The ethics committee of Babol University of Medical Science accepted this study (Ethic ID: IR.MUBABOL.HRI.REC.1398.022). The minimum sample size for this study was calculated to be around 411, which was based on the estimated prevalence of 18.6% in Tehran, Iran , with a standard score 95%, margin error of 4%, and 15% of drop-out rate.
A total of 411 singleton pregnant women, aged 18–35 years who were at 14–16 weeks of pregnancy, were recruited for this study at prenatal clinics affiliated with Babol University of Medical Sciences. The study was in progress over the period between March 2019 and February 2020. The women with a history of pre-gestational diabetes (n = 11), dyslipidemia (n = 2), chronic hypertension (n = 4), thyroid disease (n = 6) and other endocrine diseases (n = 2), fetal malformations in nuchal translucency (NT) (n = 2), and taking hyperglycemic drugs (corticosteroids and thyroid hormones) (n = 1) were excluded of our study. 24 women were initially excluded from the study, which was based on the results of the routine prenatal blood tests at the first trimester of pregnancy as well as the information obtained from their medical documents. Therefore, the remaining 387 eligible women signed the written informed consent forms. The data for all pregnant women were collected at three time-points: 14–16 gestational weeks, 24–28 weeks, and after the child birth. During the follow-up, 15 participants with diagnosed pregnancy-induced hypertension and gland thyroid disease were also excluded from the study.
During the 14–16 weeks of pregnancy, the characteristics of the participants such as age, gravidity, and parity were obtained. The women were requested to report the pre-pregnancy weight (self-report). If the woman did not remember the weight before pregnancy, the weight of the first visit (first trimester of pregnancy) was recorded. The gestational age of the participants was defined according to last menstrual period and established by early ultrasound pregnancy. The height was measured with a tape measure without shoes. The body mass index was measured by the subsequent formula: weight (kg)/ squared (m2). The neck circumference was determined through a tape from the level just below the larynx (accuracy 1 cm) with subjects standing position, straight ahead with their shoulders .
At 24–28 weeks of pregnancy, the blood pressure (BP) was measured with calibrated mercury sphygmomanometers with appropriate size cuffs after the women had rested for 15 minutes. The Korotkoff phase 1 (first sound) and Korotkoff phase 5 (fifth phase) of blood pressure was defined as systolic blood pressure and diastolic blood pressure, respectively . Pregnancy induced hypertension (PIH) was defined as blood pressure greater than or equal to 140/90 mmHg with or without proteinuria during pregnancy .
In addition, a two-hour, 75-gram oral glucose tolerance test (OGTT) was performed after a ten-hour fasting in sitting position. All blood samples were analyzed at laboratories affiliated with Babol University of Medical Sciences. If there were any of the following glucose cut-off levels: fasting ≥ 92 mg/dl or one-hour ≥ 180 mg/dl or and two-hour ≥ 153 mg/dl, the women were diagnosed with gestational diabetes .
Out of 387 women who were followed until child birth, 15 women with diagnosed pregnancy-induced hypertension and gland thyroid disease were excluded from the study for the accurate assessment of the relationship between neck circumference and gestational diabetes. In addition, the maternal weight at the end of pregnancy, the type of birth, the weight of the newborn, respiratory syndrome, and the administration in the neonatal intensive-care unit (NICU) were all collected from the records of delivery.
Analyses were performed by SPSS software version 20.0 (SPSS Inc, Chicago, IL, USA). The Kolmogorov-Smirnoff test was used as a test for evaluating the normality of the dataset. The demographic and anthropometric characteristics, blood pressure and blood glucose (gestational diabetes mellitus) were compared between the two groups using independent t-test and chi-square test. Correlation between neck circumference and risk factors of gestational diabetes mellitus was assessed by Pearson coefficient test. Adjusted age logistic regression analysis was used for present odds ratio (OR) and confidence interval (95% CI).
Also, ROC analysis was used to evaluate the predictability of gestational diabetes. The area under the curve was calculated by SPSS software, and with due sensitivity and specificity, we strove to obtain the best neck circumference cut-off points. The significance level for all tests was considered less than 0.05.