Pregnancy, the period during which a woman’s fertilized egg (embryo) gestates and becomes a fetus, lasts an average of 40 weeks from the date of the last menstrual period to delivery of the infant. 

In the first trimester (13 weeks), many pregnant women experience nausea. Usually these women report that they feel best during the second trimester. During the third (final) trimester, the increasing size of the fetus begins to place mechanical strains on the expectant mother, often causing back pain, leg swelling, and other health problems.