excluded from their analysis, which may explain the heterogeneity of the results. Similar positive association was also reported by a meta-analysis, but it only included studies with blood samples taken in the first two trimesters and the definition of preterm birth varied among studies.
The associations between increased HDL levels and decreased risks of preterm birth, macrosomia, and LGA were also observed in this study. A retrospective cohort study including 5,407 healthy women also found that maternal HDL in late pregnancy was a protective factor for macrosomia and this association was dose-depended. Previous research also reported that HDL was an independent predictor for LGA[20–22]. Furthermore, most results with regard to a decreased risk for preterm birth was found for high HDL levels[7, 23–25], and the present study confirmed this association. But no relationship between HDL and preterm birth was found in a cohort study conducted in China. Although the maternal characteristics were comparable, the discrepancy in the results might be explained by the fact that our sample was larger with higher prevalence rate of preterm birth. The protective effect of HDL on these birth outcomes may relate to its anti-inflammatory, anti-oxidant, anti-thrombotic, and vasodilator functions.
A striking finding in the present study was that high LDL concentrations during the third trimester were associated with a decreased risk of macrosomia and LGA but an increased risk of LBW. Analogous association between LDL concentrations during the third trimester and macrosomia was also found in a cohort study including 10,366 pregnant women. Similarly, R-J Hou, et alproposed that LDL-C concentrations of pregnancy women who gave birth to LGA newborns were lower than those who gave birth to appropriate-for-gestational age (AGA) newborns. However, meta-analysis and other research [12, 15, 16]failed to observe the relationships. The differences in maternal LDL levels, maternal basic characteristics, and covariates may explain the inconsistency of the findings.
The fetus mainly relies on the placenta to obtain nutrition from the mother. The normal physiological structure and function of the placenta are crucial to the growth and development of the fetus. However, abnormal maternal lipid levels during pregnancy will affect the normal transport function of the placenta[29, 30], thus damaging the intrauterine development of the fetus and increasing the risk of adverse birth outcomes. Over the course of pregnancy, due to the enrichment of triglycerides and the decrease of liver lipase activity, the particles of LDL become smaller and denser, which are more easily oxidized into oxidized LDL (Ox-LDL). Moreover, pregnancy is associated with a decrease in total antioxidant capacity (TAC), making it an inflammatory state[31, 32]. This may exacerbate changes in LDL particles. Ox-LDL may increase the risk of endothelial damage[33, 34] and the smaller, denser particle is an independent predictor of smaller birth weight. These changes during pregnancy may be responsible for the associations between maternal serum lipids and adverse birth outcomes. However, the potential mechanisms still deserve further exploration.
Some limitations should be noted. First, the study population was only from Guangdong province, limiting the generalization of conclusions. Second, the results of the sample analysis were extracted only once, thus the dynamic changes of the indicators cannot be observed. Finally, we cannot exclude the effects of potential confounding factors on the results, such as the dietary structure and supplementation of nutrients during pregnancy, weight gain and exercise during pregnancy. Therefore, more in-depth and detailed research on the influencing factors of birth outcomes is needed.