Chapter 4 From Zygote to Newborn Pages 91-97

Prenatal development is divided into 3 main periods: Germinal period – first 2 weeks Embryonic period – 3rd to 8th weeks Fetal period – from 8th week until birth Some developmentalists prefer to divide pregnancy into 3-month periods called trimesters.

At about 8 cells stage: clusters of cells begin to take on distinct traits. The first clear sign of this process, called differentiation, occurs about one week after conception, when the multiplying cells separate into outer cells that will become the placenta and inner cells that will become the embryo.

The next significant event is the burrowing of the outer cells of the organism into the lining of the uterus, a process called implantation. This process is not automatic.

At the beginning of the period of the embryo, a thin line down the middle of the developing individual forms a structure that will become the neural tube, which will develop into the central nervous system.

Major features of development during the second month: -upper arms -forearms -palms -webbed fingers

-legs -feet -webbed toes -head gets more rounded -fully formed facial features

Eight weeks after conception, the embryo weighs about 1/30 ounce (1 gram) and is about 1 inch (2.5 cm) in length. The organism now becomes known as the fetus.

The genital organs are fully formed by week 12 The genital organs are fully formed by week 12. If the fetus has a Y chromosome, the SRY gene on this chromosome sends a signal that triggers development of the male sex organs. Without that gene, no signal is sent and the fetus begins to develop female sex organs.

By the end of the 3rd month, the fetus is fully formed, weighs approximately 3 ounces (87 g) and is about 3 inches (7.5 cm) long. These figures vary from fetus to fetus.

During the 4th, 5th, and 6th months, the brain increases in size by a factor of six. This neurological maturation is essential to the regulation of such basic body functions as breathing and sucking. The brain develops new neurons in a process called neurogenesis and new connections between them in a process called synaptogenesis. (middle trimester)

By full term, brain growth is so extensive that the brain’s advanced outer areas, called the cortex, must fold into layers in order to fit into the skull.

The age at which a fetus has a least some chance of surviving outside the uterus is called the age of viability, which occurs 22 weeks after conception.

At about 28 weeks after conception, brain- wave patterns begin to resemble the sleep- wake cycles of a newborn. A 28-week-old fetus typically weighs about 3 pounds (1300 g) and has more than a 95% chance of survival.

Two crucial aspects of development in the last months of prenatal life are maturation of the respiratory and cardiovascular systems.

Beginning at 28 weeks, the fetus hears many sounds, as evidenced by increased fetal heartbeat and body movements in response to loud noises.

The average newborn weighs 7.5 pounds (3400 g)