a study by sandman and davis shows that the timing of prenatal stress is crucial to understanding how prenatal stress affects prenatal and postnatal development. cortisol is often used to measure stress as it is a hormone that is released during stressful events. if an expectant mother is experiencing a stressful event such as income insecurity or being a teenage mother, cortisol is secreted as a result. however, as demonstrated by sandman and davis, the timing of cortisol release can sometimes have a harmful effect on development and sometimes not depending on when in pregnancy stress is experienced. sandman and davis studied "125 full- term infants at 3, 6, and 12 months of age" to determine the effects of maternal cortisol timing differences on development. they found that "exposure to elevated concentrations of cortisol early in gestation was associated with a slower rate of development over the 1st year and lower mental development scores at 12 months" and "elevated levels of maternal cortisol late in gestation were associated with accelerated cognitive development and higher scores at 12 months". overall, cortisol's effects on infant cognitive development are dependent upon the timing of cortisol release.