The goal of this randomized, double-blind, placebo-controlled study included pregnant women at high risk of GDM at 6-15 weeks of gestation is to investigate whether supplementation with vitamin D (1600 IU per day) or prebiotics (inulin, 10 gram per day) during pregnancy has beneficial effects on controlling blood glucose during pregnancy.

The goal of this randomized, double-blind, placebo-controlled study included pregnant women at high risk of GDM at 6-15 weeks of gestation is to investigate whether supplementation with vitamin D (1600 IU per day) or prebiotics (inulin, 10 gram per day) during pregnancy has beneficial effects on controlling blood glucose during pregnancy.

About 500 pregnant women aged 18-45 years, at 6-15 weeks of gestation, had resided locally for at least one year, with high risk of GDM will be enrolled in the study. Pregnant women at high risk of GDM are defined as meeting any of the following criteria:

1. Pre-pregnancy body mass index (BMI) ≥24.0 kg/m2;
2. History of GDM or a family history of diabetes;
3. History of delivery of macrosomia (birth weight >4000 g);
4. Glycated hemoglobin (HbA1c) 5.7-6.4% or fasting blood glucose 5.6-7.0 mmol/L; Eligible participants must have singleton pregnancy, no infertility treatment, no history of diabetes or a definite diagnosis of diabetes, or impaired glucose tolerance at recruitment (fasting blood glucose >7.0 mmol/L or HbA1c≥6.5%), no allergic history of chicory root and they will be required to agree to limit the use of vitamin D, prebiotics, probiotics, or synbiotic supplementations and to be convenient access to study centers. Safety exclusions include serious chronic or infectious diseases, serious liver or kidney disease, abnormalities in calcium metabolism, and clinically diagnosed mental disorders, or other conditions that would preclude participation.

Eligible participants will be assigned by chance to one of four groups: (1) daily vitamin D (1600 IU) and prebiotics (10 gram); (2) daily vitamin D (1600 IU) and prebiotics placebo; (3) daily vitamin D placebo and prebiotics (10 gram); or (4) daily vitamin D placebo and prebiotics placebo. Randomization will be conducted.

At enrollment, baseline questionnaires are designed to collect data on sociodemographic factors, pregnancy-related information, lifestyle habits, health status, and medical conditions before and during pregnancy. Participants in all groups will take two capsules that contained either vitamin D or vitamin D placebo and a strip of prebiotics powder (10 g per strip, brew with warm water and take with meals) or prebiotics placebo each day until delivery.

Participants will be followed up three times during pregnancy (24-28 weeks gestation, 32-36 weeks gestation and delivery), and receive a single stage-specific dosage at any given follow-up time.

Participants will have physical measurements of height, weight, other anthropometric measurements, and blood pressure at baseline and each follow-up visit. Blood, urine, and stool samples will also be obtained in the study center, fasting blood glucose, HbA1c and other biochemical indicators will also be detected at the same time, and a 75g oral glucose tolerance test (OGTT) will be measured during 24-28 weeks of gestation to diagnose GDM.

The primary outcomes, including fasting blood glucose and HbA1c will be measured using blood samples.

Secondary outcomes in this study include incidence of GDM, changes in blood lipids (such as total cholesterol, triglycerides, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol), trends in plasma vitamin D levels, inflammatory factors, intestinal flora related indicators, delivery mode, incidence of preeclampsia and adverse pregnancy outcomes. Data will be collected and analyzed.

Inclusion Criteria:

1. 6-15 weeks of gestation.

2. Age 18-45 years old.

3. Permanent resident or resident locally at least one year.

4. GDM high-risk pregnant women (meeting any of the following criteria):

   1. Pre-pregnancy body mass index (BMI) ≥24.0 kg/m2,
   2. History of GDM or a family history of diabetes,
   3. History of delivery of macrosomia (birth weight >4000 g),
   4. HbA1c 5.7-6.4% or fasting blood glucose 5.6-7.0 mmol/L.

Exclusion Criteria:

1. Multiple pregnancy.
2. Have received infertility treatment such as in vitro fertilization or intrauterine insemination.
3. History of diabetes or current diagnosis of diabetes (including type 1 and type 2 diabetes) or abnormal glucose tolerance at recruitment (fasting blood glucose >7.0 mmol/L or HbA1c≥6.5%).
4. Serious chronic diseases (including cardiovascular and cerebrovascular diseases, cancer, and thyroid dysfunction) or infectious diseases (including hepatitis B, active tuberculosis, acquired immunodeficiency syndrome, and syphilis).
5. Severe liver disease (such as cirrhosis) or severe kidney disease (such as renal failure or requiring dialysis).
6. Kidney stones, hypercalcemia, hypercalciuria, parathyroid hormone abnormality.
7. Any mental disorders, such as schizophrenia, depression, other mental disorders, or bipolar disorder.
8. History of allergy or intolerance to vitamin D, chicory root, or starch.
9. Have participated in or are participating in other clinical trials within the past 3 months.
10. Daily vitamin D intake >800 IU.
11. Inability or refusal to answer and communicate.
12. Those who are unwilling to sign the informed consent.
13. The researcher thinks that it is not suitable to participate in this research.