Ovulation Pregnancy And Parturition Notes

Pregnancy Introduction

Ovum is released from the Graafian follicle of the ovary into the abdominal cavity at the time of ovulation. From the abdominal cavity, the ovum enters one of the fallopian tubes via the fimbriated end. The fimbriated end of the fallopian tube is placed near and around the ovary.

The inner surface of the fimbriated end of the fallopian tube has ciliated epithelium. The cilia of these cells beat continuously and facilitate the entry of the ovum into the fallopian tube.

Fertilization Of The Ovum

Fertilization refers to the fusion (union) of male and female gametes (sperm and ovum) to form a new offspring. The ovum of matured follicles in the ovary is in the primary oocyte stage with a diploid number (23 pairs) of chromosomes.

Read And Learn More: Medical Physiology Notes

• Just before ovulation, meiotic division takes place in the ovum. The primary oocyte divides into a secondary oocyte and a first polar body. The first polar body is expelled. The secondary oocyte contains only 23 chromosomes (haploid).
• The remaining 23 chromosomes are lost in the expelled first polar body. Thus, when the ovum is released into the abdominal cavity during ovulation, it is in the secondary oocyte stage with a haploid number of chromosomes.
• From the abdominal cavity, the ovum enters one of the fallopian tubes through the fimbriated end. The cilia that are present in the inner surface of the fimbriated end beat towards the lumen of the fallopian tube and facilitate the entry of the ovum.
• If sexual intercourse occurs at this time and semen is ejaculated in the vagina, the sperms travel through the vagina and uterus to reach the fallopian tube. The sperms reach the ovarian end of fallopian tube within 30-60 minutes.
• The movement of the sperm through the uterus is facilitated by the antiperistaltic contractions of uterine muscles. The uterine contractions are induced by oxytocin which is secreted from the posterior pituitary by the neuroendocrine reflex during sexual intercourse. The uterine contraction is also facilitated by prostaglandin (PGE2) present in male seminal fluid.
• Among 200-300 million of sperms entering the female genital tract, only a few thousand sperms reach the spot near the ovum. Among these few thousand sperm, only one succeeds in fertilizing the ovum.
• 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 the acrosome of sperm.
• The proteolytic enzymes from the acrosome of the successful sperm diffuse through the structures of zona pellucida and inactivate the other sperms entering the ovum.
• Immediately after fertilization, the ovum which is in the secondary oocyte stage divides into a matured ovum and a second polar body. The second polar body is expelled.
• The nucleus of the matured ovum becomes a female pronucleus with 23 chromosomes which include 22 autosomes and one sex chromosome called the X chromosome.
• Simultaneously, the head of the sperm swells and becomes a male pronucleus. Then 23 chromosomes of the sperm and 23 chromosomes of the ovum arrange themselves to reform the 23 pairs of chromosomes in the fertilized ovum.

Sex Chromosomes And Sex Determination

Sex Chromosomes

All the dividing cells in the body have 23 pairs of chromosomes. Among the 23 pairs, 22 pairs are called somatic chromosomes or autosomes. The remaining pair of chromosomes are called sex chromosomes. Sex chromosomes are X and Y chromosomes.

Sex Determination

• Sex chromosomes are responsible for sex determination. During fertilization of the ovum, 23 chromosomes from the ovum and 23 chromosomes from the sperm unite together to form the 23 pairs (46) of chromosomes in the fertilized ovum. Now, sex determination occurs.
• The ovum contains the X chromosome. Sperm has either an X chromosome or a Y chromosome. When the ovum is fertilized by a sperm with an X chromosome, the child will be female with XX chromosome.
• And, if the ovum is fertilized by a sperm with a Y chromosome, the sex of the child will be male with an XY chromosome. So, the sex of the child depends upon the male partner. The role of testosterone in sex differentiation is explained.

Implantation

Implantation is the process by which the fertilized ovum implants (fixes itself or gets attached) in the endometrial lining of the uterus.

• After fertilization, the ovum is known as a zygote. The zygote takes three to five days to reach the uterine cavity from the fallopian tube. While traveling through the fallopian tube, the zygote receives its nutrition from the secretions of the fallopian tube.
• After reaching the uterus, the developing zygote remains freely in the uterine cavity for two to four days before it is implanted. Thus, it takes about one week for implantation after the day of fertilization.
• During the stay in the uterine cavity before implantation, the zygote receives its nutrition from the secretions of the endometrium, which is known as uterine milk.
• Just before implantation, the zygote develops into a morula, and then the implantation starts. A layer of spherical cells called trophoblast cells is formed around the morula. The trophoblast cells release proteolytic enzymes over the surface of the endometrium.
• These enzymes digest the ceils of the endometrium. Now, the morula moves through the digested part of the endometrium and implants itself.

Development Of Placenta And Embryo

Already uterus is prepared by progesterone secreted from the corpus lutem during the secretory phase of the menstrual cycle. Under the influence of progesterone, the endometrial stromal cells change into large swollen cells with large amounts of proteins, glycogen, lipids, and minerals.

• When implantation occurs, there is a further increase in the thickness of the endometrium. It is because of the continuous secretion of progesterone from the corpus luteum. At this stage, the endometrial stromal cells are called decidual cells and the endometrium at the implanted area is called decidua.
• Now the trophoblastic cells of the morula develop into cords which are attached to a decidual portion of the endometrium. Blood capillaries grow into these cords from the blood vessels of the newly formed embryo.
• On about the 16th day after fertilization, the heart of the embryo starts pumping blood into the trophoblastic cords. At the same time, blood sinusoids develop around the trophoblastic cords. These sinusoids receive blood from the mother.
• The trophoblastic cells form some vascular projections into which fetal capillaries grow. These vascular projections become placental villi. Thus, the final form of the placenta has got the fetal part and the maternal part.
• The fetal part of the placenta contains the two umbilical arteries which carry fetal blood to the placental villi through the capillaries. The blood returns back to the fetus through the umbilical vein.
• The maternal part of the placenta is formed by uterine arteries through with blood flows into sinusoids that surround the villi. The blood returns back to the mother’s body through the uterine vein. The functions of the placenta are described in detail.

Maternal Changes During Pregnancy

During pregnancy, changes are noticed in various organs, body weight, 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 mammary glands during pregnancy.

1. Ovanes

• Follicular changes do not appear in the ovary and ovulation does not occur because the secretion of FSH and LH from the anterior pituitary is inhibited. The Corpus luteum enlarges and secretes a large quantity of progesterone and a little estrogen, which are essential for maintaining the pregnancy.
• It continues for three months and then, the corpus luteum degenerates. 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. Utems

When the fetus grows in the uterus, changes occur in the volume, size, shape, weight, and histology of the uterus:

1. Volume: Fetal growth increases the volume of the uterus gradually. From almost zero volume, the uterus reaches about 5-7 liters at the end of pregnancy. Out of this, 50% of the volume is due to the fetus and the rest is due to the placenta, amniotic fluid, etc
2. Size: The size of the uterus also increases. It is due to:
   • Hyperplasia (increase in the number of cells) of the myometrium
   • Hypertrophy (increase in the size of the cells) of the myometrium
3. Growth of fetus.
4. Shape: The shape of a nonpregnant uterus is pyriform. As the fetus grows, at the 12th week of pregnancy, it becomes globular. Then, once again it becomes pyriform gradually
5. Weight: The nonpregnant uterus weighs about 30-50 g. The weight increases as the pregnancy advances. At the end of pregnancy, the uterine weight alone increases to about 1,000-1,200
6. Histological changes: Endometrium shows the 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:

• • Decidua basalis which is the maternal part
  • Decidua capsularis that surrounds the fetal sac
  • Decidua parietalis which lines the rest of the uterine wall.

After the third month, the decidua capsularis and parietal are fused together.

3. Vagina

Vaginal changes during pregnancy:

1. increase in size
2. Violet coloration due to increased blood supply
3. Less cornification of superficial epithelial cells
4. Increase in the deposition of glycogen in the epithelial ceils due to increased secretion of estrogen
5. Decrease in pH below 3.5.

4. Cervix

Changes in the cervix during pregnancy:

1. Increase in the number of cervical glands
2. Hypertrophy of the endocervix which gives the appearance of a honeycomb
3. Increase in blood supply
4. Increase in mucus secretion
5. Softening of the tough cervix
6. Formation of mucus plug which closes the cervical canal.

5. Fallopian Tube

Changes in the fallopian tube during pregnancy:

1. Hyperplasia of epithelial cells
2. Increase in blood supply
3. The movement of fallopian tubes upwards is caused by the large-sized uterus.

6. Mammary Glands

The changes in breasts during pregnancy are due to the activity of estrogen and progesterone, which are secreted in large quantities.

The changes in mammary glands:

1. Development of more new ducts
2. Formation of more alveoli
3. Deposition of fat
4. Increase in the size
5. Increase in vascularization
6. Pigmentation of nipple and areola.

Increase In Body Weight

The average weight gained by the body during pregnancy is about 12 kg. The 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: 5.0 kg

If proper prenatal care is not taken, the body weight increases greatly by about 20-30 kg.

Metabolic Changes

The metabolic activities are accelerated in the body due to the increased secretion of various hormones like thyroxine, cortisol, and sex hormones.

Basal Metabolic Rate

An increase in the secretion of various hormones especially thyroxine increases the basal metabolic rate by about 15% in the later stages of pregnancy.

Protein Metabolism

The anabolism of proteins increases in pregnancy. Positive nitrogen balance occurs. The deposition of proteins increases in the uterus.

Carbohydrate Metabolism

• Blood glucose level increases. Hepatic glycogen is depleted and glucosuria occurs. Ketosis develops either due to less food or more vomiting.
• Because of all these reasons, there is hyperplasia of beta cells of islets of Langerhans in the pancreas leading to increased insulin secretion. In spite of this, there is the possibility of developing diabetes during pregnancy or latent diabetes after delivery.

Lipid Metabolism

During pregnancy, there is deposition of about 3-4 kg of fat in the maternal body. It also increases the blood cholesterol level and ketosis.

Water and Mineral Metabolism

• Estrogen and progesterone are secreted by the corpus luteum in the first trimester and by the placenta later. These hormones increase the retention of sodium and water. The 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. Calcium and phosphorus are necessary for the growing fetus.

Changes In Physiological Systems

1. Blood

The blood volume increases by about 20% or about 1 liter. This increase is mainly because of an increase in plasma volume. It causes hemodilution. Because of the 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. It is mainly because of an increase in the rate and force of contraction of the heart and blood volume. After the 3rd month, the cardiac output starts to decrease and reaches almost the normal level in the later stages of pregnancy.

Blood pressure

The 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

Pre-eclampsia is the hypertensive disorder of pregnancy. It is otherwise known as toxemia of pregnancy. About 34% of pregnant women suffer from this. It usually occurs during the last trimester of pregnancy. There is a sudden elevation of arterial blood pressure.

The exact cause for this disorder is not yet clear. Three factors are suggested to cause the elevation of blood pressure:

1. Release of some vasoconstrictor substances from the placenta
2. Hypersecretion of adrenal hormones and other hormones, which cause vasoconstriction
3. The development of some autoimmune processes is induced by the presence of the placenta or fetus.

Hypertension is associated with other symptoms like:

1. Decreased blood flow to the kidney and thickening of glomerular capillary membrane leading to a 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

Eclampsia is a serious condition of preeclampsia. It is characterized by severe vascular spasms, dangerous hypertension, and convulsive muscular contractions almost like seizures. It occurs just before, during, or immediately after delivery. It leads to death if timely treatment is not given. Treatment includes:

Features of eclampsia

1. Spasms of blood vessels
2. Very severe hypertension
3. Renal failure
4. Liver failure
5. Heart failure
6. Convulsions
7. Coma.

Treatment for eclampsia

The treatment should be immediate. It includes administration quick acting vasodilator drugs or termination of pregnancy.

3. Respiratory System

The overall activity of the respiratory system increases slightly. Tidal volume, pulmonary ventilation, and oxygen utilization are increased.

4. Excretory System

Renal blood flow and GFR increase resulting in an increase in urine formation. It is because of an increase in fluid intake and the increased excretory products from the fetus. The urine becomes diluted with a specific gravity of 1025. In the first trimester, the frequency of micturition increases because of the pressure exerted by the uterus on the bladder.

5. Digestive System

• During the initial stages of pregnancy, the morning mother. It involves nausea, vomiting and giddiness. This is because of the hormonal imbalance.
• The motility of the Gl 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

Anterior pituitary

• During pregnancy, the size of the anterior pituitary increases by about 50%. And, secretion of corticotropin, thyrotropin, and prolactin increases.
• 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 the corpus luteum initially and the placenta later on.

Adrenal cortex

There is a moderate increase in the secretion of cortisol, which helps in the mobilization of amino acids from the mother’s tissues to the fetus. 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.

Thyroid gland

Tht. and the secretory activity of the thyroid gland increase during pregnancy. The increased secretion of thyroxine helps in the preparation of mammary glands for lactation. It is also responsible for the increase in basal metabolic rate.

Parathyroid glands

Parathyroid glands also show an increase in size and secretory activity. Parathormone is responsible for the maintenance of calcium levels in the mother’s blood in spite of the loss of a large amount of calcium to the fetus.

Nervous System

There is general excitement in the nervous system during pregnancy. It leads to psychological imbalances such as changes in moods, excitement, or depression in the early stages of pregnancy. During the later months of pregnancy, the woman becomes very much excited because of anticipation of the delivery of the baby, labor pain, etc.

Gestation Period

The gestation period refers to the pregnancy period. The sickness occurs in the average gestation period is about 280 days or 40 weeks

• from the date of the last menstrual period (LMP). Traditionally it is calculated as 10 lunar months. However, in terms of the modern calendar, it is calculated as 9 months and 7 days. If the menstrual cycle is a normal 28-day cycle, the fertilization of the ovum by the sperm occurs on the 14th day after the last menstrual period.
• Thus the actual duration of human pregnancy is 280 – 14 = 266 days. If the pregnancy ends before the 28th week, it is referred to as a miscarriage, if the pregnancy ends before the 37th week, then it is considered premature labor.

Parturition

Parturition refers to 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 a fetus occurs is called labor. It involves various actions, like contraction of the uterus, dilatation of the cervix, and opening of the vaginal canal.

Braxton Hicks Contractions

Braxton Hicks contractions are weak, irregular, short, and usually painless uterine contractions that start after 6th week of pregnancy. These contractions are named after the British doctor, John Braxton Hicks who discovered them in 1872.

It is suggested that their contractions do not induce cervical dilatation but may cause softening of the 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 a fetus in the 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. False labor contractions are believed to help cervical dilatation.

Stages Of Parturition

Parturition occurs in three stages:

First Stage

First, the strong uterine contractions called labor contractions commence. The labor contractions arise from the fundus of the uterus and move downwards so that the head of the fetus is pushed against the cervix. It results in the dilatation of the cervix and opening of the vaginal canal. The exact cause of 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 from the uterus through the cervix and vaginal canal. This stage lasts for about one hour.

Third Stage

During this stage, the placenta is detached from the decidua and is expelled from the uterus. It occurs within 10-15 minutes after the delivery of the child.

Mechanism Of Labor

• The slow and weak contractions of the uterus commence about a month before parturition. Later, the contractions gradually obtain strength and finally are converted into labor contractions at the time of labor. The exact cause of the onset of labor contractions is not known.
• It is strongly believed that labor contractions are induced signals from the fetus. And during labor, reflexes from the cervix produce the powerful uterine contract- Terus itself plays an important role in the labor cervix. Many hormones are also involved during parturition.

Role Of Uterus

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 a positive feedback mechanism. It plays an important role, not only in producing more uterine contractions but also in the contractions become more and more powerful.

Role Of Cervix

The cervix also plays an important role in increasing the strength of uterine contractions. When the head of the fetus is forced against the cervix during the first stage of labor, the cervix stretches. It causes stimulation of muscles of the cervix, which in turn results in reflex contractions of the uterus.

Role Of Hormones

Many hormones are 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.

Estrogen

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:

1. Increases the force of uterine contractions
2. Increases the number of oxytocin receptors in the uterine wall
3. Accelerates the synthesis of prostaglandin iron; uterus.

Progesterone

• 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. 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.
• A sudden decrease in progesterone secretion at the end of the gestation period increases uterine contractions and PGE2 synthesis.

Oxytocin

Oxytocin:

1. Causes the contraction of the smooth muscle of the 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. Stimulates the release of prostaglandins in the decidua.

Oxytocin Is mloasod In largo quantity during labor. It Is duo to nouroondocrlno roflox. During tho movement of fotus through corvlx, tho receptors on the cervix are stimulated and start discharging a largo number of Impulses.

The Impulses are carried to hypothalamus by tho somatic norvo fibers and result In tho release of a largo quantity of oxytocin, which enhances labor. The roloaso of moro amount of oxytocin occurs due to positive feedback.

Relaxin

Relaxin Is socrelod from maternal ovary (corpus luteum) during tho initial period of pregnancy. It is secreted in largo quantity at tho litne of labor by placenta and mammary glands.

Rolaxin:

1. Helps labor by softening the cervix and loosening the ligaments of pubis symphysis, 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.

Prostaglandins

• in lot cut times, prostaglandins are considered to play a vital role in labor. Prostaglandins particularly PGE2 facilitate labor by increasing the force of uterine contractions.
• 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.
• Prostaglandins increase the force of uterine contractions by elevating the intracellular concentration of calcium ions in the uterine muscles.

Catecholamines

It is believed that the circulating adrenaline and noradrenaline also might increase the uterine contraction through alpha adrenergic receptors.

Cortisol

At the time of labor, hypothalamus releases large quantity of corticotropin releasing hormone, which increases the release of cortisol from the adrenal cortex. Cortisol enhances the uterine contraction and, plays an important role in helping the mother to withstand the stress during labor.