Pregnancy: The First Trimester

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MEDICAL ANIMATION TRANSCRIPT: Pregnancy is the time period during which a baby develops inside your uterus. It usually lasts about 40 weeks, starting from the first day of your last menstrual period. These 40 weeks are grouped into three segments called trimesters. The first trimester lasts about 13 weeks. During the first two weeks, you weren't actually pregnant because of the way your due date is calculated. But, your body is preparing for pregnancy. During the second week, an egg is released from your ovary, in a process called ovulation. Around the end of the second week, the baby is conceived in a process called conception or fertilization. This happens when a sperm from the father merges with the egg from the mother, usually in the fallopian tubes. During conception, genetic material from the mother and father combine to form a unique genetic code, instantly determining sex, hair color, eye color, and hundreds of other characteristics. This new single cell, called a zygote or embryo, is the beginning of a new human being. During the third week, the zygote divides to form a hollow ball of cells called a blastocyst. The cells of the blastocyst begin to arrange themselves into two groups. The inner group will develop into your baby, and the outer group will form tissues to nourish and protect it. During week four, the blastocyst moves into the uterus, where it hatches out of its outer layer. Once freed from this outer layer, the blastocyst can embed itself in the thickened lining of the wall of the uterus, in a process called implantation. Weeks 5 to 10 are referred to as the embryonic period. All of the major organs start to grow such as the brain, spinal cord, and heart. The heart begins to beat during this period. Structures called the placenta and umbilical cord begin providing life support for the embryo. They bring nutrients and oxygen from the mother to the embryo. Bones and muscles start to grow beneath thin, transparent skin. Toward the end of the embryonic period, the embryo starts to look more human. At the end of week 10, the embryo is now called a fetus and is about 1.25-inches long from head to rump. During the next few weeks, the arms and legs grow longer and start to move. Fingers and toes finish developing, and the baby's face becomes well-formed. By the end of the first trimester, your baby is almost three inches long. Your body experiences many changes during the first trimester. This is due to changing levels of chemicals called hormones circulating through your body. The most common first sign of pregnancy is that your menstrual periods have stopped. You may have other symptoms as well, such as nausea, often called morning sickness, that can occur at any time of day, tender, swollen breasts, mood swings, constipation, weight loss or weight gain, craving or disliking certain foods, and feeling more tired than usual. You may have only a few of these symptoms or none at all. If you have any questions about how your baby is developing, or concerns about how you're feeling, talk to your healthcare provider.

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