Cells dividing rapidly are at greatest risk. 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. Each phase of development provides required conditions to subsequent development.
Programming involves structural changes to important organs.
The placenta plays a key role in programming. The developing baby will attempt to compensate for deficiencies in the womb. But that compensation carries a price in later life. Attempts made after birth to reverse the consequences of programming might have their own unwanted consequences. Fetal cellular mechanisms often differ from adult processes.