Are maternal metabolic syndrome and lipid profile associated with preterm delivery and preterm premature rupture of membranes?

We aimed to evaluate the association of metabolic syndrome (MetS), its components and lipid profile in mid-pregnancy with preterm delivery and preterm premature rupture of membranes (PPROM). This prospective cohort study was conducted on 203 pregnant women between 24 and 28 weeks of gestation, undergoing gestational diabetes screening test with 50 g glucose challenge test (GCT). Fasting serum total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglyceride (TG) levels were measured during the week after GCT assessment. Information on the participants’ pre-pregnancy weight, demographic/reproductive characteristics, and height and blood pressure (BP) measurements were documented at baseline entry into the study. Metabolic syndrome was defined as the co-existence of 3 or more of the following criteria: Body mass index (BMI) before pregnancy ≥ 30 kg/m2, BP ≥ 130/85 mmHg, GCT ≥ 140 mg/dl, TG ≥ 150 mg/dl, and HDL-C ≤ 50 mg/dl. All participants were followed through routine prenatal care, up to delivery. Any deliveries or rupture of membranes of less than 37 weeks of pregnancy were considered preterm delivery and PPROM, respectively. Statistical analysis was performed by SPSS V.20, and p value of less than 0.05 was considered significant. MetS was detected in 10 (4.9%) of participants. Logistic regression analysis showed HDL-C levels, and hypertension were associated with spontaneous preterm delivery [(OR 0.952, 95% CI 0.910–0.995), (OR 1.629, 95% CI 1.554–1.709) respectively], but no statistically significant results were found for PPROM. Low HDL-C levels and hypertension in mid-pregnancy are associated with the occurrence of spontaneous preterm deliveries, indicating that MetS and its components should be monitored more closely in pregnancy.


Background
In 1998, the World Health Organization (WHO) defined 'metabolic syndrome' (MetS) as a combination of central abdominal (visceral) obesity, glucose intolerance, insulin resistance, dyslipidemia and hypertension [1]. This syndrome is a cluster of physiological abnormalities that accelerate the risk of type 2 diabetes and atherosclerotic cardiovascular disease [2]. Recently, the diagnostic criteria and etiology of this syndrome have become a controversial topic [3] and its prevalence is a challenging issue worldwide [4]. According to one study, the prevalence of metabolic syndrome was considerably higher in Iranian women (55.0%), and men (30.1%), than in French women (6.6%) and men (13.7%) [5].
Despite the increasing prevalence of MetS worldwide in recent years, and consequently an increased prevalence during pregnancy, there is still no established definition for metabolic syndrome in pregnancy [6].
Normal pregnancy is associated with varying degrees of resistance to insulin, dyslipidemia, and inflammation [7,8]. Studies that have assessed metabolic components in pregnancy have generally used accepted definitions for the adult population [9].
A preterm birth, described as birth under 37 weeks of gestation, can lead to adverse consequences for the newborn [10]. Although management of preterm infants has much improved, the mortality and morbidity of neonates, even in late preterm labors (34 ≤ labor ≤ 37), have remained unchanged [11]. Preterm premature rupture of membrane (PPROM) is defined as the rupture of membranes in less than 37 weeks of gestation and usually leads to preterm birth. This condition affects approximately 3% of all pregnancies and occurs in up to 30% of preterm deliveries [12].
In some studies, it has been shown that mothers who have given birth to premature babies suffer from cardiovascular disease and diabetes later in life [13,14]. Several years after delivery, prematurity is related to high blood pressure, insulin resistance, and low-grade inflammation in mothers [15], but the question is whether there is a metabolic syndrome in the mother before and during pregnancy? Inflammation and stress oxidative may be associated with increased risk of spontaneous preterm labor [16]. Increased production of proinflammatory cytokines has been shown to play an essential role in uterine activity and preterm birth; whereas, production of anti-inflammatory cytokines has been associated with uterine quiescence [17]. "All the parameters included in the diagnosis of the metabolic syndrome including dyslipidemia are associated with a low-grade inflammation state [18]. An association of elevated levels of interleukin-6 (IL-6) and CRP with the metabolic syndrome has also been found. Also, low levels of the anti-inflammatory cytokine interleukin 10 (IL-10) have been found to be associated with metabolic syndrome in women [19]. On the other hand, increased peroxidation of these elevated plasma lipids causes enhanced oxidative stress by progressively producing free radicals and lipid peroxides. Lipid peroxides are toxic compounds that have the potential to damage endothelial cells [20].
Many studies have assessed the relationship between metabolic syndrome components such as hypertension, obesity, diabetes, dyslipidemia, and adverse outcomes in pregnancy [6]. Results of a study showed that elevated maternal triglyceride levels measured during early pregnancy are associated with an increased risk of induced preterm delivery. In this study, total cholesterol was not associated with any pregnancy complications [21]. One study also demonstrated that odds of spontaneous preterm delivery were increased among women with high total cholesterol, LDL and TG [22].
However, few studies have been conducted on metabolic syndrome and preterm birth which are inconsistent. In the study by Grieger et al. metabolic syndrome of the mother at the onset of pregnancy had no significant relationship with low birth weight and preterm delivery [9], but in the study by Chatzi et al. the risk of preterm delivery in mothers with metabolic syndrome increased approximately threefold [6].
Considering the lack of confirmatory evidence on the association between the metabolic syndrome and preterm birth or PPROM, especially in Iran, we aimed to assess metabolic syndrome and its components in mid-pregnancy and determine their associations with the occurrence of preterm delivery and preterm premature rupture of membranes.

Methods
This prospective cohort study was performed on 203 pregnant women who had been referred to Niloo and Akbarabadi Laboratories in Tehran, from August 2013 through June 2014, for routine gestational diabetes screening test with 50 g glucose challenge test (GCT), between the 24th and 28th weeks of gestation. The inclusion criteria included the age range of 18-45 years, Iranian citizenship, singleton pregnancy, and no chronic physical disorders (e.g., hypertension, diabetes, cardiovascular disorder, familial hyperglycemia) according to the information provided in the mother's records.
The subjects were entered into the study by available (convenient) sampling method. Due to the fact that the prevalence of metabolic syndrome during pregnancy in Iran is unknown, 70 mothers were assessed in a pilot study, and the prevalence of each of the components of metabolic syndrome and the occurrence of pregnancy consequences (spontaneous preterm birth and PPROM) were measured, and finally the lowest prevalence was used. The lowest prevalence among metabolic syndrome components belonged to hypertension with a rate of 7.1%, and among the consequences of pregnancy belonged to PPROM which was 2.9%, these rates were considered as P1 and P2, respectively. Considering an impact factor of 0.04, the required sample size was calculated using the following formula: After explaining the study details and obtaining written consent from the participants, they were enrolled in the study. At the entry, maternal characteristics such as age, parity, educational level, height, weight and pre-pregnancy body mass index (BMI), blood pressure, nutritional complements consumed during pregnancy, family history of diabetes, hypertension, and dyslipidemia were documented.
If the mother was fasting, venous samples were obtained at the time of entry, otherwise during the following weeks (between the 24th and 28th weeks of gestation), venous samples were obtained from the participants in fasting status to measure serum total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglyceride (TG) levels.
As there is no definition of metabolic syndrome in pregnancy, we modified the Metabolic Syndrome Diagnosis Criteria of NCEP-ATP III [23,24], in which the presence of 3 or more of the following criteria sets the diagnosis of MetS: central obesity, waist circumference > 88 cm or BMI > 30 kg/ m 2 , triglycerides > 150 mg/dl, HDL-C < 50 mg/dl, systolic Blood Pressure > 130, diastolic BP > 85 mmHg, and/or fasting blood glucose (FBS) > 105 mg/dl. Since waist circumference increases naturally in pregnancy, thus is inapplicable in this study, we used pre-pregnancy BMI > 30 kg/m 2 instead of waist circumference. FBS was also replaced by impaired GCT results (glucose intolerance). Thus, our diagnosis of MetS in pregnancy was based on the presence of 3 or more of the following criteria: Pre-pregnancy BMI ≥ 30 kg/m 2 , GCT ≥ 140 mg/dl, systolic blood pressure ≥ 130 mmHg or diastolic blood pressure ≥ 85 mmHg (taken twice, at least six hours apart), plasma triglyceride levels ≥ 150 mg/dl, and HDL-C levels ≤ 50 mg/dl.
After entering into the study and obtaining the record number and giving their telephone number, all mothers went through routine prenatal care and were followed up to delivery by health services staff, as well as 2 of the researchers (SH. N., and H. S.) who gathered and recorded the data too.
Spontaneous preterm birth was defined as intact membranes, regular contractions and cervical changes in the absence of labor induction in mothers with less than 37 weeks of pregnancy that caused delivery. Preterm premature rupture of membranes (PPROM) was defined as rupture of membranes before the initiation of spontaneous contractions in mothers with less than 37 weeks of pregnancy according to the information provided in the hospital's records.
All data were registered and analyzed by SPSS software V.20. Independent Student's t test and Chi-square tests were used to detect significant differences between the preterm birth and term deliveries. Metabolic syndrome, together with its components, and cholesterol and LDL levels were compared between two groups by Fisher's exact test.
To adjust for confounding variables, we used logistic regression analysis to evaluate some potential factors affecting preterm birth and PPROM, i.e., number of pregnancies, mothers' education, use of multivitamins during pregnancy, history of preterm birth, and metabolic syndrome together with its components. p values of less than 0.05 were considered statistically significant.
This research was approved by the Ethical Committee of the Faculty of Medical Sciences in Tarbiat Modares University, Tehran, Iran.

Results
A total of 203 pregnant women were enrolled in the study. As many as 18 (8.9%) spontaneous preterm deliveries and 10 (4.9%) PPROM cases occurred among the study subjects. Demographic and reproductive characteristics of the study participants are demonstrated and compared between the 2 study groups (preterm and term deliveries) in Table 1. Using the Independent Student's t test and Chi-square tests, no statistically significant differences were detected between the two groups ( Table 1). The only different variable between the two groups, as expected, was gestational age at delivery (34.11 vs. 38.71 weeks for preterm and term deliveries respectively).
MetS was detected in 10 (4.9%) study participants. High cholesterol (≥ 200 mg/dl) and TG (≥ 150 mg/dl) levels were observed in 75.4 and 82.8% of participants, respectively. Metabolic syndrome, together with its components, and cholesterol and LDL levels were compared between preterm birth and term deliveries by Fisher's exact test ( Table 2). The only statistically significant differences between the two groups were in the impaired GCT results and high LDL levels, both occurring more in the preterm group [(61.1% vs 37.3%) and (55.6% vs 32.4%), respectively].
Metabolic syndrome, together with its components, and cholesterol and LDL levels were compared between PPROM versus term deliveries by Fisher's exact test which revealed no statistically significant differences between the two groups (Table 3).

Discussion
In this prospective cohort study of 203 Iranian pregnant women in mid-pregnancy, 8.9% of deliveries were preterm, and prevalence of MetS was 4.9%. This study showed that some of the components of MetS related with preterm birth.
This study showed that impaired glucose tolerance (impaired GCT results) was significantly associated with preterm birth. A Turkish study found that the incidence of preterm labor was not altered by GDM status [25], but most studies have suggested that high glucose levels are associated with preterm delivery. Yun et al. in Korea, showed that glucose intolerance and gestational diabetes (GD) were risk factors for preterm labor [26]. In addition, Basirat et al. reported that the prevalence of PROM and preterm labor, in Iran, was higher in pregnant women with abnormal GCT results [27]. Diverse mechanisms, such as inflammation, infection, stress and some immunologic mechanisms might explain this situation [28].
In this study, logistic regression showed that lower levels of HDL-C were also significantly associated with preterm birth. These findings are consistent with the results of some other studies [28][29][30]. Inflammation and stress oxidative may be associated with increased risk of spontaneous preterm labor [16]. All the parameters included in the diagnosis of the metabolic syndrome including dyslipidemia are associated with a low-grade inflammation state [18]. An association of elevated levels of interleukin-6 (IL-6) and CRP with   the metabolic syndrome has also been found. Also, low levels of the anti-inflammatory cytokine interleukin-10 (IL-10) have been found to be associated with metabolic syndrome in women [19]. On the other hand, increased peroxidation of these elevated plasma lipids causes enhanced oxidative stress by progressively producing free radicals and lipid peroxides. Lipid peroxides are toxic compounds that have the potential to damage endothelial cells [20]. In this study, a significant relationship was observed between gestational hypertension and preterm birth. Ghatzi et al. assessed the association between metabolic syndrome components and the risk of preterm birth in Greece, and their findings showed that the most significant risk factor was hypertension [6]. Catov et al. suggested that chronic hypertension, diabetes, body mass index, and age were related to the risk of term and preterm SGA births [31]. Gilbert reported that mothers with chronic hypertension are at risk of various neonatal morbidities such as low birth weight and prematurity [32]. In general, women with hypertension may have an inadequate vascular response and may suffer from a defect in endothelial wall and also abnormal placentation; on the other hand, gestational hypertension, independent of maternal obesity and metabolic disorders, is associated with insulin resistance and dyslipidemia.
In this study, no significant relationship was observed between metabolic syndrome and preterm birth and PPROM. Similarly, in a study by Baliutavičienė et al. gestational age at delivery in women who had three components of metabolic syndrome (impaired glucose tolerance, obesity, and hypertension) was not different from healthy women [33]. Furthermore, in the study by Grieger et al. diagnosis of MetS was not associated with increased risk at lower gestational ages at birth [9]. However, in the study by Lei et al. metabolic abnormalities in pregnancy were associated with adverse pregnancy outcomes such as preterm labor [34]. The reason for the difference in the results of these studies is that there is no single definition for the metabolic syndrome in pregnancy and different measurements have been carried out to diagnose the metabolic syndrome.
This study provides a new insight into the metabolic syndrome and its components in mid-pregnancy (24th-28th weeks of gestation) and any possible association with preterm birth; to our best knowledge, no such investigation has hitherto been conducted in Iran, measuring and evaluating precisely all these components in addition to lipid profile in mid-pregnancy and following subjects up to delivery.
A major limitation was the lack of a standard definition for gestational metabolic syndrome. Studies that have assessed metabolic components in pregnancy have generally used a pregnancy-modified version of accepted definitions for the adults. In the study by Grieger et al. random plasma glucose and abdominal circumference were considered as metabolic syndrome criteria [9]. In the study by Stekkinger et al. like our study, abdominal circumference was not considered as a criterion of obesity, although obesity was defined as a body mass index higher than 30 kg/m 2 before pregnancy [35]. However, in our study instead of performing a fasting blood sugar test (FBS), glucose challenge test (GCT), was used. Appropriate definitions of metabolic syndrome have yet to be made in pregnancy and the accuracy of previously defined variables from the non-pregnant adult population is unclear. Thus, we used a pregnancy-modified version of NCEP-ATP III definition of metabolic syndrome for a general population, which needs further investigations to be confirmed. Also, the sample size of this study is not large enough to draw definite conclusions. Further research is needed with larger samples/cohorts for clarification and confirmation of these associations. The observed protective effect of metabolic syndrome on preterm birth needs further investigations too.

Conclusion
Premature birth is associated with an increased risk to both mother and fetus during pregnancy and in later stages of life. However, etiology is not yet clear. The results of this study showed that blood pressure and dyslipidemia may be associated with preterm labor. Therefore, the necessary counseling and education on weight management and blood pressure control before pregnancy can be helpful. Also, our findings can be used for early identification of pregnancies at higher risk of preterm deliveries and to implement appropriate measures to prevent them. Finally, the identification of risk factors might provide important insights into mechanisms leading to preterm birth.
Funding This study was performed as the thesis project for a M.Sc. degree in Midwifery in Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran. No external funding sources were provided for this study.

Compliance with ethical standards
Conflict of interest The authors have no conflict of interest to declare.
Ethical approval This research was approved by the Ethical Committee of the Faculty of Medical Sciences in Tarbiat Modares University, Tehran, Iran.
Informed consent Informed written consent was obtained from all participants.