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The expression of birTh weighT is modulaTed by The breeding season in a goaT model C é s a r A . M e z a -H e r r e r a 1 , G u a d a l u p e C a l d e r ó n -L e y v a 1 , M a r i a J . S o t o -S a n c h e z 1 , J o s é A b a d -Z a v a l e t a 2 , J u a n M . S e r r a d i l l a 3 , A n t ó n G a r c í a -M a r t i n e z 3 , R a f a e l R o d r i g u e z -M a r t i n e z 4 , F r a n c i s c o G . V e l i z 4 , U l i s e s M a c i a s -C r u z 5 , H o m e r o S a l i n a s -G o n z a l e z From a biological point of view in all mammal species, goats included, there is an optimal birth weight (BW) range within which normal physiological processes for growth and development can occur, reducing the expression of health dysfunctions 238while maximizing neonatal survival (Perez-Razo et al., 1998; Mellado et al., 2000). From a productive and economic viewpoint in small ruminants, BW expression is a key phenotypic characteristic of paramount importance in kid-milk goat production systems and their commercialization schemes (Meza-Herrera, 1986; Arechiga et al., 2008). Both fetal growth and development as well as BW expression are affected by genetic, placental and maternal cues (Meza-Herrera et al., 1987; Montaldo and Meza-Herrera, 1999; Xita and Tsatsoulis, 2010; Mellado et al., 2011).While exposed to an accurate gestational environment, fetus is able to reach an adequate size according to its genetic potential. Nonetheless, perturbations at uterus level, either intrinsic or extrinsic in nature, may potentially compromise both growth and development of fetus, the BW expression as well as preweaning mortality (Meza-Herrera, 1986; Gonzalez-Bulnes et al., 2011). Prenatal development and final BW are both dependent upon a close relationship among the available nutritional level and the secretion pattern of diverse hormones and growth factors (Funston et al., 2010).In addition, environmental fluctuations across year also determine different expressions on both BW and neonatal survival (Lincoln and Short, 1980). At the beginning of the embryonic life, although the embryo's nutritional requirements are low because of a low rate in the absolute growth, the rate of the relative growth is quite high. For this reason, several physiological, metabolic and genetic dysfunctions may occur, compromising the future development of the main organs and systems, a syndrome known as intrauterine growth retardation. Therefore, both the fetal and the birth weight relative to a gestational age, are frequently considered to measure and to detect intrauterine growth restriction (Wallace et al., 2005). In addition, experimental animal studies suggest that the nutritional, hormonal and metabolic environment afforded by the mother may permanently programme differentiating target tissues of the offspring toward the development of the metabolic syndrome in adult life (Xita and Tsatsoulis, 2010).Such physiological scenario, which can be modified by environmental variations throughout the year, may not only compromise the neonatal health status ...