The medical textbook "Gray's Anatomy: The Anatomical Basis of Medicine and Surgery" explains that "the heart tubes fuse into a common loop" and "contractile activity commences" at 21 to 27 days after fertilization. Likewise, the textbook "Biology: Investigating Life on Earth" states that the human "embryo has its own circulatory system, complete with a heart that started beating only 24 days after conception…." Obstetricians measure pregnancy timeframes from the first day of the last menstrual period, which occurs an average of 2 weeks before fertilization. Thus, the hearts of pre-birth humans begin beating at 3 to 4 weeks after fertilization, which is commonly called the 5th to 6th week of pregnancy.