The onset and progression through the various stages of puberty are influenced by a number of factors (Fig. 2). In both animals and humans, the age of puberty appears to be related more to body weight than to chronologic age. Undernutrition and low body fat, or an altered ratio of lean mass to body fat, seem to delay the adolescent spurt and to retard the onset of menarche. According to Frisch, a minimum level of fatness (17% of body weight) is associated with menarche; however, a heavier minimum weight for height, representing an increased amount of body fat (22%), appears necessary for the onset and maintenance of regular menstrual cycles in girls over 16 years of age. This critical amount of body fat implies that a particular body composition, in addition to other environmental and psychosocial factors, is important in triggering and maintaining the pubertal process.
PIP: Biological factors which influence the progression through female puberty stages are delineated, and an increase in the proportion of the body's fat content is identified as a critical prerequisite for the onset and maintenance of regular ovulatory cycles. Excessive exercise or malnutrition may interfere with the normal increase in the proportion of body fat and retard the onset of menarche. Pubic hair growth and breast development begins in most American females between the ages of 8-13. Menarche follows 4.2 years later for 50% of the females, but of others, the time period ranges from 18 months to 6 mor years. Both males and females experience hormonal changes before the 1st physical signs of puberty are manifested. As sex hormones increase, changes in the body's proportion of lean, fat, and skeletal mass occur. For females an increase in body fat begins at 7 years and continues through ages 16-18 years. Studies indicate that the body's fat content must account for 17% of the body's weight before menarche can occur and that, at age 18 years, the fat content must be at least 22% for the maintenance of regular menstrual cycles. Apparently, hypothalamic sensitivity to estrogens is decreased when the critical ratio of lean mass to body fat is reached, and changes in the hypothalamic and pituitary hormones promote pubertal progression and the establishment of reproductive functions. Poor nutrition alters the ratio of lean mass to body fat and delays the onset of menarche. In the US, the age at menarche decreased by 3 years since 1840 due to improvements in the population's nutritional status. Underweight females generally experience menarche at later ages than normal weight females. In contrast overweight females often experience menarche earlier than the average weight female. Athletic females and ballet dancers frequently experience late menarche, and these delays may be due to the disruption in fat accumulation which results from excessive exercise. Physically, inactive adolescents, on the other hand, tend to experience menarche at an earlier age than normally active females. In conclusion, the body's fat content along with a variety of environmental and psychosocial factors are responsible for the development and maintenance of female reproductive functions.