Prenatal development involves all of the changes that take place from the time of conception until birth. After conception, a zygote forms that rapidly changes into a blastocyte, embryo and finally a fetus. Every week of a pregnancy, the developing fetus undergoes dramatic changes as the body forms and takes shape.

During the prenatal development, certain milestones occur every week. Experts differ on exactly when these take place. For instance, some experts contend that the fetal period begins in the eighth week of pregnancy, whereas others contend that it starts in the ninth, 11^th or 12^th weeks. As a result, the timeframe used should serve as a rough estimate rather than an exact schedule of how prenatal development will unfold.

Though many pregnancies proceed smoothly, death or disease can occur at any time during prenatal development. About 25 percent of pregnancies end in a spontaneous abortion, according to the American Society for Reproductive Medicine. This typically occurs during the first trimester and is most often the result of problems related to chromosomes or some other abnormality.

Factors that can damage the fetus’ health include:

• Maternal use of alcohol and certain prescription and recreational drugs
  
  
• Maternal infection (including rubella and cytomegalovirus)
  
  
• Radiation from x-rays or radiation therapy
  
  
• Nutritional deficiencies

After fertilization, and even beforehand, the expectant mother can take steps to ensure normal development. These include taking prenatal vitamins and folic acid supplements, exercising regularly and maintaining a healthy weight, eating healthy meals, and trying to manage stress and health conditions. Pregnant women should also avoid smoking, drinking alcohol, using recreational drugs or taking certain over-the-counter or prescription medications.