BIOL 260 Chapter Notes - Chapter 4.2: Multinucleate, Ground Substance, Primitive Streak
Document Summary
Human gestation (pregnancy) begins with conception (fertilization) and ends with parturition (childbirth) It lasts a(cid:374) a(cid:448)erage of (cid:1006)66 days (cid:894)(cid:1007)8 (cid:449)eeks(cid:895) a(cid:374)d is (cid:373)easured for(cid:373) the day a (cid:449)o(cid:373)a(cid:374)"s last menstrual period (lmp) began, and birth is predicted to occur about 280 days (40) weeks thereafter. Time periods in this chapter, however, are measured from the date of conception: conceptus: all the products of conception, pregnancy can be divided into 3-months intervals called trimesters: The first trimester (first 12 weeks): extends from fertilization through the first month of fetal life. This is the most precarious stage of development; more than half of all embryos die in the trimester. Stress, drugs, and nutritional deficiencies are most threatening to the conceptus during this time. The second trimester (weeks 13 through 24): is a period in which the organs complete most of their development. It becomes possible with sonography to see good anatomical details in the fetus.
