a woman's human chorionic gonadotropin (hCG) levels rise above 25 mIU/mL. Biologically, pregnancy starts at implantation of the fertilized egg.

Human pregnancy can be divided into three trimesters, each approximately three months long: the first, second, and third trimester. The first trimester is from the last menstrual period through the 13th week, the second trimester is 14th-28/29th week, and the third trimester is 29/30th-42nd week. Birth normally occurs at a gestational age