Germ cells provide transmission of hereditary information to offspring. They contain special protein structures (chromosomes) in which hereditary factors, or genes, are concentrated. Each species of animals and plants is characterized by a specific set of chromosomes.
The nuclei of all cells of the human body contain 46 chromosomes, i.e. 23 pairs, and only male and female germ cells are an exception, they contain half the composition – 23 chromosomes. Immature germ cells, like all others, have 46 chromosomes, but upon maturation they divide and as a result only 23 chromosomes remain in a mature germ cell. When the male and female sex cells merge, the fertilized egg again receives a complete set of chromosomes – 23 pairs, of which half are maternal and half are paternal. In this way, species constancy of a person is maintained.
The conception of a new organism occurs at a time when the nuclei of the reproductive eggs with their genetic information are combined into a single whole.
We have already said that the nucleus of any human cell contains 23 pairs of chromosomes, of which the 23rd pair determines the sex of a person. The first 22 pairs of chromosomes are indicated by numbers, and sex – by letters of the Latin alphabet X and Y. The gender of a person depends on the combination of these two chromosomes. If they are the same, that is, XX, then this is a female cell, if different, XY, is a male cell, any cell of a woman is designated 46 (XX), men – 46 (XY). When the female reproductive eggs mature, when they begin to divide and the number of chromosomes is halved, one of the X chromosomes necessarily enters any egg. Another situation occurs when spermatozoa mature: when they are divided, the X chromosome can fall into one sperm, and the Y chromosome into the other. It is easy to imagine that when a fertilized egg with an X chromosome is fertilized with a sperm with an X chromosome, the fertilized egg receives a set of 46 (XX), i.e. the fetus will be female. If the egg is fertilized by a sperm with a Y chromosome, the set of chromosomes will be 46 (XY), then a boy will be born. From the foregoing, it is clear that the sex of the offspring is determined by male sex cells – sperm.
A fertilized egg (zygote) 30 hours after the introduction of a sperm into it undergoes its first crushing. The first division is followed by a series of the following nuclear divisions with the formation of new cells. This process is called crushing a fertilized egg, resulting in the embryo becoming multicellular. However, the previous volume of the egg still does not change much, it seems to be crushed into smaller cells. This is due to the fact that the embryo at this time moves along the narrow lumen of the tube, the width of which is equal to the diameter of a human hair.
The movement of the embryo is ensured by muscle contraction of the tubes, the movement of the fringes and the flow of fluid in the tube towards the uterus. The fallopian tubes not only transport the embryo, but also nourish it. After a 3-day journey through the tube, a dense lump of constantly dividing germ cells enters the uterine cavity, the mucosa of which by this moment grows significantly in anticipation of a fertilized egg. The embryo is implanted, or implanted, into the thickness of the uterine mucosa. This occurs approximately on the 5th day after fertilization. At this time, not only active cell division occurs in the embryo, but also their formation into the rudiments of future tissues in accordance with the plan for building the future organism. Outside, the embryo is covered with cells that secrete an enzyme that melts the uterine mucosa, extracting nutrients from it, and freeing the bed to immerse the embryo in the uterus.
In the process of fetal development, it is customary to distinguish between the embryonic or embryonic period (gr. Embryon – embryo) , which lasts from the moment of fertilization of the egg to 8 weeks. pregnancy, and fetal , or fruit period (Latin fetus -. fruit), extending from 8 weeks. pregnancy until delivery. In the embryonic period, the formation of all the main organs and systems of the unborn child takes place, in the fetal period, their further growth and development continues.
The fetus lives in conditions that allow it to develop intensively. If a person spends time and energy on heat production and muscle work, then the fetus is deprived of this opportunity and necessity. He lives in conditions of optimal temperature, his movements are limited by space and facilitated by swimming in a liquid. All the energy of the fetus is spent on the process of creating his body. The intrauterine development of its organs and systems occurs constantly starting from the conception process. For 9 months of fetal life, the fetus turns into a complex human body.
Already on the 6-7th day , outgrowths (villi) are formed from the external cells of the zygote, which are embedded in the inner surface of the uterine wall, its blood vessels, forming spaces filled with mother’s blood. In villi, fetal vessels begin to form.
The blood of the fetus and the blood of the mother never mix, although only the thin membrane of the villi and the walls of the blood vessels of the embryo separate them. Among all mammals living on Earth, only in humans there is the closest connection between the blood vessels of the fetus and the blood of the mother.
Already in a 4-week embryo with a special ultrasound examination, one can clearly see the contractions of the heart, which indicates the presence of a circulatory system. At 8 wk. he has already developed all the features inherent in man: the body, head are formed, there are rudiments of the limbs, eyes, nose and mouth. From this moment, the fetus is considered the fetus. It is located in the fetal bladder, filled with amniotic fluid.
At 16 weeks (almost 4 months) the length of the fetus is 16 cm, weight 120 g. Hands, legs are clearly visible, fingers are distinguishable. All the most important organs are developed, such as the lungs, liver, kidneys, and the sex of the fetus can be determined.
From the 14th week, the fetus begins to move, but the mother does not feel its movements.
At 28 weeks (6 1/2 months) the length of the fetus reaches 35 cm, weight 1000 g. His skin begins to become covered with a special lubricant, impermeable to amniotic fluid. At this time, the fetus is very active, it has enough space in the uterus, where it moves freely. Therefore, his position is unstable, his head is often located upstairs. Mother feels his movements well. A living fetus born at this time is considered a newborn baby, not a miscarriage.
Over the past 12 weeks before giving birth, the fetus is fully developed and is ready for extrauterine life by the 40th week (9 months ). His lungs are ripe enough for breathing. The length of the full-term fetus is on average 50 cm, body weight – 3200–3400 g. It is usually head down in a stable position, since its movements in the uterus are limited due to its sufficiently large size.
By the time of birth, the fetal head falls lower, the pregnant woman feels that she has become easier to breathe. Both the fetus and the mother’s body are ready for childbirth.
We have already mentioned that the mother is the environment for the fetus. In the first 8 weeks of life, the fetus does not yet have independent functions, therefore its well-being depends entirely on the mother’s body. The hormone secreted by the mother’s ovaries – progesterone – creates the conditions for implantation of the embryo and its proper development. After 8 weeks, the fetus begins to form a placenta from the villous membrane (baby place). Its full development ends by the 16th week. The fetus is connected to the placenta with the umbilical cord, in which the blood vessels pass. The placenta is the organ through which breathing, nutrition and excretion of metabolic products of the growing fetus through the blood of the mother are carried out. In addition, it is the gland of internal secretion, since it completely produces all the hormones that were synthesized by the pituitary gland and the ovaries of the mother before pregnancy. Moreover, the number of placental hormones exceeds the level of the pituitary and ovarian hormones of the mother outside pregnancy many times over. However, the placenta is the only endocrine gland that does not have a complete set of enzymes for self-synthesis of hormones; some of these enzymes are located in the liver and adrenal glands of the fetus. Therefore, the molecules of substances involved in the synthesis of hormones carry out shuttle communication with fetal enzymes. In this sense, the placenta and the fetus are a functional unity.
The level of hormones in the urine or blood of pregnant women can reliably judge the condition of the fetus. This example shows how close the relationship is between the mother, placenta, and fetus.
Some women feel or “know for sure” about pregnancy, others only find out after a few months, especially in cases where the menstrual cycle is irregular. Pregnancy can be assumed when the following symptoms appear: the absence of menstruation on the expected days or the appearance of shorter than usual meager periods, engorgement of the mammary glands, a distortion of taste for food, a tendency to spicy foods, salty foods, nausea, and vomiting in the morning, the appearance of rapid fatigue and mild weakness, more frequent than usual urination.
Hormones , secreted by the placenta during pregnancy, the mother’s body to cause numerous physiological changes that ensure the proper development of the fetus, preparing the body for the upcoming birth and feeding.
The pregnant woman’s nervous system is rebuilt in such a way that subordinates the whole body to one goal – carrying a pregnancy. In accordance with this, physiological changes occur in the basic systems of the body, aimed at adapting it to new conditions.
For example, during pregnancy , the cardiovascular system performs more intense work, as the “second” heart actually appears in the body – this is the placental circle of blood circulation. Here the blood flow is so great that every minute 500 ml of blood passes through the placenta. The heart of every healthy woman during pregnancy easily adapts to additional loads: the mass of the heart muscle increases, the cardiac output increases. To meet the growing fetal needs for nutrients, oxygen and building materials in the mother’s body, blood volume begins to increase, reaching a maximum of 7 months of pregnancy. Instead of 4000 ml of blood, 5300–5500 ml are now circulating in the body. And if the heart of a healthy woman easily copes with such work, then in pregnant women with heart diseases this load causes complications. That is why women with heart defects in the period of 27-28 weeks of pregnancy are recommended hospitalization, in which the treatment will prepare the heart for hard work.
Blood pressure during pregnancy is practically unchanged. On the contrary, in women who have a slight increase in blood pressure before pregnancy or in its early stages, it usually decreases in mid-pregnancy. This is due to the fact that under the influence of progesterone, the tone of blood vessels decreases and blood pressure decreases. However, in the last 2-3 months of pregnancy, as a rule, in such women, the pressure rises again and can complicate its further course. In this regard, it is very important to pay attention to the increase in blood pressure in the early stages of pregnancy in order to establish careful monitoring of its change in the later stages. In a healthy woman, during pregnancy, blood pressure is considered elevated if it exceeds the figures 130 and 80 mm Hg. Art.
Due to the increased oxygen demand of a woman’s body during pregnancy, lung activity increases . Despite the fact that as the pregnancy develops, the uterus pushes the diaphragm up, the respiratory surface of the lungs does not decrease due to the expansion of the chest and increased air passage through the bronchi, which expand under the influence of placental hormones. An increase in the volume of respiratory air during pregnancy facilitates the removal of fetal gas exchange products through the placenta. The number of breaths during pregnancy does not change, remaining 16-18 times per minute, slightly increasing towards the end of pregnancy. When shortness of breath and other respiratory disorders occur, the pregnant woman must consult a doctor.
The kidneys during pregnancy function with great stress, as they remove from the body the metabolic products of the woman herself and her growing fetus. The amount of urine produced varies depending on the amount of fluid you drink. A healthy woman during pregnancy secretes an average of 1,200-1,600 ml of urine per day, while 950-1,200 ml of urine is excreted in the daytime and the remaining portion at night. Due to the increased excretory function of the kidneys, urinary sugar may be excreted, which is not associated with diabetes mellitus and passes quickly after childbirth, but requires close attention of the doctor to such a pregnant woman.
In the urine of healthy women there are white blood cells, not more than 5-6. In the last months of pregnancy, traces of protein may appear in the urine. Under the influence of progesterone, the tone of the bladder decreases, it becomes more elongated and lethargic, which often leads to stagnation of urine. Pressing the bladder with the head of the fetus also contributes to this. The created conditions facilitate the introduction of infection into the urinary tract, which is often observed in pregnant women. The presence of leukocytes in the urine of more than 8-10 in the field of view and protein indicates impaired renal function and requires serious medical examination and treatment.
In many women, in the first 2-3 months of pregnancy, a violation of the function of the digestive organs is observed : nausea appears, often vomiting in the morning, taste sensations change, and there is an attraction to unusual substances (clay, chalk). All these phenomena usually pass by the 3-4th month of pregnancy, less often – at a later date. Under the influence of placental hormones, intestinal tone decreases, which often leads to constipation. The intestine is pushed up by the pregnant uterus, the stomach also moves up and squeezes, while part of its contents can be thrown into the esophagus and cause heartburn. In such cases, it is recommended to take alkaline solutions (baking soda, borzh, medications are undesirable), eating 2 hours before bedtime and position in bed with a raised head end.
The liver during pregnancy works with a heavy load, as it neutralizes the metabolic products of the woman and her fetus. In this case, thickening of bile may be observed, which often causes itching of the skin. The appearance of severe skin itching, and especially vomiting and pain in the right hypochondrium, requires immediate medical attention and tests to determine liver function.
Under the influence of progesterone during pregnancy, the ligaments in the joints relax . The joints of the pelvis become especially mobile, which facilitates the passage of the fetus through the pelvis during childbirth. Sometimes the softening of the pelvic joints is so pronounced that there are slight discrepancies in the pubic bones. A pregnant woman has pain in the bosom, a duck walk. It is necessary to inform the doctor about this and get appropriate recommendations. Due to the need to balance the weight of the pregnant uterus, the body sagging in the lumbar spine increases, which can cause back pain, especially if a woman walks in high-heeled shoes. In connection with the increase in the pregnant uterus, the abdominal wall is stretched and longitudinal strips may appear on it.
Dark pigmentation intensifies on the skin of the face, around the nipples and the midline of the abdomen . This phenomenon is due to an increase in the pregnant coloring hormone secreted by the pituitary gland.
During pregnancy, the mammary glands are prepared for the upcoming feeding. In them, the number of glands secreting milk and adipose tissue increases, and blood supply increases. The mammary glands increase in size, become rude, droplets of colostrum can be squeezed out already at the beginning of pregnancy. All these changes are caused by the action of the hormones of the placenta and pituitary gland.
The greatest changes during pregnancy occur in the genitals and concern mainly the uterus. The pregnant uterus is constantly increasing in size, by the end of pregnancy its height reaches 35 cm instead of 7-8 before pregnancy, the mass increases to 1000-1200 g (without a fetus) instead of 50-100 g. The volume of the uterine cavity increases by 500 times by the end of pregnancy. A change in the size of the uterus occurs due to an increase in the size of muscle fibers under the influence of the hormones of the placenta, as well as an increase in the number of blood vessels that expand and seem to braid the uterus.
During pregnancy, uterine contractions are observed, usually a woman does not feel them. Towards the end of pregnancy, these contractions become more active and feel like a “squeeze”. Such normal contractions of the uterus can be considered as training exercises before the upcoming work in childbirth.
The position of the uterus changes in accordance with its growth. By the end of the 3rd month of pregnancy, she leaves the pelvic cavity and rises above the womb; By the end of pregnancy reaches the hypochondrium. The uterus is held in position by ligaments, which also thicken and stretch during pregnancy. Pain that occurs on the sides of the abdomen, especially during a change in body position, is often caused by sprains.
Blood supply to the external genitalia increases, so varicose veins can appear on the labia. Due to the difficulty of outflow of blood from the lower extremities during pregnancy, varicose veins also often appear on the legs, and the location of the dilated veins on one limb usually indicates the attachment of the placenta in the uterus on the side of the affected leg.
All the physiological changes described above lead to a change in the body weight of a woman during pregnancy. A healthy woman during pregnancy increases her weight by an average of 12 kg with fluctuations from 10 to 15 kg. Usually, in the first half of pregnancy, body weight increases by 4 kg, in the second – 2 times more.
The weight gain for pregnancy is distributed as follows: fetal mass – 3300 g, placenta – 650 g, amniotic fluid – 800 ml, uterus – 900 g, mammary glands – 405 g, blood volume – 1250 ml, tissue fluid – at least 2500 ml . The remaining mass gain is in fat. At the same time, women with a low fat deposition before pregnancy, as a rule, accumulate more fat during pregnancy than full women. Its mass can increase by 3.5-4 kg, especially in the thighs and buttocks. A healthy woman adds in the mass weekly 300 ± 30 g to 20 weeks, 330 ± 40 g from 21 to 30 weeks and 340 ± 30 g after 31 weeks before delivery.
Thus, the changes occurring in the pregnant body create a state of physiological adaptation necessary to meet the needs of the growing fetal organism.