If cells don't differentiate properly, they will never function properly. Once the window of development passes, damage can not be repaired. Many developmental changes are controlled by hormones. Hormones occur in very low concentrations in our bodies all the time. During prenatal development, subtle shifts in hormone levels cause various cells to differentiate at different times.
Some chemicals interfere with normal hormone function.
These chemicals are known as endocrine disruptors and are far more widespread than you might think. Some endocrine disruptors mimic hormones like estrogen. If an expecting mother is exposed to estrgen mimicing compounds, the estrogen level in her body will be functionally higher. This means her baby is being exposed to more estrogen than usual. Estrogen is important for development, but too much can be a very bad thing, leading to feminization of male babies and increased risk of breast cancer in females.