The first trimester of pregnancy is characterized by hormonal surges that cause changes in the woman’s body, such as growth of the uterus and breasts, and in the developing baby, such as the formation of organ systems. The first 12 weeks are usually counted from the last menstrual cycle, and discomforts such as morning sickness and constipation may occur.
During the first trimester of pregnancy, many changes take place in a pregnant woman’s body, often caused by surges or pregnancy hormones such as estrogen and progesterone. These hormonal surges are generally thought to be responsible for a variety of pregnancy symptoms and early discomforts a woman may experience. This period also brings many changes and rapid growth in the developing child.
Many pregnant women don’t know the exact day they ovulated and conceived. For this reason, the first trimester of pregnancy, i.e. the first 12 weeks, is usually counted from the day of a woman’s last menstrual cycle. This could actually be two weeks or more before the egg has been fertilized. In general, there are no specific pregnancy-related changes that occur in the body during that pre-fertilization period.
Once the egg is fertilized, it prompts a woman’s body to start producing higher-than-normal amounts of hormones, including estrogen and progesterone. These hormones help prepare the body to support a child’s growth. For example, they play a role in helping maintain the thick uterine lining to protect the developing embryo and in the developing placenta, which is the embryo’s main source of oxygen and nutrients.
Other changes in a woman’s body caused by pregnancy hormones during the first trimester often include growth of the uterus and breasts, increased blood flow, and slowing down of the digestive system. These changes can cause many of the common discomforts often felt in the first trimester of pregnancy, such as tender breasts as the milk ducts begin to grow, frequent urination as the growing uterus begins to put pressure on the bladder, and constipation as the digestive system slows down. to allow more nutrients to be absorbed. One hormone in particular, called human chorionic gonadotropin (hCG), is generally held responsible for perhaps the best-known discomfort: morning sickness. These nauseous feelings caused by hCG can actually occur at any time of day.
The first trimester of pregnancy also brings many changes to the developing baby. During the first few weeks of pregnancy, the baby develops into a tiny ball of stem cells and implants itself in the lining of the uterus. Stem cells are special cells that can develop into any other type of human cell. Around the fifth week of pregnancy, these special cells begin to differentiate and form familiar human organ systems. At this point, the fertilized egg is now called an embryo.
The first organ systems that develop normally during the embryonic stage are the nervous system, including the brain and spinal cord, and the circulatory system, including the heart and blood vessels. These generally begin to develop during the fifth week of the first trimester of pregnancy. In weeks six to eight, the embryo begins to develop a head, face, eyes, arms, and legs. Around the ninth week, fingers, toes, muscles, and bones usually begin to develop. At about 10 weeks, the genitals usually begin to develop.
In general, the last two weeks of the first trimester of pregnancy mark the beginning of the fetal stage. During this period, the fetus begins to grow rapidly. By the end of week 10, most fetuses are only about 1 inch (2.54 cm) long, but by the end of the first trimester they often have already tripled in size to about 3 inches (7.62 cm). This rapid growth generally continues in the second and third trimesters of pregnancy until the baby is born.
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