5.1 Periods of Prenatal Development
The Human Development Teaching & Learning Group
Learning Objectives
- Differentiate between development during the germinal, embryonic, and fetal periods
- Explain the process of brain development during the prenatal period
You may be familiar with the three trimesters of pregnancy, three trimesters consisting of three months each to equal the entire pregnancy of nine months. People who have recently given birth frequently refer to how they felt during their first, second, or third trimester, and this is easy to do, as the trimesters generally describe the experience of the gestational parent during these three temporally equal intervals of pregnancy. When we look at the development of the organism from zygote to fetus to newly born infant, we also examine three periods of development. However, these periods are not equal in length and they do not align with the gestational parent’s experience of the three trimesters. Actually, a full term pregnancy is considered to be 40 weeks long (which would actually be equivalent to 10, not nine, months!), and two of the three periods of development occur within the first trimester. Let’s take a look at some of the changes that take place during each of the three periods of prenatal development: the germinal period, the embryonic period, and the fetal period.
The Germinal Period (Weeks 1-2)
The germinal period (about 14 days in length) lasts from conception to implantation of the fertilized egg in the lining of the uterus (see Figure 5.3). At ejaculation millions of sperm are released into the vagina, but only a few reach the egg and typically only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. The egg is typically fertilized in the top section of the fallopian tube and continues its journey to the uterus. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a zygote.
During this time, the organism begins cell division through mitosis. After five days of mitosis there is a group of 100 cells, which is now called a blastocyst. The blastocyst consists of both an inner and outer group of cells. The inner group of cells, or embryonic disk will become the embryo, while the outer group of cells, or trophoblast, becomes the support system which nourishes the developing organism. This stage ends when the blastocyst fully implants into the uterine wall (U.S. National Library of Medicine, 2015). Approximately 50-75% of blastocysts do not implant in the uterine wall (Betts et al., 2019).
Mitosis is a fragile process and fewer than half of all zygotes survive beyond the first two weeks (Hall, 2004). Some of the reasons for this include the