What are pregnancy and gestation?
Pregnancy begins conception or fertilization with the fusion of an egg and sperm within a woman’s body and continues until childbirth, typically thirty-eight weeks later. This gestational time is divided into three approximately equal periods called trimesters, each associated with specific physical and biochemical hallmarks.
Prior to conception, a mature egg, or ovum, ruptures from a fluid-filled follicle within the ovary and is swept into the Fallopian tube by large fringes on the tube that caress the ovary’s surface. The empty follicle is transformed into a structure called the corpus luteum which secretes hormones that help to prepare the woman’s body for pregnancy. If the ovum is fertilized within twenty-four hours, pregnancy will occur. If it is not fertilized, the uterine lining will be shed during menstruation.
Sperm ejaculated into a woman’s vagina travel through the cervix and uterus and into the Fallopian tubes. Only about 200 of the original 500 million sperm delivered to the woman may reach the vicinity of the ovum. Enzymes in the sperm heads dissolve protective outer layers of the egg. When one sperm finally breaks through the plasma membrane, the innermost covering of the ovum, chemical changes on the surface of the ovum prevent additional sperm from entering. The genetic material contained in the sperm and ovum fuse. If the fertilizing sperm is a gynosperm, which carries an X chromosome, then the baby will be female; if it is an androsperm, which carries a Y chromosome, then the baby will be male.
About twelve hours after fusion of the genetic material, the first cell division occurs. Divisions continue at intervals of twelve to fifteen hours, doubling the number of cells each time, and the fertilized ovum is now called a blastocyst. The blastocyst is gently guided through the Fallopian tube to the uterus by the beating of the millions of tiny hairs, called cilia, that line the inner surfaces of the Fallopian tubes. This journey to the uterus takes about three days.
Upon arriving in the uterus, the blastocyst “explores” the endometrium, the uterine lining, for an appropriate site to settle. Prior to this implantation in the endometrium, the blastocyst ruptures from the clear protective sheath that helps to prevent it from settling in the Fallopian tube. By ten to fourteen days after fertilization, the blastocyst implants securely within the endometrium and the embryonic stage begins. At this point, it consists of several hundred cells and is about the size of the head of a pin.
After implantation, chemical signals produced by the blastocyst prevent the mother’s immune system from recognizing the blastocyst as a foreign invader and destroying it. Other chemical signals cause the endometrium to thicken and extend blood vessels to the blastocyst for nourishment from the mother. The uterine wall softens and thickens, and the cervical opening is sealed with a mucus plug.
At this point, the cells of the blastocyst divide into two distinct clusters: One part will form the embryo itself, and the other part will join with the woman’s tissue to form the placenta, the structure that will provide nourishment from the mother to the growing embryo. These early placental cells produce the hormone human chorionic gonadotropin (hCG). This hormone signals the ovary to cease ovulation and stimulates it to produce the hormone progesterone, which prevents menstruation and causes the endometrium to grow even thicker.
During the second week after fertilization, a cavity surrounding the embryo begins to form. This is destined to become the amniotic sac, which will contain the shock-absorbing amniotic fluid in which the fetus floats during development. At this time, the hCG being produced by the blastocyst and ovaries can be detected by pregnancy tests.
Three weeks after conception, the size of the embryo is about 2 millimeters (0.08 inch). The embryonic cells have divided into germ layers, distinct groupings of cells that are destined to produce specific body parts. The rudimentary brain appears at the end of a long tube. The heart is forming and will be beating within a week. At this point, the mother has missed her menstrual period and may be experiencing symptoms of pregnancy, such as nausea, heartburn, and tender breasts.
During the fifth through seventh weeks of embryonic development, massive physical changes occur. The crown-to-rump length of the embryo increases to around 1.25 centimeters (0.5 inch). The embryo’s face, trunk, and limbs grow, and by the end of this period distinct fingers and toes are formed. The backbone is in place, and ribs begin to develop, as do skin, eyes, and all of the organ systems and the circulatory system. At this point, the placenta is connected to the embryo by the umbilical cord, and placental cells penetrate the blood vessels of the endometrium to provide transit of nutrients from the mother’s bloodstream to the embryo. The placenta also filters out some potentially dangerous substances from the mother’s bloodstream and aids in disposing of embryonic waste products.
By the eighth week of development, the embryo is about 4 centimeters (1.5 inches) long, weighs about 14 grams (0.5 ounce), and is composed of millions of cells. All organs are formed, and the embryo is officially called a fetus. The woman’s uterus has increased in size, and her waistline may begin to enlarge. At this point, hormonal shifts stabilize, which frequently relieves morning sickness and other discomforts of early pregnancy.
Growth and organ system interconnection continue during the third month of fetal life, and the cells of the immune system are formed. During the fourth month, facial features develop, and the fetus may begin to respond to sound. Hair on the head and the eyebrows coarsens and develops pigment. The distinction between male and female fetuses becomes apparent with a visible vagina or penis. Sixteen to eighteen weeks after fertilization, the mother may feel the first fetal movements. She has gained several pounds, and changes in her body shape are readily visible. Frequently, this second trimester is associated with feelings of joy and minimal discomfort.
The third trimester is largely a period of growth for the fetus. Its weight increases rapidly. The fetus becomes very active within the amniotic fluid and responds to sound from both within and outside the mother. The weight gain may put incredible stress on the mother’s body, and pressure on internal organs may cause frequent urination, heartburn, and difficulty breathing and sleeping.
Normal pregnancies vary from thirty-eight to forty-two weeks long. The lungs are the final organs to mature, and by the end of the eighth month all organ systems are established and functional. The fetus continues to gain weight until the end of pregnancy. At this time, the fetus will weigh around 3 kilograms (7 pounds) and have a crown to rump length of approximately 37 centimeters (14.4 inches). The end of pregnancy is heralded by the beginning of uterine contractions, and frequently by the rupture of the amniotic sac and expulsion of its fluid. Labor leads to the birth of a unique individual created from the developmental programs contained in the genetic material inherited from each parent.
Since the developing embryo or fetus is dependent on the placental connection to the mother for nourishment, its health is directly tied to the diet and lifestyle of the mother. Any environmental substance that may cause a developmental defect is known as a teratogen. Women must take care to avoid teratogens such as drugs or nicotine during pregnancy. Pregnant women must ingest adequate levels of protein, vitamins, and iron to remain healthy and have a healthy baby. Smoking during pregnancy has been linked to to heart defects, and it decreases the amount of oxygen available to the fetus, which causes poor growth. Consumption of alcohol is associated with a host of defects collectively called fetal alcohol syndrome. Whether any level of alcohol consumption is safe during pregnancy is not yet known. The use of drugs such as marijuana and cocaine during pregnancy increases the likelihood of stillbirths and unhealthy babies.
The loss of a fetus before the twentieth week of pregnancy is called a miscarriage. The most common cause of miscarriage during the first trimester is a major genetic defect in which the embryo has missing or extra chromosomes and therefore cannot develop normally. Other common causes are physical abnormalities in the embryo, a malformed uterus in the mother, an “incompetent” cervix that opens as the fetus enlarges, scarring of the uterus, and hormonal deficiencies. Increasing age of the mother, smoking, and alcohol and drug consumption are also correlated with miscarriage. To prevent future miscarriages of a nongenetic cause, hormone therapy, medications, and surgery are options.
Ectopic pregnancies occur when the fertilized egg implants in the wall of the Fallopian tube instead of in the uterus. The growing embryo may rupture the tube, endangering the mother’s life and necessitating emergency surgery and possible loss of that Fallopian tube. Unruptured ectopics can be treated with medication or laparoscopic surgery. Scarring of the Fallopian tube from an infection can narrow this passage and cause ectopic pregnancy, as can early “hatching” of the blastocyst from its protective covering.
Neural tube defects result from a problem in the ectodermal layer of the embryo resulting in improper closure of the brain and spinal cord in early embryonic development. The outcomes of this defect are anencephaly (absence of a complete brain and part of the skull), a lethal condition, or spina bifida (portions of the spinal cord protruding from the spine), which can vary from mild to severe. These disorders seem to occur in families with a history of neural tube defects in pregnancy. Folic acid, an important component of prenatal vitamins, can reduce the risk of neural tube defects.
Neural tube defects can be detected prenatally by ultrasound, in which high-frequency sound waves are bounced off the contents of the uterus. The echoes are converted to an image, or sonogram, on a screen. Another test for neural tube defects is the alpha fetoprotein test, which measures levels of a fetal protein in the mother’s blood; high levels indicate a neural tube defect.
Down syndrome, which results from the presence of one extra chromosome number 21, is the leading cause of mental retardation in the United States and occurs in about 1 in every 800 live births. Many other genetic disorders are caused by missing or extra chromosomes. Some inherited diseases are attributable to errors in small pieces of chromosomes. Examples of these sorts of disorders include cystic fibrosis, hemophilia, Tay-Sachs disease, and sickle cell disease. Many of these genetic diseases can be detected by prenatal tests.
Amniocentesis is performed between the fifteenth and seventeenth weeks of pregnancy. A needle is inserted through the mother’s abdomen, and a sample of the amniotic fluid is removed. Fetal cells present in the fluid are cultured for two to three weeks. The fetal cells are then analyzed for chromosome complement or tested for small genetic changes that can result in specific genetic diseases. Occasionally amniocentesis is done in the third trimester to determine fetal lung maturity when delivery of a premature infant is expected. Chorionic villus sampling provides a similar means to examine the genetic material of the fetus. This test is performed as early as the eighth week of pregnancy when a small piece of the chorionic villus, a tissue of embryonic origin that surrounds the early placenta, is removed and genetic analysis is immediately performed. Prenatal diagnosis of a chromosome abnormality or genetic disease allows the parents to terminate the pregnancy or to prepare for the birth of an affected child.
Infections acquired by a pregnant woman may be only inconveniences for her, but they may have severe consequences for an unborn child. Rubella, or German measles, can cause fetal death or severe impairment during the first trimester and permanent hearing loss during the second trimester. Vaccination prevents contraction of rubella, but cannot be given during pregnancy. Cytomegalovirus (CMV) can cause physical and mental retardation, blindness, and deafness to the fetus if the mother first contracts CMV during pregnancy. Toxoplasmosis is an infection that can cause serious fetal consequences including miscarriage, stillbirth, and neonatal death if contracted in pregnancy. Avoidance of gardening where cats defecate and avoiding any contact with cat litter are measures counseled to pregnant women to reduce risk. Transmission of the sexually transmitted diseases syphilis and gonorrhea to the fetus can be prevented with antibiotic therapy. Human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency syndrome (AIDS), can be transmitted from mother to fetus through the placenta. Newborns with AIDS have a host of disorders and usually die within one or two years.
Rh factor is a substance on the surface of red blood cells. Individuals with and without this substance are Rh positive and Rh negative, respectively. If a woman is Rh negative and has an Rh-positive fetus, during delivery some of the baby’s blood cells will enter the woman, and her body will manufacture antibodies to destroy the foreign cells. If with a subsequent pregnancy the fetus is Rh positive, these antibodies could attack and destroy the fetus. This problem is avoided by bloodtyping of mother and fetus during the first pregnancy and by administering Rh0(D) immune globulin (human), which destroys the fetal red blood cells entering the mother before her body can produce antibodies. Hence, future pregnancies are not at risk for Rh incompatibility.
A host of medical problems in the mother can develop during pregnancy that could jeopardize both her health and the health of her fetus. Blood pressure problems develop in about 7 percent of pregnant women as a result of the enormous changes in blood volume and pressure. The largest danger is that the fetus will not receive enough oxygen, which may lead to growth problems or sudden death during the final months of pregnancy. Preeclampsia is a cluster of symptoms related to high blood pressure, including edema (swelling caused by water retention) and kidney malfunction. Eclampsia (convulsions and coma) is life-threatening and needs emergency treatment. Bed rest, diet modification, close monitoring by a physician, or hospitalization may be prescribed for mild to severe cases of high blood pressure during pregnancy.
The effect of the hormones induced by pregnancy on the production of insulin, which regulates sugar levels in the body, is not well understood. In some pregnancies, insulin levels are not regulated properly, which results in gestational diabetes. Untreated, this can result in loss of the fetus late in pregnancy, the birth of a baby with high body fat content and an immature pancreas, or maternal convulsions and coma. Proper medical intervention and monitoring can correct or ease the effects of gestational diabetes on both mother and fetus.
Up to half of pregnant women develop anemia, a deficiency in red blood cells, because of a lack of iron or folic acid. The demand for the production of red blood cells in both mother and fetus leads to this disorder, which can cause poor growth in the fetus and increased susceptibility to infection, fatigue, and severe bleeding during childbirth for the mother. Proper diet and dietary supplements can alleviate anemia.
Until the mid-twentieth century, what was known about fetal development was derived mainly from the study of miscarriages. The development and use of ultrasound techniques in the 1960s allowed a more accurate picture of developmental progression of normal, active fetuses, and such techniques subsequently became indispensable for the detection of some developmental abnormalities.
While physicians had been able to sample the amniotic fluid surrounding a fetus since the late nineteenth century, it was not until 1970 that they discovered that the fluid contained fetal cells which could be analyzed for chromosomal composition. At this time, amniocentesis became a tool for prenatal genetic analysis and sex determination and a routine test for pregnant women over the age of thirty-five, who are at a higher risk of carrying a fetus with a genetic abnormality. The liberalization of abortion laws in the United States in the 1960s gave parents of abnormal fetuses the option of pregnancy termination. In the early 1980s, chorionic villus sampling allowed much earlier detection of genetic defects and facilitated decision making for the expectant parents.
In the 1980s, some physicians began performing surgery on fetuses within the mother’s uterus to correct problems threatening the life of the fetus, such as kidney disorders. Advanced monitoring and intervention techniques have virtually eliminated maternal deaths during pregnancy and childbirth. Increased knowledge about infectious and toxic agents and about the ill effects of certain lifestyle habits upon a developing fetus has led to better education and prenatal care for both mother and fetus. Because of career and personal considerations, older women frequently wish to begin families. Medical advances have made it not uncommon for women past the age of forty to conceive for the first time and give birth to healthy infants.
Much research has been focused on the initial stages of pregnancy and infertility. Techniques have been developed to circumvent damaged Fallopian tubes. In the procedure called in vitro fertilization, a woman is given hormones to induce ovulation. Several mature eggs are removed from her ovaries and fertilized in a glass dish. The resulting blastocysts are then implanted into the woman’s uterus. In some cases in which a woman is unable to carry the fetus herself, a surrogate mother has been used as an “incubator” for the embryo. Unused embryos fertilized in vitro are stored in a deep freeze and may be thawed for future use. Early embryos have been separated into individual cells, which then develop into genetically identical blastocysts, each with the potential to become an infant. These technological advances have raised a host of ethical questions concerning disposal of the embryos and their genetic manipulation.
Human reproductive research is focused on the reversal or circumvention of infertility, the alleviation of maternal and fetal distress, and the prenatal detection and treatment of genetic disease. The knowledge gained about reproduction serves to enhance a sense of awe and wonder at the beauty and complexity of the gestational process.
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