Module 4 Lesson 3 - 2
Lesson 3 — Fetal Development
Fetal Development
Read pages 520 - 521
Although prenatal development is divided into the embryonic and fetal development periods, the full pregnancy is divided into three trimesters; each trimester is three-months in duration. Trimester 1 is the embryonic development period and the beginning of fetal development. The first three months are the most critical time of development, the embryonic development. All organ systems begin. For example, the heart begins beating, blood cells flow through blood vessels, limbs are formed, genitals are present, and the brain, sense organs, and nervous tissue are formed.
Trimesters 2 and 3 have significant developmental events that consist of further system development and maturation. Following are some interesting developments that occur by the end of the next 6 months.
Second Trimester
End of month 4
- Baby is developing reflexes, such as sucking and swallowing, and may begin sucking thumb in utero.
- Tooth buds are developing.
- Sweat glands are forming on palms and soles.
- Hair begins to grow on baby's head.
- Eyebrows, eyelids, and eyelashes appear.
- Skin is covered with a protective coating called vernix.
- Baby is able to hiccup.
Third Trimester
End of month 7
- Taste buds have developed.
- Fat layers are forming.
- Tremendous brain growth occurs at this time.
- Fingernails can now extend beyond fingertips.
- Most body organs are now developed, except for the lungs.
- The lungs are mature.
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Baby now is fully developed and can survive outside the mother's body.
Teratogens
Read pages 521 - 523
Substances and circumstances can affect prenatal development at any time. Although maternal blood does not pass through the placenta to the fetus, many of these substances can diffuse through the placenta to enter the fetal circulatory system. During the first trimester, the developing offspring is most vulnerable to teratogens, substances that can alter normal development. Teratogens are agents that cause structural abnormalities due to exposure during pregnancy.
Examples of some teratogens are cigarette smoke, alcohol, drugs, and x-rays, but there are many others. Although some harmful agents may be transferred to the embryo and fetus, other substances are essential for proper growth and development.
Cigarette smoke prevents the fetus from receiving enough oxygen in its lungs; this can result in underweight or premature babies. Alcohol is one of the most damaging teratogens, and it is also the easiest to avoid. Alcohol consumption has significant effect on the developing central nervous system of the fetus and can result in fetal alcohol spectrum disorder (FASD). Alcohol increases the likelihood of decreased head size, weight, and height. As well, facial malformation, learning difficulties, and personality disorders are evident in babies affected by alcohol consumption during pregnancy.
The picture below shows the MRI images three individuals. Panel A is the side view of a 14-year-old with normal corpus callosum (indicated by the white arrow). Corpus callosum connects the two hemispheres of the brain and transfers information between
them. Panel B shows a thin corpus callosum of a 12-year-old with fetal alcohol syndrome. Panel C shows absence of corpus callosum in a 14-year-old subject with fetal alcohol syndrome.
Examples of dangers that teratogens pose include the following.
| Teratogens | Effects |
| prescription medication (such as thalidomide) | structural abnormalities, such as deformed limbs |
| cigarette smoke | constriction of blood vessels, restricting oxygen supply and increasing risk of underweight babies, premature birth, stillbirth, and miscarriage |
| alcohol | damage to the brain, central nervous system, and physical development, leading to decreased weight, height, head size, capacity to learn, memory problems, and personality disorders |
| excess amounts of vitamin C | baby being prone to scurvy, easy bruising, and infections |
| exposure to environmental contaminants | increased danger of miscarriage, low birth-weight babies, difficulty fighting infection and disease, developmental delay |
Proper nutrition, which includes the right amounts of vitamins and minerals, is fundamental to the healthy development of the embryo and fetus. One of the most important vitamins is vitamin B9, also known as folate and commonly called folic acid. Folic
acid helps to prevent neural tube defects. Having an adequate amount of folate is very important for a woman's diet before conception. One of the activities in this lesson will help you understand why this is so important.