Pregnancy Introduction.
[1]. Pregnancy term refers to as the fusion or fertilization of Sperm and ovum together to form Zygote. Fusion of sperm and Ovum leads to form zygotes leads to pregnancy.
[2]. Ovum is released from graafian follicle of ovary into the abdominal cavity at the time of ovulation. From abdominal cavity, ovum enters fallopian tube through the fimbriated end. Entry of ovum is facilitated by movement of cilia present in the inner surface of fimbriated end.
[3]. Ovum of matured follicle in the ovary is in primary oocyte stage with diploid number (23 pairs) of chromosomes. Just before ovulation, meiotic division takes place in the ovum. Primary oocyte divides into a secondary oocyte and a first polar body.
[4]. First polar body is expelled out. Secondary oocyte contains only 23 chromosomes (haploid). Remaining 23 chromosomes are lost in the expelled first polar body. Thus, when the ovum is released into abdominal cavity during ovulation, it is in the secondary oocyte stage with haploid number of chromosomes.
Fertilization of the Ovum.
[1]. Fertilization refers to fusion (union) of male and female gametes (sperm and ovum) to form a new offspring. If sexual intercourse occurs at ovulation time and semen is ejaculated in the vagina, the sperms travel through the vagina and uterus to reach the fallopian tube.
[2]. Sperms reach the ovarian end of fallopian tube within 30 to 60 minutes. Movement of the sperm through uterus is facilitated by the antiperistaltic contractions of uterine muscles. Uterine contractions are induced by oxytocin, which is secreted from posterior pituitary by neuroendocrine reflex during sexual intercourse .
[3]. Uterine contractions are also facilitated by prostaglandin (PGE2) present in male seminal fluid. Among 200 to 300 millions of sperms entering female genital tract, only a few thousand sperms reach the spot near the ovum. Among these few thousand sperms, only one succeeds in fertilizing the ovum.
[4]. During fertilization, the sperm enters the ovum by penetrating the multiple layers of granulosa cells known as corona radiata present around the ovum. It is facilitated by hyaluronidase and proteolytic enzymes present in acrosome of sperm.
[5]. Proteolytic enzymes from acrosome of the successful sperm diffuse through the structures of zona pellucida and inactivate the other
sperms entering the ovum. Penetrating movement of sperm is enabled by a protein called CatSper present in the tail portion of the
sperm. It is a tunnel-shaped protein and forms the ion channel for entry of calcium into sperm cell.
[6]. Immediately after fertilization, ovum, which is in secondary oocyte stage, divides into a matured ovum and a second polar body. Second polar body is expelled. Nucleus of matured ovum becomes female pronucleus with 23 chromosomes, which include 22 autosomes and one sex chromosome called X chromosome.
[7]. Simultaneously, head of sperm swells and becomes male pronucleus. Then 23 chromosomes of the sperm and 23 chromosomes of ovum arrange themselves to reform the 23 pairs of chromosomes in the fertilized ovum.
Sex Chromosomes.
[1]. All the dividing cells in the body have 23 pairs of chromosomes. Among the 23 pairs, 22 pairs are called somatic chromosomes or autosomes.
[2]. Remaining one pair of chromosomes is called sex chromosomes. Sex chromosomes are X and Y chromosomes.
Sex Determination.
[1]. Sex chromosomes are responsible for sex determination. During fertilization of ovum, 23 chromosomes from ovum and 23 chromosomes from the sperm unite together to form the 23 pairs (46) of chromosomes in the fertilized ovum.
[2]. Now, sex determination occurs. Ovum contains the X chromosome. Sperm has either X chromosome or Y chromosome. When the ovum is fertilized by a sperm with X chromosome, the child will be female with XX chromosome.
[3]. And, if the ovum is fertilized by a sperm with Y chromosome, the sex of the child will be male with XY chromosome. So, the sex of the child depends upon the male partner.
Implantation.
[1]. Implantation is the process by which the fertilized ovum called zygote implants (fixes itself or gets attached) in the endometrial lining of uterus. After the fertilization, the ovum is known as zygote.
[2]. Zygote takes 3 to 5 days to reach the uterine cavity from fallopian tube. While travelling through the fallopian tube, the zygote receives its nutrition from the secretions of fallopian tube.
[3]. After reaching the uterus, the developing zygote remains freely in the uterine cavity for 2 to 4 days before it is implanted. Thus, it takes about 1 week for implantation after the day of fertilization.
[4]. During the stay in uterine cavity before implantation, the zygote receives its nutrition from the secretions of endometrium, which
is known as uterine milk. Just before implantation, the zygote develops into morula and then the implantation starts.
[5]. A layer of spherical cells called trophoblast cells is formed around morula. Trophoblast cells release proteolytic enzymes over the surface of endometrium. These enzymes digest the cells of the endometrium. Now, morula moves through the digested part of endometrium and implants itself.
Placenta & Embryo Development.
[1]. Already uterus is prepared by progesterone secreted from the corpus luteum during secretory phase of menstrual cycle. After implantation, placenta develops between morula and endometrium.
[2]. When implantation occurs, there is further increase in the thickness of endometrium because of continuous secretion of progesterone from corpus luteum. At this stage, the endometrial stromal cells are called decidual cells and the endometrium at the implanted area is
called decidua.
[3]. Now the trophoblastic cells of morula develop into cords, which are attached with decidual portion of endometrium. Blood capillaries grow into these cords from the blood vessels of the newly formed embryo.
[4]. At about 16th day after fertilization, heart of embryo starts pumping the blood into the trophoblastic cords. At the same time, blood sinusoids develop around the trophoblastic cords. These sinusoids receive blood from the mother.
[5]. Trophoblastic cells form some vascular projections into which fetal capillaries grow. These vascular projections become placental villi.
Thus, the final form of placenta has got the fetal part and the maternal part.
[6]. Fetal part of placenta contains the two umbilical arteries, which carry fetal blood to the placental villi through the capillaries. The blood returns back to fetus through umbilical vein.
[7]. Maternal part of placenta is formed by uterine arteries through which blood flows into sinusoids that surround the villi. The blood returns back to mother’s body through uterine vein.
Maternal Changes During Pregnancy.
During pregnancy, the changes are noticed in various organs, body weight, the metabolic activities and functional status of different physiological systems in the mother.
Structural Changes.
Various structural changes are noticed in the primary sex organs, accessory sex organs and in the mammary glands during pregnancy.
1. Ovaries.
[1]. Follicular changes do not appear in ovary and ovulation does not occur because the secretion of FSH and LH from anterior pituitary is inhibited.
[2]. Corpus luteum enlarges and secretes a large quantity of progesterone and little estrogen, which are essential for maintaining
the pregnancy. It continues for 3 months and then, corpus luteum degenerates.
[3]. By this time placenta develops fully and takes over the function of secreting estrogen and progesterone. It continues throughout the
period of pregnancy thus inhibiting the secretion of FSH and LH.
2. Uterus.
When the fetus grows, uterus undergoes changes in volume, size, shape and weight.
i. Volume .
Volume of uterus increases gradually due to fetal growth. From almost zero volume, uterus reaches about 5 to 7 liters at the end of pregnancy. Out of this, 50% of the volume is due to the fetus and rest is due to the placenta, amniotic fluid, etc.
ii. Size.
Size of the uterus also increases due to:
a. Hyperplasia (increase in number of cells) of myometrium.
b. Hypertrophy (increase in size of the cells) of myometrium.
c. Growth of fetus.
iii. Shape.
The shape of non-pregnant uterus is pyriform. As the fetus grows, at the 12th week of pregnancy, it becomes globular. Then, once again it becomes pyriform gradually.
iv. Weight.
Non-pregnant uterus weighs about 30 to 50 g. The weight increases as the pregnancy advances. At the end of pregnancy, the uterine weight increases to about 1,000 to 1,200 g.
v. Histological changes.
Endometrium shows formation of decidua, which is the bed for the fertilized ovum during the initial stages of pregnancy. Later, by the end of 3 months, three layers of decidua are formed:
a. Decidua basalis, which is the maternal part.
b. Decidua capsularis that surrounds fetal sac.
c. Decidua parietalis, which lines rest of uterine wall. After the 3rd month, the decidua capsularis and parietalis fuse together.
3. Vagina.
Vagina increases in size and its color changes to violet due to increased blood supply. There is deposition of glycogen in the epithelial cells.
4. Cervix.
In cervix, the number of glands, blood supply and mucus secretion increase. The tough cervix becomes soft and it is closed by mucus plug.
5. Fallopian Tube.
The number of epithelial cells and blood supply increase in fallopian tubes.
6. Mammary Glands.
Size of the mammary glands increases because of development of new ducts and alveoli, deposition of fat and increased vascularization. Pigmentation of nipple and areola occurs.
Changes in Body Weight During Pregnancy.
Average weight gained by the body during pregnancy is about 12 kg. Approximate weight of various structures, which adds to the weight gain:
1. Fetus : 3.5 kg.
2. Amniotic fluid : 2.0 kg.
3. Placenta : 1.5 kg.
4. Increase in maternal body weight : 5.0 kg
If proper prenatal care is not taken, the body weight increases greatly by about 20 to 30 kg.
Metabolic Changes During Pregnancy.
The metabolic activities are accelerated in the body due to increased secretion of various hormones like thyroxine, cortisol and sex hormones.
1. Basal Metabolic Rate.
Increase in the secretion of various hormones especially thyroxine increases the basal metabolic rate by about 15% in the later stages of pregnancy.
2. Protein Metabolism.
The anabolism of proteins increases during pregnancy. Positive nitrogen balance occurs. The deposition of proteins increases in the uterus.
3. Carbohydrate Metabolism.
[1]. Blood glucose level increases leading to glucosuria. Ketosis develops either due to less food or more vomiting.
[2]. Because of all these reasons, there is hyperplasia of beta cells of islets of Langerhans in pancreas leading to increase in secretion of insulin. Inspite of this, there is possibility of developing diabetes in pregnancy or latent diabetes after delivery.
4. Lipid Metabolism.
During pregnancy, there is deposition of about 3 to 4 kg of fat in the maternal body. It also increases the blood cholesterol level and ketosis.
5. Water and Mineral Metabolism.
[1]. Estrogen and progesterone are secreted by corpus luteum in the first trimester and by placenta later. These hormones increase the retention of sodium and water.
[2]. Secretion of aldosterone increases during pregnancy. Aldosterone in turn increases the reabsorption of sodium from renal tubules. Apart from water and sodium retention, there is retention of calcium and phosphorus as well.
[3]. Calcium and phosphorus are necessary for the growing fetus.
Physiological Changes in the body System.
1. Blood.
[1]. The blood volume increases by about 20% or about 1 L. This increase is mainly because of increase in plasma volume. It causes hemodilution.
[2]. Because of great demand for iron by the fetus, the mother usually develops anemia. It can be rectified by proper prenatal
care and iron replacement.
2. Cardiovascular System.
Cardiac output.
Generally, cardiac output increases by about 30% in the first trimester. After the 3rd month, cardiac output starts decreasing and reaches almost the normal level in the later stages of pregnancy.
Blood pressure.
Arterial blood pressure remains unchanged during the first trimester. During the second trimester, there is a slight decrease in blood pressure. It is due to the diversion of blood to uterine sinuses. And, hypertension develops if proper prenatal care is not taken.
Pre-eclampsia.
[1]. Pre-eclampsia is the hypertensive disorder of pregnancy. It is otherwise known as toxemia of pregnancy.
[2]. About 3% to 4% of the pregnant women suffer from this. It usually occurs during last trimester of pregnancy.
Cause for hypertension.
1. Release of vasoconstrictor substances from placenta.
2. Hypersecretion of adrenal hormones and other hormones, which cause vasoconstriction.
3. Development of autoimmune processes induced by the presence of placenta or fetus.
Other symptoms associated with hypertension.
1. Decreased blood flow to kidney and thickening of glomerular capillary membrane, leading to reduction in GFR and urinary output.
2. Retention of sodium and water.
3. Decreased urinary output along with retention of sodium and water results in increased extracellular fluid volume and edema.
4. Excretion of proteins through urine.
Eclampsia.
[1]. Eclampsia is the serious condition of pre-eclampsia characterized by severe vascular spasm, dangerous hypertension and convulsive muscular contractions almost like seizures.
[2]. It occurs just before, during or immediately after delivery. It leads to death, if timely treatment is not given.
Features of eclampsia.
1. Spasm of blood vessels.
2. Very severe hypertension.
3. Renal failure.
4. Liver failure.
5. Heart failure.
6. Convulsions.
7. Coma.
Treatment for eclampsia.
Treatment should be immediate. It includes administration of quick acting vasodilator drugs or termination of pregnancy.
3. Respiratory System.
Overall activity of respiratory system increases slightly. Tidal volume, pulmonary ventilation and oxygen utilization are increased.
4. Excretory System.
[1]. Renal blood flow and GFR increase resulting in increase in urine formation. It is because of increase in fluid intake and the increased excretory products from fetus.
[2]. The urine becomes diluted with the specific gravity of 1,025. In the first trimester, the frequency of micturition increases because of the pressure exerted by the uterus on bladder.
5. Digestive System.
[1]. During the initial stages of pregnancy, the morning sickness occurs in mother. It involves nausea, vomiting and giddiness. This is because of the hormonal imbalance.
[2]. The motility of GI tract decreases by progesterone and constipation is common. Indigestion and hypochlorhydria (decrease in the amount of hydrochloric acid in gastric juice) also occur.
6. Endocrine System.
a. Anterior pituitary action.
[1]. During pregnancy, the size of anterior pituitary increases by about 50%. And secretion of corticotropin, thyrotropin and prolactin increases.
[2]. However, the secretion of FSH and LH decreases very much. It is because of negative feedback control by estrogen and progesterone, which are continuously secreted from corpus luteum initially and placenta later on.
b. Adrenal cortex action.
[1]. There is moderate increase in secretion of cortisol, which helps in the mobilization of amino acids from the mother’s tissues to the fetus.
[2]. Aldosterone secretion also increases. It reaches the maximum at the end of pregnancy. Along with estrogen and progesterone,
aldosterone is responsible for the retention of water and sodium.
c. Thyroid gland action.
[1]. The size and the secretory activity of thyroid gland increase during pregnancy.
[2]. The increased secretion of thyroxine helps in the preparation of mammary glands for lactation. It is also responsible for increase in basal metabolic rate.
d. Parathyroid glands.
[1]. Parathyroid glands also show an increase in the size and secretory activity.
[2]. Parathormone is responsible for maintenance of calcium level in mother’s blood in spite of loss of large amount of calcium to fetus.
7. Nervous System.
[1]. There is general excitement of nervous system during pregnancy. It leads to the psychological imbalance such as change in the moods, excitement or depression in the early stages of pregnancy.
[2]. During the later months of pregnancy, the woman becomes very much excited because of anticipation of delivery of the baby, labor pain, etc.
Gestation Period.
[1]. Gestation period refers to the pregnancy period. The average gestation period is about 280 days or 40 weeks from the date of last menstrual period (LMP). Traditionally, it is calculated as 10 lunar months. However, in terms of modern calendar it is calculated as 9 months and 7 days.
[2]. If the menstrual cycle is normal 28 day cycle, the fertilization of ovum by the sperm occurs on 14th day after LMP. Thus the actual
duration of human pregnancy is 280 – 14 = 266 days.
[3]. If the pregnancy ends before 28th week, it is referred as miscarriage. If the pregnancy ends before 37th week, then it is considered as premature labor.
Parturition.
[1].Parturition is the expulsion or delivery of the fetus from the mother’s body. It occurs at the end of pregnancy. The process by which the delivery of fetus occurs is called labor.
[2]. It involves various activities such as contraction of uterus, dilatation of cervix and opening of vaginal canal.
Braxton Hicks Contractions.
[1]. Braxton Hicks contractions are the weak, irregular, short and usually painless uterine contractions, which start after 6th week of pregnancy. These contractions are named after the British doctor, John Braxton Hicks who discovered them in 1872.
[2]. It is suggested that these contractions do not induce cervical dilatation but may cause softening of cervix. Often called the practice
contractions, Braxton Hicks contractions help the uterus practice for upcoming labor. Sometimes these contractions cause discomfort.
Braxton Hicks contractions are triggered by several factors such as:
1. Touching the abdomen.
2. Movement of fetus in uterus.
3. Physical activity.
4. Sexual intercourse.
5. Dehydration.
False Labor Contractions.
While nearing the time of delivery, the Braxton Hicks contractions become intense and are called false labor contractions. The false labor contractions are believed to help cervical dilatation.
Stages of Parturition.
Parturition occurs in three stages:
First Stage.
[1]. First, the strong uterine contractions called labor contractions commence. Labor contractions arise from fundus of uterus and move downwards so that the head of fetus is pushed against cervix.
[2]. It results in dilatation of cervix and opening of vaginal canal. Exact cause for the onset of labor is not known. This stage extends for a variable period of time.
Second Stage.
In this stage, the fetus is delivered out from uterus through cervix and vaginal canal. This stage lasts for about 1 hour.
Third Stage.
During this stage, the placenta is detached from the decidua and is expelled out from uterus. It occurs within 10 to 15 minutes after the delivery of the child.
Mechanism of Labor.
[1]. The slow and weak contractions of uterus commence at about a month before parturition. Later, the contractions gradually obtain strength and finally are converted into labor contractions at the time of labor. Exact cause for the onset of labor contractions is not known.
[2]. It is strongly believed that the labor contractions are induced by the signal from fetus. And during labor, reflexes from uterus and cervix produce the powerful uterine contractions. Thus, uterus and cervix play an important role in labor. Many hormones are also involved during parturition.
Role of Uterus.
[1]. Once started, the uterine contractions cause the development of more and more strong contractions. That is, the irritation of uterine muscle during initial contraction leads to further reflex contractions. It is called positive feedback mechanism.
[2]. It plays an important role, not only in producing more number of uterine contractions but also the contractions to become more and more powerful.
Role of Cervix.
[1]. Cervix also plays an important role in increasing the strength of uterine contractions. When the head of fetus is forced against the cervix during the first stage of labor, the cervix stretches.
[2]. It causes stimulation of muscles of cervix, which in turn results in reflex contractions of uterus.
Role of Hormones.
Hormones involved in the process of parturition:
Maternal Hormones.
1. Oxytocin.
2. Prostaglandins.
3. Cortisol.
4. Catecholamines.
5. Relaxin.
Fetal Hormones.
1. Oxytocin.
2. Cortisol.
3. Prostaglandins.
Placental Hormones.
1. Estrogen.
2. Progesterone.
3. Prostaglandins.
a. Estrogen.
[1]. Estrogen is continuously secreted along with progesterone throughout the gestation period. However, in the later period, the quantity of estrogen released is much greater than that of progesterone.
Estrogen:
i. Increases the force of uterine contractions.
ii. Increases the number of oxytocin receptors in uterine wall.
iii. Accelerates the synthesis of prostaglandin from uterus.
b. Progesterone.
[1]. Progesterone plays an important role in labor indirectly by its sudden withdrawal at the end of pregnancy. Throughout the period of gestation, progesterone suppresses uterine contractions.
[2]. It also inhibits the synthesis of prostaglandin (PGE2), which is necessary for uterine contraction. Progesterone inhibits prostaglandin synthesis by inhibiting the release of the enzyme phospholipase A, which is essential for prostaglandin synthesis.
[3]. Sudden decrease in progesterone secretion at the end of gestation period increases the uterine contractions and PGE2 synthesis.
c. Oxytocin.
[1]. Oxytocin Causes contraction of smooth muscle of uterus and enhances labor. During the later stages of pregnancy, the number of receptors for oxytocin increases in the wall of uterus by the influence of estrogen. Because of this, the uterus becomes more sensitive to oxytocin.
[2]. Oxytocin Stimulates the release of prostaglandins in the decidua. Oxytocin is released in large quantity during labor. It
is due to neuroendocrine reflex.
[3]. During the movement of fetus through cervix, the receptors on the cervix are stimulated and start discharging a large number of
impulses.
[4]. Impulses are carried to hypothalamus by the somatic nerve fibers and result in the release of a large quantity of oxytocin, which enhances labor. The release of more amount of oxytocin occurs due to positive feedback.
d. Relaxin.
Relaxin is secreted from maternal ovary (corpus luteum) during the initial period of pregnancy. It is secreted in large quantity at the time of labor by placenta and mammary glands .
[1]. Relaxin helps labor by softening the cervix and loosening the ligaments of symphysis pubis, so that the dilatation of cervix occurs
[2]. Increases the number of receptors for oxytocin in the myometrium.
[3]. Simultaneously suppresses the inhibitory action of progesterone on uterine contraction so that the uterus starts contracting.
[4]. Facilitates the development of mammary glands.
e. Prostaglandins.
[1]. In recent times, prostaglandins are considered to play a vital role in labor. Prostaglandins particularly PGE2 facilitate labor by increasing the force of uterine contractions.
[2]. The prostaglandins are secreted from uterine tissues, fetal membranes and placenta. Their concentration is increased in maternal blood and amniotic fluid at the time of labor.
[3]. Prostaglandins increase the force of uterine contractions by elevating the intracellular concentration of calcium ions in the uterine muscles.
f. Catecholamines.
It is believed that the circulating adrenaline and noradrenaline also might increase the uterine contraction through alpha adrenergic receptors.
g. Cortisol.
[1]. At the time of labor, hypothalamus releases large quantity of corticotropin-releasing hormone, which increases the release of cortisol from the adrenal cortex.
[2]. Cortisol enhances the uterine contraction and plays an important role in helping the mother to withstand the stress during labor.
Thanks for Visiting us.
