Under normal circumstances, pregnancy lasts approximately 40 weeks, although a pregnancy of only 38 weeks is thought of as a full term. The start of pregnancy is considered the first day of a woman’s last period. Pregnancy is divided into three periods called trimesters. All of them are quite different for the developing baby and the mother, and the environmental risks differ greatly.
First Trimester
The time from the start of pregnancy to the end of the 12th week of pregnancy is called the first trimester. During fertilization, the egg and the sperm join to form a zygote. It travels down the Fallopian tubes and arrives in the uterus after about seven to nine days. There, the zygote implants itself to the uterine lining and is then called an embryo. The placenta, yolk sac and umbilical cord are then formed to support, nourish and protect the developing embryo. Among the first organs are the brain, the spinal cord and the heart. The brain, for example, begins to form at approximately the third week of pregnancy. The embryo develops a definitive sex during the 11th week and by the end of the first trimester, the embryo is able to move and has formed legs and arms.
Second Trimester
At the beginning of the second trimester, the embryo has become a fetus. One major event during the second trimester is that the mother is finally able to feel the movement of the fetus. Now that all major organs have developed, the fetus undergoes a period of intense growth. During the second trimester, the skin of the fetus is red, wrinkled and covered with soft hair called lanugo. The skin of the fetus eventually smooths out once he develops more fat. A creamy substance called vernix appears on top of the skin to protect it. New reflexes, such as swallowing and sucking, appear during this period. The new life now has fully separated fingers and toes, and even has nails.
Third Trimester
The 24th week of pregnancy usually marks the start of the age of vitality; the first time the fetus could potentially survive outside the uterus. Yet the fetus still needs more time to develop because her lungs, for example, are still immature. If she were to be born, she would most likely need a machine to help her breathe. Until now, the fetus has mainly slept, but now, she has becomes more active. Toward the end of pregnancy, she spends roughly 16 percent of her time awake. The third trimester is also a period of development for sight and hearing. A study by doctors J.P. Lecanuet and B. Schaal that appeared in he European Journal of Obstetrics, Gynecology and Reproductive Biology reported that even though the visual system develops rapidly during this time, it is still very frail and only able to respond to light after about the 26th week of pregnancy. On the other hand, the auditory system matures much more rapidly and can respond to sounds from the 28th week on.
Pregnancy and the Mother
The three trimesters of pregnancy are quite different for the mother. The first is characterized by a general feeling of nausea called morning sickness. Despite its name, it can happen at any time of day. Another typical change during early pregnancy is that a new mother’s breasts tend to grow in size and become tender. These changes are likely caused by rapid increases in the levels of progesterone and estrogen. Toward the end of the first trimester, most women stop vomiting and no longer have nausea. The second trimester is typically the easiest time of the pregnancy. Early pregnancy symptoms are over and the baby is still quite small and doesn’t cause too much discomfort for the mother. The last of the trimesters tends to be the hardest one. The growing fetus may make it difficult for the mother to sleep or even breathe. Many women also develop varicose veins and leg cramps. There's often an increase in the amount of practice contractions.
Environmental Risks
From the moment the embryo implants and connects itself with the mother’s bloodstream, anything the mother eats or drinks can now either nourish or hurt the very delicate embryo. The developing life is at the biggest risk during the first two trimesters from exposure to drugs, alcohol, radiation and environmental toxics. Such chemicals as lead, mercury and pesticides can harm an unborn baby, according to the March for Dimes. The time during which the fetus was exposed to these chemicals and the duration of exposure greatly affect the outcome, because different organs develop during different times of pregnancy.
References
- "Child Development": Fetal Neurobehavioral Development; DiPietro, Hodgson, Costigan; vo. 67, 2553-2567, 1996.
- "European Journal of Obstetrics and Gynecology and Reproductive Biology": Fetal Sensory Competencies.; Lecanuet, Schaal; vol. 68, 1-23, 1996.
- National Library of Medicine: Fetal development
- "Langman's Medical Embryology"; T.W. Saddler, M.D.; 2006.
- Children's Hospital of Pittsburg: Pregnancy
