Study design and population
The hospital-based prospective cohort study was designed and conducted among pregnant women who were registered in the obstetric archives and completed their first perinatal visit in the obstetrics clinic of the First Affiliated Hospital of Shantou University Medical College and Beijing Friendship Hospital of Capital Medical University from March 2014 to December 2015. The purpose of this cohort was to assess the association between increased hematological parameters in early pregnancy and GDM incidence. All participants had provided written informed consent before participation. This study was designed and conducted in accordance with the Declaration of Helsinki (2000) of the World Medical Association and approved by the Institutional Research Review Board of the National Research Institute for Family Planning, Beijing, China.
Pregnant women aged 20–49 years old, of Han nationality, with singletons, registered in the obstetric archives of the two hospitals, and living in local areas for more than half a year without tendency to move out during pregnancy were enrolled at the baseline of this cohort study. Participants were excluded if they were more than 12 gestational weeks, have missing values of hematological parameters, or had a history of chronic diseases (including diabetes, hypertension, or chronic nephritis) at the baseline. After the baseline interviews, all participants were followed up at 24, 32, 36 gestational weeks and the time of delivery, respectively. The flowchart of the recruited population is provided in Fig. 1.
Data collection
Baseline information about demographic characteristics, medical history of diseases, and lifestyle behaviors of each participant were collected using a structured questionnaire by trained staff through face-to-face interviews. Demographic characteristics included age, educational level, occupation, pre-pregnancy weight, height, and monthly income. Medical history of disease regarded history of GDM, hypertension, adverse pregnancy outcomes, anemia, thyroid disease, and infection of hepatitis B virus. Family history of diseases included diabetes mellitus, hypertension, and GDM. Medication history covered the use of iron contained health products, oral contraceptives, antibiotics, and painkillers. Lifestyle behaviors included activities of smoking, drinking, passive smoking, psychological pressure, and transportation to work. All questionnaires were reviewed and entered independently by two persons under the settings for logical error correction. Physical examinations, previous obstetric history, laboratory parameters, and diagnoses were extracted from the medical records. Information of physical examinations contained height, weight, waist circumference, and blood pressures. Laboratory parameters recorded indicators of blood tests, liver function tests, renal function tests, fasting blood glucose tests, and Hepatitis B antigen tests. All these laboratory indicators were tested from fasting blood samples which collected and immediately stored at 4–8°C in 24 h.
Follow-up information in prenatal medical care tests about maternal and neonatal health status were all documented. Recorded information included GDM diagnosis, lifestyle behaviors during pregnancy (smoking, passive smoking, drinking, and diet), gestational weight, uterine height, abdominal circumference, and blood pressures.
Definitions
Age was classified as < 35 and ≥ 35 years old; education level was categorized as below college and equal or above college; occupation was sorted as farmers and workers; monthly income was grouped as RMB ≤ 5000 and RMB > 5000. Pre-pregnancy body mass index (BMI) was calculated as weight in kilograms divided by the square of height in meters. Participants with BMI < 18.5, 18.5–23.9, 24–27.9, and > 28.0 kg/m2 at pre-pregnancy were regarded as underweight, normal, overweight, and obese, respectively, according to the guidelines for Chinese adults [17]. Gestational weeks were further categorized as ≤ 8 and 8–12 groups. Weight gain in the first trimester was calculated by weight before 12 gestational weeks minus weight in pre-pregnancy. Weight gain in the second trimester was calculated by weight in 24 gestational weeks minus weight in pre-pregnancy. The information about whether used iron contained health products, such as Elevit or Kingsley, were also collected. Early pregnancy fasting glucose was divided into < 6.1mmol/L and ≥ 6.1 mmol/L levels. Self-reported dietary taste was categorized as light, moderate, and salty. Work transportation was grouped as non-working, driving, taking subway, and riding bicycle.
As recommended by Abbassi [18], the normal range of RBC count, Hb, WBC, and PLT in the first trimester were defined as 3.42–4.55 (×1012 /L), 116–139 (g/L), 5.7–13.6 (×109 /L), and 174–391 (×109 /L), respectively. Then, all categorized hematological parameters in the first trimester were classified into two levels (Class I), namely, normal RBC (3.42–4.55 ×1012 /L) and abnormal RBC (< 3.42 or > 4.55 ×1012 /L); normal Hb (116–139 g/L) and abnormal Hb (< 116 or > 139 g/L); normal WBC (5.7–13.6 ×109 /L) and abnormal WBC (< 5.7 or > 13.6 ×109 /L); normal PLT (174–391 ×109 /L) and abnormal PLT (< 174 or > 391 ×109 /L). Successively, all categorized hematological parameters in the first trimester were classified into three levels (Class II), RBC count was categorized as < 3.42 (×1012 /L), 3.42–4.55 (×1012 /L), and > 4.55 (×1012 /L) groups; Hb concentration was stratified into < 116 (g/L), 116–139 (g/L), and > 139 (g/L) levels; WBC count was classified as < 5.7 (×109 /L), 5.7–13.6 (×109 /L), and > 13.6 (×109 /L) groups; and PLT count was grouped into < 174 (×109 /L), 174–391 (×109 /L), and > 391 (×109 /L) levels.
GDM was diagnosed if at least one value of plasma glucose concentration was equal to or exceeded the thresholds of 5.1, 10.0, and 8.5 mmol/L for fasting, 1 h, and 2 h post-glucose load values, respectively, after performing a 75 g oral glucose tolerance test (OGTT) at gestational 24–28 weeks according to the Guidelines for Diagnosis and Treatment of Diabetes in Pregnancy (2014) in China [19].
Statistical analysis
Kolmogorov–Smirnov test was used to determine the normality of hematological parameters, and baseline characteristics were described by median and interquartile range. Mann–Whitney U test or Kruskal-Wallis test was conducted to evaluate the differences in hematological parameters among participants with categorized basic characteristics. An independent sample t-test or χ2 test was also performed to evaluate the difference in baseline characteristics between GDM and non-GDM cases. The changes of RBC and Hb in the first and the second trimester were also described and the differences of GDM incidence within RBC or Hb levels were examined by χ2 test. Afterwards, the association (odds ratio [OR] and corresponding 95% confidence interval [95%CI]) between continuous hematological parameters at the first or the second trimester and the risk of GDM were assessed using age-adjusted and stepwise multivariable-adjusted logistic regression models, respectively. To further evaluate the association between categorized hematology parameters in the first trimester and the incidence of GDM, numeric hematological parameters were grouped into two classes, and each class used two multivariable-adjusted logistic regression models. Covariates in the multivariable-adjusted model I included age, gestational weeks, BMI before pregnancy, weight gain in the first trimester, dietary taste, iron contained health product consumption, passive smoking status, transportation to work, fasting glucose, history of GDM, and family history of GDM at the baseline. In model II, weight gain in the first trimester was replaced with weight gain in the second trimester based on model I. All analyses were used with R software 3.5.0, and two-sided p < 0.05 values were considered statistically significant.