Gestational diabetes mellitus (GDM) can be defined as glucose intolerance either first developed or found during pregnancy regardless of the degree of symptoms. It is a type of diabetes or impaired glucose tolerance developed during pregnancy and one of the most common medical complications of pregnancy [1]. GDM develops in approximately 3% to 4% of pregnant women, and mothers with GDM have a high risk of developing diabetes after delivery as well as their children who also risk developing obesity. It has been reported that GDM increases the frequency of pre-eclampsia, eclampsia, infectious disease, and the birth rate of large for gestational age fetuses in mothers. It also increases perinatal morbidity by increasing the risk of infantile respiratory distress syndrome, metabolic disorders, and hyperbilirubinemia in fetuses [2]. For the tests to diagnose GDM as a screening test, a 50 g oral glucose tolerance test (OGTT) during the 24 to 28 weeks of pregnancy is utilized. Subjects with blood glucose levels measured 1 hour after oral ingestion of glucose of 140 mg/dL (7.8 mol/L) or more are considered positive for the screening test, and for a confirmatory test, a 100 g OGTT is conducted following 12 hours of fasting [3]. Once GDM is confirmed, depending on the level of fasting blood glucose and post-meal (2-hour) blood glucose, diet therapy and exercise or insulin therapy can be conducted. Through such therapies, GDM-related complications in both mothers and their fetuses can be reduced [4].

The age of a mother, her pre-pregnancy body mass index (BMI), the amount of obesity, family history, and weight gain during pregnancy are GDM-related factors, but among these, obesity is the most serious risk factor [5,6]. Obesity is known to be highly correlated to abdominal subcutaneous fat thickness (ASFT) [7]. Although computed tomography (CT) is the most credible method for measuring ASFT, it is avoided for pregnant women due to its high cost and X-ray exposure. On the other hand, ultrasonography (US) imaging-based measurement of ASFT is very useful for pregnant subjects because the results are highly correlated to CT and there is no risk of X-ray exposure due to its relatively simple measuring process [8].

In this study, we intended to analyze the correlation between ASFT as measured by US in pregnant subjects during the first trimester of pregnancy in order to suggest an ASFT cut-off value as a predictive factor for GDM during the second trimester of pregnancy.

Although the frequency of GDM varies depending on the country, ethnicity, populations screened, and diagnostic criteria, it is commonly reported as 1% to 10% and its incidence rate is generally increasing due to factors such as the increase in maternal age and obesity among mothers. This can further increase the incidence of GDM-related complications and can also cause fatal conditions to fetuses such as dyspnea at birth. For this reason there is presently a dire need to be able to predict and prevent GDM [12]. The most common method for diagnosing GDM is to conduct a 100 g OGTT for those showing blood glucose levels of 140 mg/dL or more in the 50 g OGTT conducted during the 24 to 28 weeks of pregnancy. This test is conducted at the second trimester of pregnancy, so it is difficult to detect GDM at its early stage. Because the incidence rate of obstetric complications varies depending on the time of diagnosis, early detection and active management is required to prevent GDM-related obstetric complications [13].

This study was conducted in the hopes of finding a predictive factor relating to GDM in pregnant Korean women using as independent variables the ages of the mothers, pre-pregnancy BMI, weight gain during pregnancy, and ASFT as measured by ultrasound imaging conducted during the first trimester of pregnancy. To date, studies of the early detection of GDM have been ongoing. A large scale study indicated that pre-pregnancy BMI significantly increases along with the incidence of GDM during the second trimester of pregnancy as the age of a mother increases and when categorized based on ethnicity, the age of a mother and her BMI play more significant roles as risk factors in South Asian and black African women compared to white European or black Caribbean women [14]. Nevertheless, there was no significant correlation between the increased age of the mothers and GDM in this study (P=0.210), and for pre-pregnancy BMI, the incidence of GDM significantly increased as BMI increased from the Asian standard criterion. In particular, the impact was more significant in the overweight group (P=0.005) suggesting the importance of pre-pregnancy weight management for mothers. Further, although Choi [6] reported that the excess weight (or obesity) of a mother or excessive weight gain during pregnancy induced GDM, there was no significant correlation between the weight gain and GDM in this study (P=0.500).

Recently, as numerous metabolic and cardiovascular diseases related to obesity have been observed, methods to accurately evaluate abdominal fat have been gaining attention. Among such methods, despite its drawback of possible measurement error due to interoperator variation, measuring abdominal fat thickness using ultrasound is actively being studied because it can easily be applied to pregnant women and its reproducibility has been reported [15]. The results of a longitudinal cohort study of 1,510 pregnant women in Australia suggested the potential of ASFT as an independent predictor of GDM [16]. The correlation between maternal obesity and GDM has already been reported, and in particular, a study was recently published that examined the relationship between central obesity and the progression of GDM during the second trimester of pregnancy [17].

In a study of Caucasian subjects by De Souza et al. [18], the mean ASFT depth was 1.9±0.80 cm (range, 0.56 to 5.1 cm), consistent with the mean depth of 1.9±0.5 cm (range, 0.9 to 4.1 cm) in this study. However, although De Souza et al. [18] reported that ASFT was not a statistically significant predictor of GDM based on composite outcomes, this study observed a significant difference in GDM according to ASFT as indicated by the significant increase in GDM (P<0.001).

Based on these results, the cut-off value of 2.4 cm (AUC=0.90, P<0.001) was determined and an elevated ASFT was significantly associated with a positive GDM (unadjusted odds ratio, 2.91; 95% CI, 1.07 to 7.92; P=0.034). Therefore, when ASFT at the first trimester of pregnancy is 2.4 cm or more, adequate management is required and care practitioners should be alerted to the possibility of GDM and make efforts to manage diabetes during pregnancy.

One limitation of this study was the inclusion of pregnant women who were admitted to a single general hospital; therefore, these subjects are not representative of the entire population of pregnant women in Korea. Furthermore, various studies have reported different lengths of US-determined visceral fat and subcutaneous fat, and our results did not yield sufficient intraobserver reliability. This suggests the need for standardized measurement methods. In future studies, an identical ASFT measurement method must be applied to a larger population of patients, and this process should be used to standardize the measurement methods.

In conclusion, the results of this study demonstrated that ASFT as measured by ultrasound during the first trimester of pregnancy can be a valuable indicator for predicting GDM during the second trimester. Thus, ASFT, information that can be easily obtained during the first trimester of pregnancy, is expected to be increasingly utilized as an auxiliary diagnostic criterion for predicting the risk of GDM during the second trimester of pregnancy.