growth (the first visit underestimating and the second visit overestimating) in the same elapsed period between measurements. Note there is a discontinuity in the growth curves at age 2, which reflects the difference in recumbent length (with the child on his or her back), used in measuring infants and toddlers, and standing height typically measured from age 2 onwards. Height, like other phenotypic traits, is determined by a combination of genetics and environmental factors. A child's height based on parental heights is subject to regression toward the mean, therefore extremely tall or short parents will likely have correspondingly taller or shorter offspring, but their offspring will also likely be closer to average height than the parents themselves. Genetic potential and several hormones, minus illness, is a basic determinant for height. Other factors include the genetic response to external factors such as diet, exercise, environment, and life circumstances. Humans grow fastest (other than in the womb) as infants and toddlers, rapidly declining from a maximum at birth to roughly age 2, tapering to a slowly declining rate, and then, during the pubertal growth spurt (with an average girl starting her puberty and pubertal growth spurt at 10 years and an average boy starting his puberty and pubertal growth spurt at 12 years), a rapid rise to a second maximum (at around 11–12 years for an average female, and 13–14 years for an average male), followed by a steady decline to zero. The average female growth speed trails off to zero at about 15 or 16 years, whereas the average male curve continues for approximately 3 more years, going to zero at about 18–19. These are also critical periods where stressors such as malnutrition (or even severe child neglect) have the greatest effect. Moreover, the health of a mother throughout her life, especially during her critical period and pregnancy, has a role. A healthier child and adult develops a body that is better able to provide optimal prenatal conditions. The pregnant mother's health is essential for herself but also the fetus as gestation is itself a critical period for an embryo/fetus, though some problems affecting height during this period are resolved by catch-up growth assuming childhood conditions are good. Thus, there is a cumulative generation effect such that nutrition and health over generations influence the height of descendants to vary degrees. The age of the mother also has some influence on her