This is a randomized controlled trial of different glycemic targets during tratment of women with GDM with assessement of epygenetic aspects of their effects on the fetus and pregnancy outcomes. This study is interventional, randomised controlled trial, open-label.

The study aims to clarify the effect of hyperglycemia and its correction, the level of physical activity and consumption of major macro- and micronutrients by women during pregnancy on DNA methylation and expression of genes involved in neuroendocrine regulation and development of metabolic diseases in offspring, as well as functional characteristics of human umbilical vein endothelial cells (HUVECs). For the purpose of the study women with GDM are randomised to 2 treatment groups per glycemic targets ( very tight and tight-moderate glycemic targets). Data on glycenmic levels during the study and consumption of major macro- and micronutrients will be collected using a mobile application with electronic dairies report forms.

This clinical trial record primarily describes the overarching observational cohort study "Genetic and Epigenetic Mechanisms of Developing Gestational Diabetes Mellitus and Its Effects on the Fetus" including women with GDM and healthy pregnant women.

Substudy Protocol: A key predefined component of this larger study is an interventional, randomized controlled substudy entitled "Tight versus less tight glycaemic targets for women with gestational diabetes mellitus: a randomised controlled trial" (also known as the GEM GDM Trial).

The GEM GDM Trial is an open-label, randomized controlled trial (RCT) that compares the effects of tight (very tight) versus less tight (tight-moderate) glycemic control on maternal and neonatal outcomes in women diagnosed with Gestational Diabetes Mellitus (GDM). Participants from the main GEM cohort who are diagnosed with GDM according to International Association of the Diabetes and Pregnancy Study Groups (IADPSG) criteria are eligible for screening and subsequent randomization into this substudy.

The substudy aims to enroll 650 pregnant women with a singleton pregnancy and GDM, randomly assigning them in a 1:1 ratio to one of two glycemic target groups.

• Tight Target Group: Fasting capillary glucose <5.1 mmol/L (<92 mg/dL); 1-hour postprandial glucose <7.0 mmol/L (<126 mg/dL).
• Less Tight Target Group: Fasting capillary glucose <5.3 mmol/L (<95 mg/dL); 1-hour postprandial glucose <7.8 mmol/L (<140 mg/dL).

Inclusion Criteria:

• Pregnant women with GDM diagnosed according to the Russian national consensus and the recommendations of the International Association of Diabetes and Pregnancy Study Groups (fasting glucose of ≥5.1 mmol/L, and/or ≥10.0 mmol/L after 1 h, and/or ≥8.5 mmol/L after 2 h in oral glucose tolerance test (OGTT) with 75 g of glucose).
• Gestational age at the time of inclusion in the study 12 weeks 0 days - 31 weeks 6 days
• For control group: pregnant women with normal glucose tolerance confirmed by OGTT at 24-31 weeks of gestation.

Exclusion Criteria:

• Diabetes mellitus type 1 and tipe 2
• Other deseases that affect methabolism of carbohydrates
• Use of drugs that affect methabolism of carbohydrates
• Malformations of the fetus identifired prior to inclusion to the study.

Gestation