The Germinal Stage
The germinal stage begins with conception, when the sperm and egg cell unite in one of the two fallopian tubes. The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete. Cell division begins approximately 24 to 36 hours after conception.Cell division continues at a rapid rate and the cells then develop into what is known as a blastocyst. The blastocyst is made up of three laters: the ectoderm (which will become the skin and nervous system), the endoderm (which will become the digestive and respiratory systems), and the mesoderm (which will become the muscle and skeletal systems).
Finally, the blastocyst arrives at the uterus and attached to the uterine wall, a process known as implantation.
The Embryonic Stage
The mass of cells is now know as and embryo. The embryonic stage begins after implantation and continues until cell differentiation has been mostly completed. Structures important to the support of the embryo develop, including the placenta and umbilical cord.During this time, cells begin to differentiate into the various body systems. The basic outlines of the organ, body, and nervous systems are established. By the end of the embryonic stage, the beginnings of features such as fingers, eyes, mouth, and ears become visible.
The Fetal Stage
Once cell differentiation is mostly complete, the embryo enters the next stage and becomes known as a fetus. The early body systems and structures established in the embryonic stage continue to develop. The neural tube develops into the brain and spinal cord and neurons form. Sex organs begin to appear during the third month of gestation. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the later stages of pregnancy.Problems During Prenatal Development
Most prenatal develop occurs normally, following the established patterns with little variation. However, there are a number of things that can go wrong during this time, which are usually caused by genetics or environmental problems.Genetic Problems
- Down Syndrome – Also known as trisomy 21, Down syndrome is the most common genetic anomaly during prenatal development. Down syndrome is caused by and extra copy of the 21 chromosome (meaning there are three chromosomes instead of the usual two) and impacts approximately 1 out of every 1,000 infants. Typical features of Down syndrome include flattened facial features, heart defects, and mental retardation. The risk of having a child with Down syndrome increases with maternal age.
- Inherited diseases – A number of illnesses can be inherited if one or both parents carries a gene for the disease. Examples of inherited diseases include Sickle-cell anemia, Cystic fibrosis, and Tay-Sachs disease. Genetic tests can often determine if a parent is a carrier of genes for a specific disease.
- Sex-Chromosome Problems – A third type of genetic problems involves sex-chromosomes. These includes conditions such as Klinefelter’s syndrome (an extra X-chromsome) and Turner syndrome (a single X-chromosome).
Environmental Problems
Harmful environmental elements that can effects the fetus are known as teratogens. There a number of teratogens that can harm the fetus, including:
- Maternal Drug Use – The use of substances by the mother can have devastating consequences to the fetus. Smoking is linked to low birth weight, which can result in a weakened immune system, poor respiration, and neurological impairment. Alcohol use can lead to fetal alcohol syndrome, which is linked to heart defects, body malformations, and mental retardation. The use of illicit drugs such as cocaine and methamphetamine is also linked to low birth weight and neurological impairment.
- Maternal Disease – There are a number of maternal diseases that can negatively impact the fetus, including herpes, rubella, and AIDS. Herpes virus is one of the most common maternal diseases and can be transmitted in the fetus, leading to deafness, brain swelling, or mental retardation. Women with herpes virus are often encouraged to deliver via cesarean to avoid transmission of the virus.
