Diploma in Midwifery
Physiology of pregnancy
Table of Contents
Physiology of pregnancy refers to the normal natural changes that occur in a woman’s body due to pregnancy. These changes result mainly from alterations in hormones and metabolism.
CHANGES IN THE ENDOCRINE SYSTEM
1. Hormonal Changes:
The placenta produces several hormones that cause a number of physiological changes.
Successful physiological adaptation during pregnancy is due to alterations in hormone production by the maternal endocrine system and the trophoblast (the outer layer of cells of the developing embryo that will later form part of the placenta).
2. Human Chorionic Gonadotropin (hCG):
Produced by the trophoblast.
hCG levels increase rapidly in early pregnancy, peaking at 8-10 weeks of gestation.
Main Function: Maintains the corpus luteum (a temporary structure in the ovary), ensuring the secretion of progesterone and estrogen until placental production is adequate (around 10-12 weeks).
hCG levels gradually decrease after 10-12 weeks and disappear completely about 2 weeks after birth.
3. Progesterone:
Primarily produced by the corpus luteum in early pregnancy and later by the placenta.
Functions:
Thickens the decidua (the lining of the uterus during pregnancy) to receive a fertilized ovum.
Increases glandular tissue and ducts in the breasts.
Promotes myometrial (uterine muscle) relaxation.
4. Estrogen:
Produced by the ovaries and later by the placenta.
Functions:
Stimulates the growth of the uterus.
Promotes the development of the duct system in the breasts.
Excreted in urine, and the amount present can indicate fetal well-being.
5. Relaxin:
Produced by the corpus luteum, decidua, and placenta.
Functions:
Acts on ligaments and joints in the last weeks of pregnancy, increasing the flexibility of the pelvis (“give”).
Promotes myometrial relaxation.
May play a role in cervical ripening (softening) and rupture of membranes.
6. Human Placental Lactogen (hPL):
Produced by the placenta.
Functions:
Stimulates breast growth.
Has lactogenic properties (promotes milk production).
Affects metabolic changes that ensure glucose is readily available for fetal growth and development, protecting against nutritional deficiencies.
7. Pituitary Hormones:
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are suppressed by high levels of estrogen and progesterone during pregnancy.
The adrenal gland increases slightly in size due to hypertrophy (enlargement) and widening in the glucocorticoid area, suggesting increased hormone secretion.
8. Thyroid Function:
The thyroid gland increases in size due to hyperplasia (increased cell number) of glandular tissue and increased vascularity.
There is an increased uptake of iodine during pregnancy, possibly to compensate for increased renal clearance of iodine, which can lead to reduced plasma iodine levels.
CHANGES IN THE REPRODUCTIVE SYSTEM
CHANGES IN THE UTERUS:
The uterus stretches and expands to accommodate and nurture the growing fetus. These changes primarily occur in the myometrium (the muscular wall of the uterus). The body of the uterus grows to provide a nutritive and protective environment for fetal development.
Uterine Muscle Layers:
1. Endometrium:
Menstruation stops.
Transforms into the decidua during pregnancy.
Becomes thick, soft, spongy, and richly supplied with blood.
2. Myometrium:
The enlargement of the uterine body is due to two factors:
Hypertrophy: Existing muscle fibers increase in length (about 10 times) and width (about 3 times).
Hyperplasia: New muscle cells develop and grow alongside the original muscle cells.
Size: The uterus grows from its normal size (approximately 7.5 cm long, 5 cm wide, and 2.5 cm thick) to about 30 cm long, 23 cm wide, and 20 cm thick. Its weight increases from about 60g to 960g.
Shape:
Pre-pregnancy: Pear-shaped.
12 weeks: Globular.
12-38 weeks: Oval.
After 38 weeks (with lightening): Returns to a more globular shape.
Muscle Layers of the Myometrium:
Outermost Longitudinal Layer: Begins in the anterior wall of the upper uterine segment, passes over the fundus, and down the posterior wall. Contraction and retraction of this layer help expel the fetus during labor.
Middle Oblique Layer: Muscle fibers are arranged in a crisscross pattern, surrounding blood vessels in a figure-of-eight. After placental separation and expulsion, these fibers compress blood vessels, helping to prevent postpartum hemorrhage. They are sometimes called “living ligatures.”
Inner Circular Layer: The weakest layer. Muscle fibers run transversely around the uterus and are more developed around the cervix, lower uterine segment, and fallopian tubes. They contribute to cervical dilatation.
3. Perimetrium:
The layer of peritoneum that partially covers the uterus.
It is reflected over the bladder anteriorly, forming the utero-vesical pouch, and posteriorly, forming the pouch of Douglas.
After 12 weeks, the uterus rises out of the pelvis and becomes an abdominal organ. It loses its anteverted and anteflexed position, becoming more erect and leaning slightly to the right.
CLINICAL OBSERVATIONS OF THE GROWING UTERU
Uterine Growth and Adaptations
12 Weeks:
The uterus transitions from a pelvic organ to an abdominal organ, becoming palpable just above the symphysis pubis.
It adopts a more upright position, losing its initial anteverted and anteflexed orientation.
Its size is comparable to that of a large grapefruit.
16 Weeks:
The fundus (top portion) of the uterus assumes a dome shape.
As the uterus ascends, it rotates slightly to the right (dextrorotation), a phenomenon attributed to the presence of the rectosigmoid colon on the left side of the pelvis.
This growth exerts tension on the broad and round ligaments, which support the uterus.
The expanding conceptus (embryo/fetus and associated tissues) causes the isthmus (lower segment) to open, contributing to a more globular uterine shape.
20 Weeks:
The uterine fundus reaches the level of the umbilicus.
The uterus exhibits a more rounded shape.
30 Weeks:
The fundus is located midway between the umbilicus and the xiphoid process (the lower tip of the sternum).
The growing uterus displaces the intestines laterally and superiorly.
The abdominal wall supports the uterus, maintaining alignment between the long axis of the uterus and the pelvic inlet.
When the woman lies on her back (supine position), the uterus presses against the vertebral column, aorta, and inferior vena cava.
36 Weeks:
The uterus nearly fills the abdominal cavity, with the fundus reaching the tip of the xiphoid cartilage.
38-40 Weeks:
The lower uterine segment softens and thins further.
The uterus becomes more rounded, and the fundal height decreases.
This descent of the presenting part, known as lightening, often provides some relief from upward pressure.
Changes in Blood Supply
Uterine blood vessels enlarge, and new vessels develop under the influence of estrogen.
Blood flow through the uterine and ovarian arteries increases significantly, reaching about 750 ml/min at term to meet the demands of the growing uterus and functioning placenta.
Changes in the Fallopian Tubes
The fallopian tubes become elongated and more vascular.
The uterine end of each tube is typically closed, while the fimbriated end remains open.
Changes in the Isthmus
The isthmus softens and lengthens from approximately 7 mm to 23 mm, forming the lower uterine segment in late pregnancy.
Changes in the Ovaries
Follicle-stimulating hormone (FSH) production ceases due to elevated estrogen and progesterone levels secreted by the ovaries and corpus luteum. This prevents ovulation and menstruation.
The ovaries are displaced upward as the uterus enlarges.
Both ovaries enlarge due to increased vascularity and become edematous, especially the one containing the corpus luteum.
The corpus luteum enlarges in early pregnancy and may even form a cyst. It produces progesterone, which is essential for maintaining the endometrial lining.
The corpus luteum functions until around 10-12 weeks of pregnancy when the placenta takes over progesterone and estrogen production. After this point, the corpus luteum gradually shrinks and its function declines.
Changes in the Cervix
The cervix remains tightly closed during pregnancy, protecting the fetus and resisting pressure from above when the woman is upright.
Slight cervical growth occurs, and it becomes softer due to increased vascularity and the relaxing effects of hormones.
Under the influence of progesterone, racemose glands in the cervix secrete thick, viscous mucus that fills the cervical canal, forming a protective barrier against infection (the operculum).
Towards the end of pregnancy or at the onset of labor, the cervix becomes part of the lower uterine segment (a process called effacement).
The external os (opening) of the cervix may admit a fingertip. A short, softened cervix that admits a fingertip at term is considered “ripe.”
Changes in the Vagina
The vaginal muscle layer hypertrophies (enlarges), and the vaginal capacity increases, becoming more elastic to accommodate delivery.
The vaginal epithelium thickens, and there is increased shedding of superficial cells, contributing to a white vaginal discharge called leukorrhea.
These epithelial cells contain high levels of glycogen. They interact with Döderlein’s bacilli (normal vaginal flora) to create a more acidic environment. This provides some protection against certain organisms but increases susceptibility to others, such as Candida albicans (yeast).
The vagina becomes more vascular and may appear violet in color.
Changes in the Vulva
The vulva may appear bluish due to increased vascularity and pelvic congestion.
Breast Changes
Early Pregnancy: Breasts may feel full or tingly. They increase in size as pregnancy progresses.
Nipples: Become more erectile.
Areola: Darkens, and the diameter increases.
Montgomery’s Tubercles: Sebaceous glands around the areola enlarge and protrude. They secrete sebum to lubricate the breast during pregnancy and breastfeeding.
Superficial Veins: Become more visible, appearing as a bluish tint due to increased circulation.
Colostrum: A clear, sticky fluid (colostrum) may be expressed from the nipples after the first trimester. It later becomes yellowish.
Changes in the Cardiovascular System
Heart: The heart hypertrophies, particularly the left ventricle, due to increased workload. The growing uterus displaces the heart upward and to the left. Heart sounds may change, and murmurs are common.
Cardiac Output: Increases by 40%.
Heart Rate: Increases by an average of 15 beats per minute.
Stroke Volume: Increases from about 64 ml to 71 ml.
Effect on Blood Pressure
First Trimester: Blood pressure remains relatively constant.
Second Trimester: Blood pressure tends to decrease due to the vasodilatory effects of progesterone, reaching its lowest point at 16-20 weeks.
Third Trimester: Blood pressure gradually returns to first-trimester levels.
Supine Hypotensive Syndrome: Lying flat on the back (supine position) should be avoided in later pregnancy, as the weight of the uterus can compress the inferior vena cava, reducing venous return to the heart and causing a drop in blood pressure.
Venous Return: Poor venous return in late pregnancy can contribute to edema in the lower limbs, varicose veins, and hemorrhoids.
Blood Flow
Blood flow increases to the uterus, kidneys, breasts, and skin but not to the liver or brain.
Uteroplacental blood flow increases by 10-15%, reaching about 75 ml per minute at term.
Renal blood flow increases by 70-80%.
Blood Volume
Total blood volume increases steadily from early pregnancy, reaching a maximum of 35-45% above non-pregnant levels. This increase varies depending on the woman’s size, number of previous pregnancies, and whether the pregnancy is singleton or multiple.
Reasons for Increased Blood Volume:
Provide adequate blood flow for placental circulation.
Supply the extra metabolic needs of the fetus.
Increase perfusion to the kidneys and other organs.
Compensate for blood loss during delivery.
Counterbalance the effects of increased venous and arterial capacity.
Plasma Volume and Red Cell Mass
Plasma volume increases by 40%, while red cell mass increases by only 20%. This disproportionate increase leads to hemodilution (physiological anemia of pregnancy).
These changes begin around 6-8 weeks of pregnancy.
The acceptable hemoglobin (Hb) level during pregnancy is 11-12 g/dl.
Iron Metabolism
Pregnancy requires approximately 1000g of additional iron:
500g for increased red cell mass
300g for the fetus
200g for daily iron loss
Only about 20% of ingested iron is absorbed in normal pregnancy.
Iron supplementation aims to prevent iron deficiency anemia, not necessarily to raise Hb levels.
Plasma Protein
During the first 20 weeks of pregnancy, plasma protein concentration decreases due to increased plasma volume.
This leads to lower osmotic pressure, contributing to edema (swelling) in the lower limbs, often seen in late pregnancy.
Moderate edema in the absence of other medical conditions is considered physiological.
Clotting Factors
Levels of fibrinogen and factors 7, 8, 9, and 10 increase, shortening the coagulation time from 12 to 8 minutes.
This increased clotting capacity helps prevent postpartum hemorrhage after placental separation.
White Blood Cells
White blood cell count increases slightly during pregnancy, from around 7,000/mm³ to 10,500/mm³, and may reach 16,000/mm³ during labor.
The increase is primarily due to a rise in neutrophils, polymorphonuclear leukocytes, monocytes, and granulocytes, which are active phagocytes (cells that engulf and destroy foreign particles).
The rise in white blood cells begins around 8 weeks, peaking at 30 weeks of gestation.
Erythrocytes
Erythrocyte (red blood cell) count decreases from about 4.5 million/mm³ to 3.7 million/mm³.
Hemoglobin (Hb)
Hb concentration falls from around 14 g/dl, which is considered a physiological change.
A high Hb level during pregnancy could indicate an underlying medical condition.
RESPIRATORY SYSTEM
Basal Metabolic Rate (BMR): Increases.
Tidal Volume: The volume of air entering and leaving the lungs during normal breathing increases slightly due to increased oxygen consumption by the fetus and the increased workload of the maternal heart and lungs.
Late Pregnancy: The ribs flare out, limiting the expansion of the thoracic cavity. The enlarging uterus elevates the diaphragm and compresses the lower lobes of the lungs.
CHANGES IN THE URINARY SYSTEM
Renal Blood Flow and Glomerular Filtration Rate (GFR): Increase by 50%.
Frequency of Micturition: Common in early and late pregnancy due to hormonal changes (early) and the pressure of the enlarging uterus on the bladder (late).
Ureters: Become elongated and kinked due to progesterone, leading to urine stagnation and an increased risk of urinary tract infections (UTIs).
CHANGES IN THE GASTROINTESTINAL TRACT (GIT)
Gums: May become edematous, soft, spongy, and prone to bleeding.
Salivation: Increased salivation (ptyalism) is common.
Nausea and Vomiting: Common in up to 70% of pregnancies.
Taste: Changes in taste, sometimes described as metallic, may occur.
Pica: Craving for unusual substances like soil or plaster.
Appetite: Often increased.
Heartburn: Common due to the relaxation of the cardiac sphincter and upward pressure from the growing uterus.
Motility: Reduced gastrointestinal motility can lead to constipation.
Changes in Metabolism
Metabolic Rate: Increased to provide nutrients for both the mother and the fetus.
Maternal Weight: Weight gain is a normal and expected part of pregnancy, reflecting fetal growth.
First 20 Weeks: Approximately 4 kg gain (0.2 kg/week ).
Last 20 Weeks: Approximately 8.5 kg gain (0.4 kg/week ).
Total: Approximately 12.5 kg weight gain during pregnancy.
Factors Influencing Weight Gain During Pregnancy
Several factors contribute to weight gain during pregnancy, including:
Fluid Retention (Edema): The accumulation of excess fluid in the body’s tissues, known as edema, can contribute to increased weight.
Maternal Metabolic Rate: The mother’s metabolic rate, which is the rate at which her body burns calories, can influence weight gain. A higher metabolic rate may lead to greater energy expenditure and potentially less weight gain.
Dietary Intake: The quantity and quality of food consumed by the mother play a crucial role in weight gain. A balanced and nutritious diet supports healthy weight gain during pregnancy.
Gastrointestinal Issues: Frequent vomiting or diarrhea can interfere with nutrient absorption and overall caloric intake, potentially leading to weight loss or inadequate weight gain.
Amniotic Fluid Volume: The amount of amniotic fluid surrounding the fetus contributes to overall weight. An increased volume of amniotic fluid may result in higher weight gain.
Fetal Size: The size and growth rate of the developing fetus influence maternal weight gain. A larger fetus generally corresponds to greater weight gain.
Physical Activity Level: The mother’s level of physical activity and exercise can impact weight gain. Regular exercise can help manage weight during pregnancy.
Genetic Predisposition: Genetic factors can influence an individual’s tendency to gain or retain weight during pregnancy.
Changes in the Musculoskeletal System
During pregnancy, the hormones progesterone and relaxin cause relaxation of the pelvic ligaments, joints, and muscles. This relaxation allows the pelvis to expand, increasing its capacity to accommodate the presenting part of the fetus as term approaches and during labor. This process of pelvic expansion is known as “give”. The symphysis pubis and sacroiliac joints soften, and the mother’s gait may change as her balance is altered by the weight of the growing uterus.
Changes in the Integumentary System (Skin)
Pregnancy leads to several noticeable changes in the skin:
Increased Pigmentation: Increased activity of melanin-stimulating hormone (MSH) from the pituitary gland causes varying degrees of skin darkening (hyperpigmentation) in pregnant women, starting from the end of the second month and continuing until term. This is either due to increased sensitivity of melanocytes (pigment-producing cells) to the hormone, or a greater number of melanocytes in those areas. Commonly affected areas include:
Areola of the breasts
Abdominal midline
Perineum
Axilla (armpits)
Linea Nigra: A dark line that develops from the umbilicus to the symphysis pubis and may extend up to the sternum. This is also caused by hormone-induced pigmentation. After delivery, the linea nigra gradually fades but may not completely disappear.
Mask of Pregnancy (Chloasma or Melasma): Brownish hyperpigmentation that appears on the face, particularly the forehead, nose, cheeks, and neck. It is more noticeable in women with darker complexions.
Striae Gravidarum (Stretch Marks): These are caused by the stretching and thinning of the skin as the body grows during pregnancy.
Increased Sweat Gland Activity: Sweat gland activity generally increases throughout the body, causing pregnant women to perspire more profusely.
Increased Body Temperature: A slight rise in body temperature (around 0.5°C or 0.9°F) and increased blood supply cause vasodilation (widening of blood vessels), making pregnant women feel warmer.