The medical community divides the fetal growth and development stages (which occur during pregnancy) into three trimesters. A completely full-term pregnancy lasts a total of 40 weeks, and these 40 weeks are divided into three trimesters which each denote a period of significant milestones, characteristics, and development for the fetus.
The first 12 weeks of pregnancy are also known as the first trimester of pregnancy. However, the developing embryo will not be known as a “fetus” until after around the 10th week of gestational development. The embryonic stage precedes the fetal development stage. By the end of the 10th week of gestation, the basic formations for the organs have been established, and the embryo has entered the fetal stage of development.
During this time of beginning fetal stage of development, the heartbeat can be heard with a Doppler stethoscope and the heart can be seen beating during an ultrasound examination. Other characteristics of this stage include: Urine secretion from the kidneys, teeth buds, more developed eyelids, more distinctive face formation, more defined external ear formation, and red blood cell production in the liver.
The second trimester of pregnancy occurs during weeks 13 through 27. (Be aware that different charts may vary slightly in the exact amount of weeks that determine each trimester). During this second trimester, fetal development includes: Sexual differentiation, harder bones, finger and toe nail development, liver and pancreatic fluid secretions, continued formation of foot and fingerprints, and Lanugo (fine hairs that cover the fetus).
The third and final trimester of pregnancy is roughly weeks 28 through 40. Fetal development during this stage includes: More complex brain connections, increase in body fat, iron and calcium storage, sensitivity to light, tear production, eye movements, fully developed hearing, and body temperature regulations.