The Normal Events of Pregnancy

Introduction

There processes and series of changes that takes place in the woman's organs and tissues as a result of a developing fetus. The entire process from fertilization to birth takes an average of 266–70 days, or about nine months. (For pregnancies other than those in humans, see gestation.)

The normal events of pregnancy

Initiation of pregnancy

• Major structures and hormones involved in the initiation of pregnancy. Also seen, at right, is the …

A new individual is created when the elements of a potent sperm merge with those of a fertile ovum, or egg. Before this union both the spermatozoon (sperm) and the ovum have migrated for considerable distances in order to achieve their union. A number of actively motile spermatozoa are deposited in the vagina, pass through the uterus, and invade the uterine (fallopian) tube, where they surround the ovum. The ovum has arrived there after extrusion from its follicle, or capsule, in the ovary. After it enters the tube, the ovum loses its outer layer of cells as a result of action by substances in the spermatozoa and from the lining of the tubal wall. Loss of the outer layer of the ovum allows a number of spermatozoa to penetrate the egg's surface. Only one spermatozoon, however, normally becomes the fertilizing organism. Once it has entered the substance of the ovum, the nuclear head of this spermatozoon separates from its tail. The tail gradually disappears, but the head with its nucleus survives. As it travels toward the nucleus of the ovum (at this stage called the female pronucleus), the head enlarges and becomes the male pronucleus. The two pronuclei meet in the centre of the ovum, where their threadlike chromatin material organizes into chromosomes.

Originally the female nucleus has 44 autosomes (chromosomes other than sex chromosomes) and two (X, X) sex chromosomes. Before fertilization a type of cell division called a reduction division brings the number of chromosomes in the female pronucleus down to 23, including one X-chromosome. The male gamete, or sex cell, also has 44 autosomes and two (X, Y) sex chromosomes. As a result of a reducing division occurring before fertilization, it, too, has 23 chromosomes, including either an X or a Y sex chromosome at the time that it merges with the female pronucleus.

After the chromosomes merge and divide in a process termed mitosis, the fertilized ovum, or zygote, as it is now called, divides into two equal-sized daughter cells. The mitotic division gives each daughter cell 44 autosomes, half of which are of maternal and half of paternal origin. Each daughter cell also has either two X-chromosomes, making the new individual a female, or an X- and a Y-chromosome, making it a male. The sex of the daughter cells is determined, therefore, by the sex chromosome from the male parent.

Fertilization occurs in the uterine tube. How long the zygote remains in the tube is unknown, but it probably reaches the uterine cavity about 72 hours after fertilization. It is nourished during its passage by the secretions from the mucous membrane lining the tube. By the time it reaches the uterus, it has become a mulberry-like solid mass called a morula. A morula is composed of 60 or more cells. As the number of cells in a morula increases, the zygote forms a hollow bubble-like structure, the blastocyst. The blastocyst, nurtured by the uterine secretions, floats free in the uterine cavity for a short time and then is implanted in the uterine lining. Normally, the implantation of the blastocyst occurs in the upper portion of the uterine lining.

Diagnosis of pregnancy

Symptoms and signs; biological tests

Outward early indications of pregnancy are missed menstrual periods, morning nausea, and fullness and tenderness of the breasts; but the positive and certain signs of gestation are the sounds of the fetal heartbeat, which are audible with a stethoscope between the 16th and the 20th week of pregnancy; ultrasound images of the growing fetus, which can be observed throughout pregnancy; and fetal movements, which usually occur by the 18th to the 20th week of pregnancy.

Persons who note their body temperature upon awakening, as many women do who wish to know when they are ovulating, may observe continued elevation of the temperature curve well beyond the time of the missed period; this is strongly suggestive of pregnancy. During the early months of pregnancy, women may notice that they urinate frequently, because of pressure of the enlarging uterus on the bladder; feel tired and drowsy; dislike foods that were previously palatable; have a sense of pelvic heaviness; and are subject to vomiting (which can be severe) and to pulling pains in the sides of the abdomen, as the growing uterus stretches the round ligaments that help support it, singly or together. Most of these symptoms subside as pregnancy progresses. The signs and symptoms of pregnancy are so definite by the 12th week that the diagnosis is seldom a problem.

Biological tests for pregnancy depend upon the production by the placenta (the temporary organ that develops in the womb for the nourishing of the embryo and the elimination of its wastes) of chorionic gonadotropin, an ovary-stimulating hormone. In practice, the tests have an accuracy of about 95 percent, although false-negative tests may run as high as 20 percent in a series of cases. False-negative reports are frequently obtained during late pregnancy when the secretion of chorionic gonadotropin normally decreases. The possibility not only of false-negative but also of false-positive tests makes the tests, at best, probable rather than absolute evidence of the presence or absence of pregnancy. Chorionic gonadotropin in a woman's blood or urine indicates only that she is harbouring living placental tissue. It does not tell anything about the condition of the fetus. In fact, the greatest production of chorionic gonadotropin occurs in certain placental abnormalities and disorders that can develop in the absence of a fetus.

Tests using immature mice (the Aschheim-Zondek test) and immature rats have been found to be extremely accurate. Tests using rabbits (the Friedman test) have been largely replaced by the more rapid and less expensive frog and toad tests.

The use of the female South African claw-toed tree toad, Xenopus laevis, is based on the discovery that this animal will ovulate and extrude visible eggs within a few hours after it has received an injection of a few millilitres of urine from a pregnant woman. The male common frog, Rana pipiens, will extrude spermatozoa when treated in the same way. Both of these tests are considered somewhat unsatisfactory because false-positive reactions are not uncommon.

Several immunological reaction tests in common use are based upon the inhibition of hemagglutination (clotting of red cells). A positive test is obtained when human chorionic gonadotropin (HCG) in the woman's urine or blood is added to human chorionic gonadotropin antiserum (rabbit blood serum containing antibodies to HCG) in the presence of particles (or red blood cells) coated with human chorionic gonadotropin. The hormone from the woman will inhibit the combination of coated particles and antibody, and agglutination does not occur. If there is no chorionic gonadotropin in her urine, agglutination will occur and the test is negative.

Several “signs” noted by the physician during an examination will suggest that a patient may be in the early months of pregnancy. Darkening of the areola of the breast (the small, coloured ring around the nipple) and prominence of the sebaceous glands around the nipple (Montgomery's glands); purplish-red discoloration of the vulvar, vaginal, and cervical tissues; softening of the cervix and of the lower part of the uterus and, of course, enlargement and softening of the uterus itself are suggestive but not necessarily proof of pregnancy.

Additional Reading

General texts include Human Embryology, 4th ed. by W.J. Hamilton and H.W. Mossman (1972); and Williams Obstetrics, 19th ed. by F. Gary Cunningham et al. (1993), a standard textbook in the field. Additional information may be found in the following specialized texts: Irwin R. Merkatz and Joyce E. Thompson (eds.), New Perspectives on Prenatal Care (1990); Robert K. Creasy and Robert Resnik (eds.), Maternal-Fetal Medicine: Principles and Practice, 3rd ed. (1994); Burwell and Metcalfe's Heart Disease and Pregnancy, 2nd ed. by James Metcalfe, John H. McAnulty, and Kent Ueland (1986); and Richard S. Abrams, Handbook of Medical Problems During Pregnancy (1989).

John W. Huffman Ed.

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• MLA Style:   "pregnancy." Encyclopædia Britannica. Encyclopaedia Britannica Student and Home Edition.  Chicago: Encyclopædia Britannica, 2010.
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