Discussion
In the present study, among the South Chinese pregnant cohort, 19.2%
were pre-pregnant underweight women according to the WHO criteria for
Asian populations, while 19.4% were overweight/obese women. We found
that the proportion of underweight women was almost equivalent to the
proportion of overweight/obese women. This finding showed that the
phenomenon of underweight was actually common in South China. As was
expected, a lower BMI cut-off at 25 kg/m2 for defining
obesity would be better than BMI cut-off at 28kg/m2 for defining obesity
for pregnant women in South China, which had been demonstrated in our
previous study12.
We found underweight women had significant higher TC level than
normal-weight women and overweight/obese women (Table 3 or Table 4). In
line with Kulkarni et al13 , a research in Pune showed
that TC concentrations went higher in rural underweight women at 18 and
28 gestational weeks. Kulkarni,et al13 demonstrated in
rural, undernourished, normoglycemic Indian pregnant women a significant
association between maternal circulating lipids and fetal growth, which
was at least as strong as that of glucose. During gestation, maternal
lipid alters form anabolic to a catabolic state during, which causes
maternal physiological hyperlipidemia(MPH) 14. This
phenomenon could be ascribed to an adaptive response to satisfy the
increasing fetal demand15, 16. Cholesterol is
essential for fetal growth, steroid synthesis and neurodevelopment, at
least 1g cholesterol is essential for placenta to synthesis
placenta17. Fetus is unable to synthesize cholesterol
in the early pregnancy, since their liver and adrenal gland is immature,
most of them utilize endogenously cholesterol at term. Facing such
greatly demanded, increasing maternal serum TC seems like significant to
satisfy fetus rapid growth. Moreover, fetus need to uptake maternal
cholesterol to synthesis cell membrane 18 to maintain
neurodevelopment. Abnormal cholesterol metabolism appears to related to
impair neurological development19 and low birthweight17 or IUGR 20. Insufficiency of
LDL-C and TG may associate with FGR21. Besides, a
thin-fat phenotype was reported in South Asia22, which
was caused by undernourished state, thin-fat women has low body mass
with a high concentration of TC. This phenomenon probably indicate that
cholesterol is more necessary for underweight pregnant women.
According to the study by Butte NF15, Cholesterol is
used by the placenta for steroid synthesis to meet maternal requirement.
Estrogen production is 3-8 times higher at term than non-pregnancy,
cholesterol as an essential material of steroid is in greatly demanded.
Estrogen also strongly stimulates LDL receptor expression and improve
uptaking of cholesterol, which results in maternal hypercholesterolemia23, 24. Thus, we presume that lipocatabolic enhance in
underweight women than normal weight women and overweight/obese women to
meet maternal and fetal demand.
Our results showed that underweight women with high TC level had
significantly higher occurrence of LGA, and lower occurrence of SGA,
while there was no significant association between serum TC and LGA or
SGA in normal-weight women and overweight/obese women. Meanwhile, the
increase of LDL-c and TG/HDL-c ratio decreased risk for SGA in
underweight women, but no significant relationship was found in
overweight/obese women. In line with Adu‐Afarwuah25,
TC level, and a lipid‐based nutritional supplement intake could result
in increase of birth weight. A research in Pune showed that TC
concentrations went higher in rural underweight women at 18 and 28
gestational weeks and positively related to the newborn birth size.
Serum cholesterol concentration was directly associated with all newborn
measurements except head circumference. A 1-SD-higher maternal TC
concentration was associated with a 54-g-higher birth weight. According
to the study by Krstevska B26, LGA also attributed to
maternal serum LDL-c. High concentrations of HDL-c seemed like a
protector for LGA27, 28. However, this conclusion is
still controversial. In the contrary, Eslamian L29 and
Wang J28 did not find significant association of
maternal LDL-C levels and LGA newborn. Our results revealed an
association between high TC level and LGA in underweight women, but the
potential mechanisms were unknown. We presume that cholesterol plays a
more significant and complex role for underweight women in gestation,
because it is essential material to maintain maternal pregnancy and
fetal development. Moreover, we should pay more attention on this
‘thin-fat’ phenotype, since the proportion of underweight women and
overweight/obese women is equivalent. Unfortunately, we had paid much
more attention on the diet and weight control of overweight/obese women,
while ignored the management of underweight women on their serum lipid
profile. Furthermore, underweight women may tend to take more food
during gestation, especially in China, and this may aggravate the
hypercholesteremia of underweight women, which may lead to adverse
outcomes.
In addition, our results are in line with other researchers who reported
associations between serum TG levels and adverse maternal and fetal
outcomes. Maternal TG was an independent predictor for
GDM30 and birth weight31-33.
Meanwhile, the prevalence of GDM, PIH, PTL and macrosomia were higher in
pregnant women with high TG level34. Thus, the results
above remind us the necessity to control serum lipid level.
The main strengthen of our study was that we used a more proper criteria
to stratify the underweight women in South of China, and investigate the
lipid level and corresponding maternal and fetus outcome. This makes our
results more reliable. Additionally, we focus the relationship of
underweight women and cholesterol level, which has not been well
investigated before. Few researchers explored and interpreted the
relationship among underweight women, high level cholesterol and newborn
size. At last our study has a huge sample size about 6,000 which seemed
like to have compelling results. Thus, this study can contribute new
insights into controlling of underweight women serum lipid counseling.
There were some limitations in our study. The height and weight of
gravidas were recalled and self-reported by patients, and so it was
likely there were more error variation and potential bias than measured.
The second limitation was that diet before the blood drawing may affect
the serum lipid profile. The amount and composition of dietary fat seem
to be associated with serum concentrations of lipids35, 36. In consideration of this, we had given them
proper diet guidance before their blood tests.