American Medical Association Family Medical Guide - American Medical Association [273]
A full-term pregnancy lasts about 40 weeks from the time of conception to delivery. Although conception usually occurs halfway through a woman’s menstrual cycle, the delivery date is calculated from the first day of her last period because the exact day the egg is fertilized is difficult to know.
Confirming a Pregnancy
If you have missed a menstrual period and want to know if you are pregnant, you can use a home pregnancy test. Home pregnancy tests work by detecting the hormone human chorionic gonadotropin (HCG), which is present in the urine of a pregnant woman about 2 weeks after her first missed period. The results are most accurate when you follow the manufacturer’s instructions exactly and test your urine first thing in the morning before you drink anything. The results of a home pregnancy test may not be reliable if you are taking ntidepressants (see page 712), are approaching menopause, or have irregular or infrequent periods. Also, a negative result (indicating you are not pregnant) is less reliable than a positive result (indicating you are pregnant). You should see your doctor if you have a negative result on a home test but still suspect that you are pregnant. Your doctor, a family planning clinic, or your local health department can perform a more accurate pregnancy test.
Eggs, sperm, and fertilization
A woman′s ovaries, one on each side of her uterus, contain thousands of immature eggs, with which she was born. After puberty, one of the eggs matures each month in one of the ovaries and, about halfway through the menstrual cycle, it is released into the adjacent fallopian tube. In males, sperm are produced continuously inside the testicles and are stored in the seminal vesicles until ejaculation.
Fertilization of an egg by a sperm takes place in one of the fallopian tubes shortly after the egg is released into the fallopian tube. The sperm’s nucleus joins with the egg’s nucleus, combining their genes, and the cell divides into two cells. Each of these cells then divides into two, which divide again, and so on. The group of dividing cells travels along the fallopian tube toward the uterus and, about 2 to 7 days after fertilization, is implanted in the lining of the uterus. In a few weeks, the fertilized egg develops into an embryo and placenta.
Stages of embryo development
The fertilized egg is called an embryo until about the 12th week of pregnancy, when it is called a fetus. At 6 weeks, the embryo is about the size of a grain of rice and the most developed organ is the heart. At 7 weeks, the arms and legs can be seen as limb buds. The eyes and ears are more obvious by 9 weeks. At 10 weeks, the embryo is about 1¼ inches long and growing rapidly. The detailed features of an embryo and its actual size are shown here at different stages of development.
Placenta: The fetal lifeline
Inside the uterus, the fetus is attached to the placenta by the umbilical cord. Fetal blood flows to and from the placenta, absorbing nutrients from and expelling waste into the pregnant woman′s blood. Tiny fingerlike projections called villi provide a large surface area in the placenta to help maximize the exchange of these substances.
If you have missed two menstrual periods and your periods are usually regular, see your doctor right away. Doctors can usually confirm the diagnosis of a pregnancy after two missed menstrual periods by performing a pelvic examination. An earlier diagnosis at the time of the first missed period can be made with a blood test that detects HCG.
Terminating Pregnancy
Learning that you are pregnant may require you to make some serious decisions. You may want to consider having a medical or surgical abortion if the fetus has a severe or fatal genetic disorder or birth defect or if your life or the life of the fetus