**1. Introduction**

Obesity has become a major global health challenge because of the established health risks and substantial increases in prevalence. Urgent global action and leadership is needed to help countries to more effectively intervene [1]. This increase runs in parallel to an increase in the

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

obesity during pregnancy; moreover, due to the adverse effects that this condition has on both the mother's and offspring's health, infant obesity has become a highlight topic of study [2].

It is well known that the physiology during pregnancy differs between obese and normalweight women. Obesity is associated with increased insulin resistance, adverse effects in implantation and placentation processes, growth, development and metabolism alterations of the fetus, and even impact on the offspring gut microbiota [3].

Until now, studies focused on the origins of obesity were oriented towards dietary excesses (processed sugars, fat, and proteins) [4] or host genes [5]. But recent studies have shown changes in gut microbiota associated to different diseases, like obesity, metabolic syndrome, or type I [6] and type II diabetes [7]. The community of microorganisms living in a specific environment is known as microbiota. These microorganisms include bacteria, Archaea, viruses, and some unicellular eukaryotes [8]. The collective genomes of the microorganism that constitute the microbiota are known as microbiome [9]. The normal gut microbiota imparts specific function in host nutrient metabolism, xenobiotics, and drug metabolism, maintenance of structural integrity of the gut mucosal barrier, immunomodulation, and protection against pathogens [10]. In fact, some of these microorganisms residing in the gut encode proteins involved in functions important for the host's health, such as enzymes required for the hydrolysis of otherwise indigestible dietary compounds, and the synthesis of vitamins [9]. Since the 1990s, our knowledge of the complexity of this ecosystem has increased due to the advances in culture-independent techniques. These new techniques are fast, facilitate high throughput, and identify organisms that are uncultured to date and present in the gut microbiota; recently, by using these techniques, it has been shown that alterations in the gut microbiota composition and function are associated with certain disease states, such as obesity [11]. With the increase in knowledge about gut microbiome functions, it is becoming increasingly more possible to develop novel diagnostic, prognostic, and most important therapeutic strategies based on gut microbiota manipulation.

Focused on obesity, it has been shown that certain bacteria metabolize different nutrients more efficiently than others, increasing the absorption of calories from the diet and the amount of energy usable for the host, which contributes to fat deposition [12]. Many studies have been performed in order to link this disease with changes in the composition of the intestinal microbiota [13]. Several studies have shown increased ratio in the proportion of *Firmicutes*/*Bacteroidetes* in genetically obese mice (ob/ob) and obese humans [14, 15]. However, other studies have failed to confirm these findings and showed variable patterns in the composition of the microbiota in obese humans [13]. Within the studies cited above, it is clear that the gut microbiota plays a role in obesity and metabolic disease, but it is difficult to draw definitive conclusions about the importance of certain bacterial groups. It is therefore very important to identify the active bacteria that cause dysbiosis in the gut microbiota in order to design therapeutic strategies for long-term protection against obesity. Quantitative and qualitative alterations in the composition of the gut microbiome could lead to pathological dysbiosis.

The microbiota colonization of the maternal intestine influences offspring's metabolic and immune system development [16]. Besides, although the microbiota-gut-brain axis is not a new concept [17], in the last years there are growing interest in studying the influence of the microbiota in children neurodevelopment by analyzing the microbiome impact on eating behavior, infant cognitive function, and brain structure and function [18]. However, the mechanisms by which maternal microbiota may contribute to health programming in the offspring are still unknown. The type of delivery (vaginal or caesarean section), diet [breast milk or formula], and antibiotics exposure have an influence on the offspring's immune system that may promote the development of chronic inflammation, leading to allergies, autoimmune diseases, like diabetes mellitus or rheumatoid arthritis, or noncommunicable diseases such obesity and their comorbidities in children [19–21].

In the present chapter, we aimed to update the knowledge about the factors involved in gut microbiota establishment during perinatal life, infancy and early childhood, and the relationship to obesity development.
