Читать книгу Child Development From Infancy to Adolescence - Laura E. Levine - Страница 217

The first stage of prenatal development, called the germinal stage, begins when the sperm penetrates the egg. Once fertilization occurs and there is a zygote or fertilized egg, the outside thickens so no other sperm will be able to enter. As the zygote continues the journey through the fallopian tube, the process of cell division begins (see Figure 4.1). It takes about 15 hours for that single cell to become 2 cells, and then the process continues with 2 cells becoming 4, then 8, and so on until there is a ball of 32 cells at 4 to 5 days following conception, which is then ready to implant in the lining of the uterus. During the woman’s menstrual cycle, her hormones have prepared the lining for just this purpose. If the ball of cells fails to implant for any reason, it passes out of the woman’s body without her even realizing there had been a conception. This is not at all uncommon. In fact, it is estimated that close to 50% of conceptions fail to implant and do not survive (Brosens et al., 2014).

Germinal stage: The prenatal stage that lasts from conception to 2 weeks postconception.

Description

Figure 4.1 The germinal stage.

Source: BFIP/Photo Researchers, Inc.

As the cells continue to proliferate, the solid ball becomes a hollow ball called a blastocyst, which has a group of cells at one end called the inner cell mass and an outer ring of cells called the trophoblast (see Figure 4.2). The inner cell mass will go on to become the embryo and part of the amnion that surrounds the embryo, and the outer ring will become the support system for the pregnancy, which includes the placenta and the chorion. We describe these structures and their functions in the next stage of prenatal development.

Blastocyst: A hollow ball of cells that consists of the inner cell mass, which becomes the embryo, and an outer ring of cells, which becomes the placenta and chorion.

Inner cell mass: A solid clump of cells in the blastocyst, which later develops into the embryo.

Trophoblast: The outer ring of cells in the blastocyst that later develops into the support system for the pregnancy.

Embryo: The multicellular organism that develops as the zygote divides, lasting until the end of the second month of a pregnancy when it is called a fetus.

Figure 4.2 Development of the blastocyst.

Source: Jim Dowdalls/Photo Researchers, Inc.

Cells in the trophoblast secrete an enzyme that digests some of the lining in the uterus so that the blastocyst can securely embed itself there. After implantation, fingerlike extensions from the trophoblast grow into the uterus, and a connection between the embryo and the woman is established (Butkus, 2015). Once an outside source of nourishment is available, the blastocyst can really begin to increase in size.

Couples who engage in frequent, unprotected sex can expect to conceive a child within 1 year, so when this does not happen it may mean that the couple is dealing with infertility. About 12% of married women in the United States have difficulty either getting pregnant or carrying a pregnancy to term (RESOLVE: The National Fertility Association, 2017). One third of the cases are attributable to female factors and one third to male factors; in the remaining cases the cause is mutual or cannot be determined (RESOLVE, 2017). Although infertility can be the result of physical problems in either partner, one of the most significant factors is maternal age. Women in the United States are waiting longer to have children, and today 20% don’t have their first child until they are 35 or older (Centers for Disease Control and Prevention [CDC], 2017l). About one third of these women will have a problem with fertility, but medical science today can offer infertile couples a wide range of interventions. Several are described in Table 4.1.

Infertility: The inability to conceive within 1 year of frequent unprotected sex.

Table 4.1

Sources: American Pregnancy Association (2017); CDC (2017l).

T/F #2

Using techniques that are available today, it is possible for some parents to choose the sex of their baby with 100% accuracy. True

The term assisted reproductive technology (or ART) describes all procedures in which both the egg and the sperm are handled outside of the body (CDC, 2017l). It does not include procedures that involve only the sperm, such as artificial insemination, or in which the woman receives medication intended to stimulate egg production. By this definition, 1.6% of infants born in the United States in 2014 were conceived using ART (Sunderam et al., 2017).

Despite the emotional toll of infertility, less than one third of infertile couples seek counseling, but when they do, it is important that they receive the specific type of support that they need (Read et al., 2014). For one couple, that might be emotional support to help them deal with what they see as an unforeseen and uncontrollable event in their lives, for another it might be information about the options they have, and another couple may need both types of help.