During development in the womb, there are critical periods of vulnerability to sub-optimal conditions. Vulnerable periods occur at different times for different tissues Programming has permanent effects that alter responses in later life and can modify susceptibility to disease. Fetal development is activity dependent .
Normal development is dependent on continuing normal activity.
Programming involves several different structural changes in important organs . The placenta plays a key role in programming Compensation carries a price. In an unfavorable environment, the developing baby makes attempts to compensate for deficiencies. However, the compensatory effort often carries a price Attempts made after birth to reverse the consequences of programming may have their own unwanted consequences. Problems may arise when postnatal conditions prove to be other than those for which the fetus prepares.