Interpretation
During pregnancy, the increased secretion of hormones such as estrogen, progesterone, insulin and human placental lactogen in maternal blood promotes the absorption of lipids in intestine.1 Other maternal factors such as BMI, GWG, maternal nutrition, pre-pregnancy lipid levels and various pregnancy complications may also have significant effects on lipid metabolism and serum lipid levels.15 It follows that there was a significant elevation of all maternal blood lipids during pregnancy, including TG, which helps fetus to obtain maternal lipid nutrients through placenta for the sake of fetal development. Several population-based studies have found that serum levels of lipids, including TG, TC, LDL-C, HDL-C and phospholipids, increase gradually starting at gestational week 12, and show more prominent increases during the second and third trimesters.1, 16 The present study demonstrates that maternal TG increase with gestational weeks throughout pregnancy, especially rise by 3-fold from the first to third trimester, which is consistent with previous findings.1, 17 Many factors can affect serum lipids. Farias et al.11 found that pre-BMI was the main factor associated gestational TG level, on account of overweight and obese women showed higher mean TG compared with their normal weight counterparts during pregnancy. Our results made this conclusion even more favorable. Additionally, we found women with GDM had higher TG level throughout pregnancy, which reached the coincident conclusion as the study of Herrera et al..18
However, postpartum TG outcomes remain controversial. Hansen et al.19 reported that maternal serum total lipids at 3 days postpartum was higher than the second trimester, while was declined at 6 weeks postpartum. Mbadugha et al.20 found maternal serum lipids dropped significantly within a week after delivery, but returns to normal levels after about a year. Our study indicated that maternal TG level decreased at 42 days postpartum, but is still relatively high, which suggested that although most maternal organs and systems can return to a normal state of non-pregnancy at 42 days postpartum, it seems that blood lipids cannot back to normal that easily. It should be noted that non-fasting blood samples were tested at 42 days postpartum. Although fasting blood lipid test was deeply rooted, the clinical value of non-fasting blood lipid detection has been paid more and more attention due to support of several large population studies.21-24 Bansal et al.23 found that non-fasting TG levels were associated with cardiovascular events, independent of traditional cardiac risk factors, levels of other lipids, and markers of insulin resistance; by contrast, fasting triglyceride levels showed little independent relationship. The European Atherosclerosis Society (EAS) and European Federation of Clinical Chemistry and Laboratory Medicine (EFCC) reached a joint consensus statement to recommended that non-fasting blood samples be used as a routine method for detecting blood lipids in April, 2016.25 The effects of pre-BMI and GDM on serum TG continued to postpartum. We found overweight or obese and suffering from GDM can resulted in higher TG level at 42 days postpartum, compared to normal women, which backed up previous studies.10, 18
According to existing studies, association between postpartum TG and gestational TG is not clear. We firstly find that TG level at 42 days postpartum was positively associated with gestational TG. It was reported that plasma TG metabolism is altered by age in humans.26 Previous study found a positive relationship between maternal GWG and several pregnancy complications, most of which were accompanied by TG disorder.27 Considering these factors, including maternal age, pre-BMI, GWG, GDM and gestational age might have effects on postpartum TG level, the models of linear regression and logistic regression analysis were adjusted for them. Pre-BMI and GDM showed positive effects for postpartum TG in relation to TG level at each studied gestational week in the present study.
There is no consistent clinical diagnosis for non-fasting TG level, we defined postpartum hypertriglyceridemia as serum TG level ≥ P75 of all participants at 42 days postpartum. Hypertriglyceridemia has been regarded as an independent risk factor for cardiovascular disease and lipid metabolic disorders,28 which threatens the long-term health of women. In the present study, we proved that gestational TG levels were positively associated with higher risk of postpartum hypertriglyceridemia, especially in early pregnancy. The result suggested that elevated gestational TG can be a biomarker for postpartum hypertriglyceridemia for pregnant women. Enough attention should be paid to the supervision and control of blood lipid level early in the first trimester.
Considering the longitudinal design of the study, the positive associations between gestational TG trend of change and postpartum TG level and risk of hypertriglyceridemia were found overall. The LME model was used instead of the difference value between TG level in the third and first trimester, for the reason that TG level during pregnancy is a continuous process of change. The results indicated pregnant women whose gestational TG level elevated more significant and faster have a greater risk of postpartum hypertriglyceridemia. It suggests that it is necessary for pregnant women to pay attention to the elevation of lipid level and managing through diet and exercise during pregnancy, so as to make for postpartum health.
As a biomarker of postpartum hypertriglyceridemia for pregnant women, gestational TG level should be concreted as a threshold or a cutoff point to meet clinical significance. As for there is no consensual recommended reference values of gestational serum lipids, it is always doubtful for obstetricians to determine the status of blood lipids of pregnant women. Wang et al.29 took adverse pregnancy outcomes as the main outcome and presented the lipid reference range and abnormal lipid values in early and middle pregnancy, using the cut-off of the 95th percentiles. Our study newly set up a prediction model of gestational TG level on postpartum hypertriglyceridemia by using ROC curves and the Hosmer-Lemeshow goodness-of-fit test, and found four TG cutoff points of each studied gestational week. To find the best gestational predictive value for postpartum hypertriglyceridemia, we selected TG cut-off by the point nearest to the top-left most corner of the ROC curve instead of the 95th percentiles as diagnostic value, so as to ensure optimum sensitivity and specificity. Our results provide a reference standard for pregnant women in different trimester to pay attention to postpartum and lifelong lipid health.
For the first time, we compared the postpartum TG outcomes in pregnant women with different pre-BMI and between GDM and normal women. Interestingly, the positive association between gestational TG levels and risk of postpartum hypertriglyceridemia remained in normal weight and overweight & obese group, but not in low weight women. Furthermore, the risk of postpartum hypertriglyceridemia for women of normal weight was higher than overweight & obese group in the second and third trimester. Over the years we have been talking about the hazard of overweight and obesity,11 but the newfangled results suggested that the risk of gestational TG to postpartum health is even greater for normal women. A similar case was found in the comparison between women with and without GDM. Previous study found that high TG levels during gestation were independently associated with an increased risk of GDM.30 Free fatty acids (FFAs) potentially derived from elevated TGs might decrease insulin sensitivity, creating a vicious cycle between TG levels and insulin resistance.31, 32 Overweight or obese pregnant women and women with GDM themselves have lipid metabolism disorders, which weakened some risks of adverse outcomes of elevated gestational TG. Besides, treatments and nutrition interventions taken by pregnant women after diagnosis of GDM may be responsible for the reduced risk of postpartum hypertriglyceridemia. The study suggested that pregnant women with normal pre-BMI and normal pregnancies should pay more attention to monitoring and control of serum TG level throughout gestation, by means of diet control, nutrition guidance and fitting exercises.