Pregnancy in Humans

Pregnancy in Humans


the physiological process of development of an embryo from an egg cell in the body of a woman, terminating in the birth of an infant. Pregnancy begins at the moment of union of an egg cell and spermatozoon (fertilization), which occurs in the oviduct. The fertilized egg cell (ovum) begins to divide and move along the oviduct to the uterus, mainly because of muscle contractions of the duct and movements of the fibers of the ciliated epithelium. The ovum reaches the uterus in seven or eight days. As a result of the division of the ovum, a complex of cells is formed—the blastomeres or cleavage cells, whose outer layer is called the trophoblast. These cells liquefy the mucous membrane of the uterus by means of enzymes, and the ovum is embedded (implanted) in the membrane. From that moment, the nourishment of the ovum proceeds at the expense of the mother’s body. If the ovum is implanted outside the uterus, in the oviduct, the ovary, or the abdominal cavity (extrauterine pregnancy), surgery is necessary.

Pregnancy, especially a first pregnancy, is a physiological process that exerts a favorable influence on the woman’s entire body. However, the requirements of the developing fetus place increased stress on all the woman’s organs and tissues. From the first days of pregnancy the corpus luteum, an endocrine gland formed during ovulation which produces progesterone, begins to function in the ovary. Under the influence of the hormones progesterone and estrogen, which are secreted by the ovaries, changes occur in the entire body of the woman. The Graafian follicles stop ripening in the ovaries. The pituitary body, especially its anterior lobe, increases to three times its normal size because of its increased functioning. Under the influence of the hormones from the pituitary, the mammary glands increase in size, become more taut and indurated; colostrum may be discharged from the nipples. The mucous membrane of the uterus hypertrophies and is transformed into the so-called deciduous membrane. (After the birth it drops away and is discharged from the uterus.) New muscle fibers develop in the muscle layer; each fiber increases ten times in length and four times in width. The contractile capacity of the muscles increases. By the time of birth the size of the uterus increases 500 times and its weight increases 25 times. The length of the uterus increases from 6–7 cm at the beginning of pregnancy to 38 cm, its breadth from 4 cm to 26 cm, and its thickness from 2.5–3 cm to 24 cm. Especially great changes occur in the vascular system of the uterus: the vessels increase in length and width, and new vessels appear. The quantity of blood flowing to the uterus per unit of time increases five or six times. Pregnancy makes great demands on the cardiovascular system. Blood (arterial) pressure may increase under the influence of hormones from the pituitary. Usually, blood pressure ranges from 70–80 mm of mercury, or 9.3–10.7 kilonewtons per sq m (minimum) to 120–130 mm of mercury, or 16–17.3 kilonewtons per sq m (maximum).

Hypertrophy of the left ventricle is observed, owing to increased stress on the heart. The quantity of blood pumped into the pulmonary arteries and the aorta during heart contraction (systolic volume) is increased by 30–50 percent. Varicose dilation of the veins, usually in the lower extremities, may develop owing to changes in the walls of the venous vessels. The quantity of blood may be increased by 400–500 milliliters. The number of leukocytes may reach 15,000, and the ESR (erythrocyte sedimentation rate) almost always increases to 20–30 mm per hour. Coagulability of the blood is increased. Metabolic changes occur: enzyme processes are intensified and oxidative processes are decreased. Protein and fat metabolism decrease, leading to the accumulation of carbon dioxide, nitrogenous compounds, and products of the incomplete burning of fat in the body. Carbohydrate metabolism is intensified and, because of the increased permeability of renal epithelium, sugar may appear in the urine. Water-salt metabolism is heightened. The need for water is increased. Metabolism of the mineral salts phosphorus, potassium, magnesium, iron, and so forth is intensified. These salts are necessary to the building of the skeleton and nervous system of the fetus. When there is an insufficient intake of these substances, the fetus takes them from the mother’s body. For this reason dental cavities, brittle nails, and so forth are often observed in pregnant women. The growing body of the fetus needs a large quantity of oxygen; therefore, a pregnant woman experiences an increased need for it. A constant supply of vitamins A, B, C, D, and E entering the mother’s blood is necessary to the development of the fetus. During pregnancy the weight of the woman’s body is increased by 10–11 kg; from 12 weeks on, it increases by approximately 450 g per week. At the beginning of pregnancy, nausea, vomiting on an empty stomach, salivation, and taste distortion (revulsion to meat and fat) may be observed. There is often a desire for spicy and salty food; sometimes a desire to use inedible substances in the diet develops (chalk, clay, sand, and so forth), owing to changes in the secretory function of the stomach and in metabolism. In the first months of pregnancy, the woman’s psyche becomes more vulnerable: irritability increases, tearfulness appears, and normal sleep is disrupted. As the fetus develops, the pregnant woman becomes more even-tempered and less emotional, and her maternal feelings develop. In the second half of pregnancy, especially great changes are observed in all her systems and organs. As the uterus grows, the woman’s spine and shoulders tilt backward somewhat, giving her a more stable posture. Her gait changes (the so-called “proud” gait). The thorax expands owing to the high position of the diaphragm. Because of changes in the ligamentous apparatus of the pelvis, the mobility of its joints is increased. This facilitates passage of the fetus at birth. The abdominal wall stretches. In some women, especially with first pregnancies or multiple pregnancies, reddish-pink stripes appear on the skin of the abdomen and thighs, which become lighter after the birth. Dark spots often appear on the face. The white line of the abdomen, the external sex organs, and the mammillary ring become darker. Changes occur in the skin glands of the woman.

From the fourth month of pregnancy, there is an increase in the secretion of sex hormones, especially estriol, produced by the placenta, and progesterone, also produced by the placenta after atrophy of the corpus luteum. Substantial changes are also noted in the thyroid and adrenal glands.

Pregnancy may be complicated by a number of factors (toxicoses of pregnancy, exacerbation of certain diseases, and so forth). For this reason, a woman must be under systematic observation at a women’s consultation center, which she should visit at the first symptoms of pregnancy.

The symptoms of pregnancy may be subjective (for example, nausea, vomiting, changes in taste, and increased fatigability and irritability). More understandable symptoms are changes in the sex organs (such as an increase in the size of the uterus, changes in its consistency, the cessation of menstruation, or a change in the vagina) and in the mammary glands. Authentic signs of pregnancy include movement of the fetus, fetal heartbeat heard or recorded by a special apparatus, the determination by palpation of parts of the fetus (head, feet, back, and so forth), and X-ray determination of the fetal skeleton.

The development of the fetus depends greatly on such factors as the health of the parents, their physical condition and age, and the number of previous deliveries by the mother.

The age of the fetus or the term of the pregnancy is determined principally by the fetus’s length (see Table 1). In the first two months the embryo does not have a human appearance; only later do the characteristic features of the human fetus appear.

Table 1. Development of the fetus
Term of pregnancy in lunar months1Length of fetus (In cm)Weight of fetus (in g)
1 lunar month = 28 days
End of  
 1st month0.8
 2nd month3–3.5
 3rd month8.920–30
 4th month16–17100–120
 5th month25–27250–300
 6th month30–32600–700
 7th month35–361,000–1,200
 8th month38–401,500–1,600
 9th month43–452,300–2,500
 10th month49–503,200–3,500

In the uterus, the fetus lies in the cavity of the bag of waters formed by the fetal membranes. The internal, amniotic sac is distended by the amniotic’fluid, which ensures the free movement of the fetus and its protection against external impact, pressure from the umbilical cord, concretion of the amniotic sac and fetal skin, and so forth. During pregnancy, the fetus floats freely in the amniotic fluid. Toward the end of pregnancy, approximately one or two months before delivery, it assumes a lengthwise, or normal, position. In 96 percent of cases, its position is head down (cephalic presentation); in 3.5 percent, pelvis down (breech presentation); in 0.5 percent, abnormal (pathologic) positions are noted (transverse or shoulder), which require surgery or obstetrical assistance for delivery.

Pregnancy has an average duration of 280 days (ten lunar months) and terminates in birth (or, within the period of 28 weeks from conception, in abortion). With the object of ensuring a normal course of pregnancy during the final period and normal development of the fetus, a working woman in the USSR is granted a leave for pregnancy and delivery of 56 days before and 56 days after delivery.


Petchenko, A. I. “Fiziologicheskie izmeneniia v organizme zhenshchiny pri beremennosti.” In Mnogotomnoe rukovodstvo po akusherstvu i ginekologii, vol. 2, [book 1]. Moscow, 1963.