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Week 3 If you've just found out you're pregnant, you might want to start by reading the previous sections. How Big Is Your Baby? The embryo growing inside you is very small. At this point, it is only a group of cells, but one that is multiplying and growing very rapidly. The embryo is the size of the head of a pin and would be visible to the naked eye if you could see it. The group of cells doesn't look like a fetus or baby; it looks like the illustration on page 39. During this first week, the embryo is about 0.006 inch (0.150mm) long. How Big Are You? In this third week of pregnancy, you won't notice any changes. It's too soon! Few women know they have conceived. Remember, you haven't even missed a period yet. How Your Baby Is Growing and Developing A great deal is happening, even though your pregnancy is in its very earliest stage. Ovaries lie free in your pelvis or peritoneal cavity. They are close to the uterus and Fallopian tube. At the time of ovulation, the end of the tube (called the fimbria) lies close to the ovary. Some researchers believe this tube opening covers the area on the ovary where the egg (ovum) is released at the time of ovulation. The release site on the ovary is called theástigma. During intercourse, an average of 0.06 to 0.15 ounce (2 to 5ml) of semen is deposited in the vagina. Each milliliter contains an average of 70-million sperm; each ejaculation contains 140- to 350-million sperm. Only about 200 sperm actually reach the egg in the tube. Fertilization is the joining together of one sperm and an egg. Fertilization of the Egg Fertilization is believed to occur in the middle part of the tube called the ampulla, not inside the uterus. Sperm travel through the uterine cavity and out into the tube to meet the egg. When the sperm and egg join, the sperm must pass through the outer layer of the ovum, the corona radiata. The sperm must then digest its way through another layer of the ovum, the zona pellucida. Although several sperm may penetrate the outer layers of the ovum, usually only one sperm enters the ovum and fertilizes it. After the sperm penetrates the ovum, the sperm head attaches to its surface. The membranes of the sperm and ovum unite, enclosing them in the same membrane or sac. The ovum reacts to this contact with the sperm by making changes in the outer layers so no other sperm can enter. Once the sperm gets inside the ovum, it loses its tail. The head of the sperm enlarges and is called the male pronucleus; the ovum is called the female pronucleus. The chromosomes of the male and female pronuclei intermingle. When this happens, extremely small bits of information and characteristics from each partner unite. This chromosomal information gives each of us our particular characteristics. The usual number of chromosomes in each human is 46. Each parent supplies 23 chromosomes. Your baby is a combination of chromosomal information from you and your partner.
Embryonic Development Begins The developing ball of cells is called a zygote. The zygote passes through the uterine tube on its way to the uterus as the division of cells continues. These cells are called a blastomere. As the blastomere continues to divide, a solid ball of cells is formed, called a morula. The gradual accumulation of fluid within the morula results in the formation of a blastocyst, which is very small. During the next week, the blastocyst travels through the uterine tube to the cavity of the uterus (3 to 7 days after fertilization in the tube). The blastocyst lies free in the uterine cavity as it continues to grow and to develop. About a week after fertilization, it attaches to the uterine cavity (implantation). The cells burrow into the lining of the uterus.
Changes in You Some women can tell when they ovulate. They may feel mild cramping or pain, or they may have an increased vaginal discharge. Occasionally at the time of implantation of the fertilized egg into the uterine cavity, a woman may notice a small amount of bleeding. It's too early for you to notice many changes. Your breasts haven't started to enlarge and you won't be starting to ôshow.ö That lies ahead! How Your Actions Affect Your Baby's Development Exercise is an important part of life for many women. The more we learn about health, the more the advantages of regular exercise become evident. Regular exercise may decrease your risk of developing several medical problems, including cardiovascular disease, osteoporosis (softening of bones), depression, premenstrual syndrome (PMS) and obesity. There are many types of exercise or physical activity to choose from. Each offers its own advantages. Aerobic exercise is very popular with women today who want to keep in shape. Aerobic Exercise For cardiovascular fitness, aerobic exercise is the best. Aerobic exercise must be performed at least 3 times a week at a sustained heart rate of 110 to 120 beats a minute and maintained for at least 15 continuous minutes. The rate of 110 to 120 beats a minute is an approximate target for people of different ages. Many tables show target heart rates; I often use the one above with my patients.
Nancy, pregnant for the first time, was concerned about exercising during pregnancy. She was healthy and her pregnancy was normal, but she hadn't exercised much in the past. I told her good choices for aerobic exercise during pregnancy include brisk walking, stationary bicycling, swimming and aerobic exercise designed especially for pregnant women. I helped Nancy design an exercise program that would work for her. Muscle Strength Some women exercise for muscle strength. To strengthen a muscle, there has to be resistance against it. There are three different kinds of muscle contractionsùisotonic, isometric and isokinetic. Isotonic exercise involves shortening the muscle as tension is developed, like lifting a barbell. Isometric exercise occurs when the muscle develops tension but doesn't change its length, such as pushing against a stationary wall. Isokinetic exercise occurs when the muscle moves at a constant speed, as when you swim. Cardiac and skeletal muscles cannot usually be strengthened at the same time. Strengthening skeletal muscles requires lifting heavy weights, but you can't lift these heavy weights long enough to strengthen the cardiac muscle. Weight-bearing exercise is the most effective at promoting increased bone density to help avoid osteoporosis. Other advantages of exercise include flexibility, coordination, improvement in mood and alertness. Stretching and warming up muscles before and after exercise help you improve flexibility and avoid injury. Exercise during Pregnancy? As a pregnant woman, you are probably concerned about the risks of exercise. Can you or should you exercise when you're pregnant? It's desirable for a pregnant woman to have cardiovascular fitness. Women who are physically fit are better able to perform the work of labor and delivery. Exercise during pregnancy is not without some risk, however. Risks to the developing baby can include increased body temperature, decreased blood flow to the uterus and possible injury to the mother's abdominal area. You can exercise during pregnancy if you do it wisely. Avoid raising your body temperature above 102F (38.9C). Aerobic exercise can raise your body temperature higher than this, so be careful. A rise in body temperature can be increased by dehydration. Avoid prolonged aerobic exercise, particularly during hot weather. While exercising aerobically, blood can be diverted to the exercising muscle or skin and away from other organs, such as the uterus, liver or kidneys. A lower workload during pregnancy is advised to avoid potential problems. Now is not the time to try to set new records or train for an upcoming marathon! During pregnancy, keep your pulse below 140 beats a minute.
If you exercised aerobically before pregnancy, you can probably continue exercising at a somewhat lower rate. If you have any problems, such as bleeding or premature labor, you and your doctor will have to choose another program. It is unwise to start an aerobic exercise program during pregnancy or to increase training. If you haven't been involved in regular, strenuous exercise before pregnancy, walking and swimming are probably about as involved as you should get with exercise. Before you begin, discuss exercising with your doctor. Together you can develop a program consistent with your current level of conditioning and your exercise habits. You Should also Know Bleeding during Pregnancy Bleeding during pregnancy causes concern. In the first trimester, bleeding can make you worry about the well-being of your baby and the possibility of a miscarriage. (Miscarriage is discussed in Week 8.) Bleeding during pregnancy is not unusual. Some researchers estimate that 1 in 5 pregnant women bleeds during the first trimester. Although it makes you worry about possible problems, not all women who bleed have a miscarriage. Bleeding at the time of implantation is mentioned on page 40. This can occur as the blastocyst ôburrowsö into the uterine lining. At this point, you won't know you are pregnant because you haven't missed a period. If this happens, you may just think your period is starting early. As your pregnancy grows and the uterus grows, the placenta forms and vascular connections are made. Bleeding may occur at this time. Strenuous exercise or intercourse may also cause some bleeding. If this occurs, stop your activities and check with your doctor, who will advise you what to do. If bleeding causes your doctor concern, he or she may order an ultrasound exam. Sometimes ultrasound can show a reason for bleeding, but during this early part of pregnancy, there may be no discernible reason for it. Most doctors suggest rest, decreasing activity and avoiding intercourse when bleeding occurs. Surgery or medication are not helpful and are unlikely to make a difference. Call your doctor if you experience any bleeding. He or she will advise you what to do. Aspirin Use Almost any medication taken during pregnancy can have some effect on your baby. This includes aspirin, a drug taken frequently for many reasons, either alone or in combination with other medications. Aspirin use can increase bleeding. Aspirin causes changes in the platelet function; platelets are important in blood clotting. This is particularly important to know if you are bleeding during pregnancy or if you are close to delivery at the end of your pregnancy. Small doses of aspirin may be acceptable during pregnancy; check with your physician. Read labels on any medication you take to see if they contain aspirin. Avoid using aspirin or any products that contain aspirin during pregnancy unless you have discussed it with your doctor first. If you need a pain reliever or a medication to reduce fever, and you cannot reach your physician for advice, acetaminophen (Tylenol«) is one over-the-counter medication you can use for a short while with little fear of complications or problems for you or your baby. For further information about over-the-counter medication use during pregnancy (see Week 17.) Folic Acid Use and Neural-Tube Defects Neural-tube defects are defective closures of the neural tube during early pregnancy. These birth defects occur in about 1 in 1000 births in the United States and Canada. In general, the cause of these defects is unknown. Defects include spina bifida, when the base of the spine remains open, exposing the spinal cord and nerves; anencephaly, congenital (present at birth) absence of the brain and spinal cord; and encephalocele, a protrusion of the brain through an opening in the skull. Recent studies indicate taking folic acid during pregnancy may help prevent or decrease the incidence of neural-tube defects. However, these studies are continuing. Recommended dietary allowance of folic acid during pregnancy is 0.4mg a day. The American College of Obstetricians and Gynecologists recommends any woman who has had a child born with a neural-tube defect take 4mg of folic acid a day, preferably beginning 1 month before she plans to become pregnant. Continue taking this amount through the first trimester of pregnancy. It is unnecessary to supplement beyond the first trimester because the neural tube closes within 4 weeks after conception. Previous Week > Contents > Next
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