**Abstract**

Pregnancy is characterized by hormonal changes, critical for the mother's physiological adaptation, exercising a role in the fetus's development, maintenance, protection, and nutrition. Since born, the neuroendocrine system's involvement is necessary to prevent the embryo from being rejected by the mother's immune system. These changes are regulated by fluctuations in hormones such as Progesterone, Testosterone, Androstenedione, Dehydroepiandrosterone, Estradiol, Prolactin, human Placental Lactogen, human Chorionic Gonadotropin, and Thyroid hormones, which promote the mother's development and the fetus (maternal-fetal development). Therefore, given the great importance of these hormones during pregnancy, this chapter will explain the preclinical and clinical participation of sex hormones in maternal-fetal development.

**Keywords:** pregnancy, hormonal changes, maternal-fetal development

### **1. Introduction**

During pregnancy, the neuroendocrine system undergoes significant hormonal fluctuations determined by stimulatory and inhibitory inputs from the mother and fetus to maintain the internal environment (milieu). This process is regulated mainly by both the maternal brain and the placenta, acting through the maternalplacental-fetal unit (MPFU). It also serves as a protection system against stress and immune responses [1, 2].

Interestingly, the neuroendocrine responses generate a feedback circuit regulated by the placenta. This organ begins its development in days six-seven after conception. It has been considered a passive organ for many years, acting as a barrier between the mother and the fetus, provide nourishing and eliminate metabolism products such as urea, uric acid, and creatinine. However, the placenta is a neuroendocrine organ that can synthesize and release hormones, neuroactive factors, and other mediators, allowing the proper development of the fetus's maternal tissues to ensure an optimal pregnancy, allowing the fetus to adapt and survive under conditions of stress, infections, hypoxia, and malnutrition [3, 4]. This neuroendocrine mechanism involves at least three different endocrine axes; the hypothalamus-pituitary-gonads axis (HPG), the hypothalamus-pituitary–adrenal gland axis (HPA), and the hypothalamuspituitary-thyroid axis (HPT), to ensure optimal maternal-fetal development [1].

Specifically, the HPG axis, which is the central axis involved in regulating the reproductive function in vertebrates by a releasing pulsing of GnRH at the hypothalamus and placenta, has a decisive role in the different stages of pregnancy. In this sense, it plays a central role in regulating MPFU development through positive and negative regulation of sex hormones [1].
