The baby's organs begin developing as early as 17 days after conception and the fertilized egg begins to grow even before the first day of the missed period. Some women continue to have menstrual periods during the first few months of pregnancy and may not even realize that they are pregnant. Others may not recognize that they are pregnant until they experience a weight gain or abdominal enlargement. By then, they may have already been exposed to medications or substances potentially harmful to the fetus.

In addition to avoiding medications and substances that are potentially harmful to the fetus, other important health issues are addressed during pre-pregnancy planning. Conditions such as diabetes, high blood pressure, kidney disease, and heart valve disease in the mother are controlled to optimize pregnancy outcome. The status of the woman's immunity against German measles (rubella) and varicella (chickenpox) is also determined. Even though many women will have normal pregnancies without any planning, pregnancy planning improves the chances of a smooth pregnancy and a healthy baby.