Estrogen and progesterone levels increase dramatically, reaching levels by late pregnancy that are several hundred-fold higher than usual menstrual cycle levels. Estrogen and progesterone cause the secretion of high levels of prolactin from the anterior pituitary, which reach levels as high as 20 times greater than normal menstrual cycle levels. IGF-1 and IGF-2 levels also increase dramatically during pregnancy, due to secretion of placental growth hormone (PGH). Further ductal development, by estrogen, again in conjunction with GH/IGF-1, occurs during pregnancy. In addition, the concert of estrogen, progesterone (again specifically through PRB), prolactin, and other lactogens such as human placental lactogen (hPL) and PGH, in conjunction with GH/IGF-1, as well as insulin-like growth factor 2 (IGF-2), acting together, mediate the completion of lobuloalveolar development of the breasts during pregnancy. Both PR and prolactin receptor (PRLR) knockout mice fail to show lobuloalveolar development, and progesterone and prolactin have been found to be synergistic in mediating growth of alveoli, demonstrating the essential role of both of these hormones in this aspect of breast development. Growth hormone receptor (GHR) knockout mice also show greatly impaired lobuloalveolar development. In addition to their role in lobuloalveolar growth, prolactin and hPL act to increase the size of the nipple-areolar complex during pregnancy.
Signs and symptoms
Fear, discomfort or anxiety may be triggered both by the presence and the anticipation of the specific object or situation. The main behavioral sign of a specific phobia is avoidance. The fear or anxiety associated with specific phobia can also manifest in physical symptoms such as an increased heart rate, shortness of breath, muscle tension, sweating, or a desire to escape the situation. Causes
The exact cause of specific phobias is not known. The mechanisms for development of specific phobias can be distinguished between innate (genetic and neurobiological) factors, and learned factors. In neurobiology, one explanation proposed for specific phobia is that the typical activation of the amygdala in response to stimuli may be exaggerated due to pathological changes. According to this theory, a deficiency in amygdala habituation may also contribute to the persistence of non-experiential phobia. Certain phobias that are less lethal (e.g. dogs) seem to be more frequently observed and easily acquired in comparison to potentially lethal fears which are more relevant to developed human society (e.g. cars and guns). This was theorised to be due to biological adaptation being passed through evolution which makes recent threats less prone to easy acquisition. However, a 2014 study found evidence against this evolutionary theory, which stated: "Our findings are inconsistent with the hypothesis that fears/phobias of individual stimuli result from genetic and environmental factors unique to that stimulus. Instead, we observed substantial sharing of risk factors across individual fears."