Reproductive System

Physiological Changes In Pregnancy

  • Physiology
  • Reproductive System
  • 2020-07-04 11:34:12
  • 10 minutes, 42 seconds

Physiological Changes In Pregnancy

During pregnancy, major changes have to be made in the various organ systems of the mother to adapt to the new demands. Initially, the demands are minimal; however, as the fetus grows, the demands greatly increase.

These physiological changes in pregnancy aim to maximize nutrition and oxygen to the developing fetus and help the maternal system adjust to the extra stress.

Lack of appreciation of this difference may lead to inappropriate management of clinical problems in obstetrics.

The major maternal physiological adaptation to pregnancy

First, changes occur in the reproductive organs and breasts.

Second, the metabolic functions are increased to provide sufficient nutrients to the fetus.

Third, hormones secreted by the placenta produce their own effects.

Major physiological changes in pregnancy occur in the following systems.

    • Reproductive system changes
    • Breasts changes
    • Weight gain
    • Hematological changes
    • cardiovascular system changes
    • Metabolic changes
    • Respiratory changes
    • Urinary system changes.
    • Endocrine changes.
    • Reproductive organs

The uterus

There is an enormous growth of uterus during pregnancy. non-gravid uterus weighs about 50gm and measures about 7.5 cm in length. At term weighs 900-1000gm and measures 35cm in length. The ratio of muscle to connective tissue increase from the lower part of the uterus to the fundus (the upper part).

In early pregnancy uterine growth results from both hyperplasia and hypertrophy while later hypertrophy accounts for most of the increase.

The muscle fibers are arranged in three layers-outer longitudinal, inner circular and intermediate arranged in a criss-cross fashion through which blood vessels run.

As the pregnancy advances the uterus is divided into upper and lower uterine segments. The lower uterine segment is composed of the lower parts of the uterus and the upper cervix composed mainly from connective tissue. Because of this the lower uterine segment becomes stretched in late pregnancy.

The cervix

The cervix becomes softer and swollen in pregnancy with the result columnar epithelium lining cervical canal becomes exposed to vaginal secretion.

Oestradiol stimulates the growth of columnar epithelial of the cervical canal so it becomes violet and is called ectropine.

The mucus gland becomes distended and secretes mucus which forms a mucous plug that is expelled in labor as the show.

Prostaglandins and collagenase, especially in the last weeks of pregnancy act on collagen fiber, make the cervix softer.

The vagina

The vaginal mucosa becomes thicker during pregnancy.

The vaginal discharge during pregnancy is increased due to increased desquamation of the superficial vaginal mucosal cells.

The pH becomes more acidic (3.5-6) due to more conversion of glycogen into lactic acid by the lactobacillus as a result of high estrogen. The acidic pH prevents the multiplication of pathogenic organisms.

The breast changes in pregnancy

These changes are best evident in a primigravida.
Increase in breast size due to marked hypertrophy and proliferation of the ducts (estrogen) and the alveoli (estrogen and progesterone).

Nipples and areola become larger, erectile and deeply pigmented.
Sebaceous glands in the areola become hypertrophied (Montgomery's tubercles).

The breast becomes active as early as the 12th week. The demonstration of secretion from the breast of a woman who has never lactated is an important sign of pregnancy.

Estrogen leads to an increase in the number of glandular ducts and progesterone leads to a proliferation of glandular epithelium of the alveoli.
Prolactin leads to the active secretion of milk after birth.

Changes on The Skin

Chloasma gravidarum or pregnancy mask is an extreme form of pigmentation around the cheek, forehead and around the eyes.

Linea nigra-brownish black pigmented area in the midline stretching from the xiphisternum to symphysis pubis. These changes are probably due to melanocyte-stimulating hormone from the anterior pituitary.

Stria gravidarum- slightly depressed linear marks with varying length and breadth. This occurs as a result of mechanical stretching of the skin.

Weight gain in pregnancy

The total weight gain during the course of a singleton pregnancy for a healthy woman averages 10-12 kg.

The distribution is 1kg in 1st trimester, 5kg each in 2nd and 3rd trimester. During this time of pregnancy there is variable amount of retention of electrolytes as a result of increased estrogen and progesterone.

There is increased retention of water to the tune of 6.5 liters

Rapid gain of more than 0.5kg a week or more than 2kg a month in later months of pregnancy may be an early manifestation of pre-eclampsia.

Stationary of falling weight may suggest intrauterine growth retardation or intrauterine death of the fetus.

Volume changes

The sex hormones(estrogen) and adrenocortical (aldosterone)hormones produced during pregnancy cause the mother to gain weight by fluid retention.

Fluid retention is the most fundamental systemic changes in normal pregnancy. The total blood volume is increased during pregnancy 30%. The most marked expansion occurs in extracellular volume (ECV) with some increase in intracellular water.

There is an increase in the number of red blood cells(by bone marrow), as well as plasma, in the blood. As a result, the blood volume increases as much as 1 liter .
This, in turn, increases the amount of blood pumped by the heart (CO) by 30%.

At the time of the birth of the baby, the mother has about 1 to 2 liters of extra blood in her circulatory system. At the time of labor, the mother loses about 200–300 mL of blood.

The changes in the hormonal secretion soon after delivery bring the fluid levels close to normal.
The total amount of electrolytes (ions such as sodium, potassium, and calcium) in the blood is increased.

The factors contributing including:

  • Increase in sodium retention.
  • A decrease in plasma osmotic pressure.
  • A decrease in the thirst threshold.
  • Resetting of osmostate.
  • Decrease in plasma oncotic pressure.
  • Hematological ( blood) changes.
  • The marked increase in plasma volume associated with normal pregnancy causes dilution of many circulating factors.

Hematological changes

Decrease in:

  • red cell count.
  • hemoglobin concentration.
  • haematocrit.
  • plasma folate concentration.

Increase in :

  • White cell count.
  • Erythrocyte segmentation rate .
  • Fibrinogen concentration.
  • Blood volume –markedly increased, to maximum of 40-50% above the nonpregnant level at 30-32 weeks.
  • Plasma volume-increases
  • RBC and hemoglobinin- RBC volume increased to the extent of 20-30%. Reticulocyte count increases by 2%.

Disproportionate increase in plasma and RBC volume produces a state of hemodilution during pregnancy.

The relative hemodilution results in diminished blood viscosity that ensures optimum gaseous exchange between the maternal and fetal circulation.

The increased blood volume protects the mother against the adverse effects of blood loss during pregnancy.

There is increase in neutrophil count.

Blood coagulation factors- pregnancy is hypercoagulable state. Fibrinogen level is raised by 50% from 200-400mg% to 300-600mg.

Cardiovascular changes:

Earliest changes is peripheral vasodilatation resulting in decreased systemic vascular resistance→ ↑CO 6 L/ min.   Max. (22-28)weeks.

Heart rate increase (10-20%).

Stroke volume increase (10%).

Cardiac output increase (30-50%).

Mean arterial blood pressure decrease (10%).-

Peripheral resistance decrease (35%).-

Normal changes in heart sounds during pregnancy:

  • Increase loudness of both S1 & S2.
  • More than 95% of women develop systolic murmur which disappears after delivery.
  • 20% have a transient diastolic murmur.
  • 10% develop continues murmur due to increase mammary blood flow.
  • Ectopics
  • Relative tachycardia
  • Collapsing pulse

Supine Hypotension Syndrome (Postural Hypotension)

Late in pregnancy the gravid uterus produces a compression effect on the inferior vena cava  when the patient is in supine position. This  may result in serious curtailment of venous return of the heart producing hypotension, tachycardia and syncope.

The normal pressure is quickly  restored by turning the patient to lateral position.

Respiratory changes

The respiratory rate increases together with the volume of air inhaled with every breath (tidal volume).

Flaring out of the ribs and increased movement of the diaphragm initiates this increase in volume.

The anterior-posterior and transverse diameter of the chest increases by about 2 cm (0.8 in).

It is also believed that the high levels of progesterone during pregnancy increase the minute ventilation even more, because progesterone increases the respiratory center’s sensitivity to carbon dioxide.

As the fetus grows and occupies more space in the abdominal cavity late in pregnancy, the mother’s breathing relies more on the movement of the ribs than the diaphragm.

The diaphragm is elevated by about 4 cm (1.6 in) as a result of the abdominal contents, as well as the flaring out of the ribs.

The changes in hormonal secretion often alter the caliber of the bronchi and, sometimes, mothers prone to asthma feel better during pregnancy.

All these changes result in an increased intake of oxygen with improved supply to the fetus and a decreased level of carbon dioxide, which enables easier transfer of carbon dioxide from fetal to maternal blood.

Increased oxygen tension and reduced carbon(IV) oxide pressure with compensatory ↓HCO3(mild compensated respiratory alkalosis).

ventilatory changes:

Thoracic anatomy changes.

Tidal volume increases.

Vital capacity increase.

Functional residual capacity decrease.

The urinary tract and renal function

The kidney increases in length by about 1 cm (0.4 in).
As a result of the increase in waste production, the fluid filtered in the kidney (glomerular filtration) is significantly increased. (50%)

In early and late stages of pregnancy, there is frequency of micturition.
In early pregnancy, the enlarging uterus is still in the pelvis, compressing the bladder.
In late pregnancy, the fetal head descends into the pelvis, irritating the bladder.

The ureters appear to be dilated and the sphincter between the bladder and the ureter is more relaxed, resulting in reflux of urine into the ureter and predisposing the individual to urinary tract infection.
As a result of fluid retention, the total urine volume excreted is less than that of nonpregnant individuals.

Clearance of most substances is enhanced.

Plasma creatinine ,urea,urate are reduced.

Glycosuria is normal.

Alimentary system changes

The gums becomes spongy.

The increasing levels of progesterone tend to reduce the motility of the gut and relaxation of sphincters. As a result, relaxation of the sphincter in the lower end of the esophagus can produce regurgitation of food into the esophagus from the stomach and cause heartburn.

Toward the later part of pregnancy, the growing fetus exerts pressure on the gut, reducing the capacity for large meals.
The slower movement in the small intestine may aid better absorption of nutrients, while the slower activity of the large intestines aids better absorption of water.
However, the latter may be responsible for the constipation often experienced by pregnant women.

Endocrinological changes:

Prolactin concentration increases markedly but act after delivery.

Human growth hormone is suppressed .

Insulin resistance develop.

Thyroid function changes little.

Trans placental calcium transport is enhanced.

Corticosteroid concentration increased.

Aldosterone concentration increased.

Angiotensin and renin increased

Hormones produced within the uterus

Human chorionic gonadotrophin (HCG): This hormone is secreted by trophoblast and can be detected in serum 10 days after conception (RIA). There is a high level of circulating HCG in early pregnancy (to provide a suitable environment for implantation and development).

To support corpus luteum secretion of estrogen and progesterone in the first trimester until the placenta becomes able to produce these hormones. The peak level normally occurs in the 12th week.

A constant level of HCG in late pregnancy is useful in controlling the placental secretion of Estrogen progesterone and suppressing the maternal immune system against the fetus.

The human chorionic gonadotrophin normally disappears from urine 7-10 days after the delivery of placenta.

human placental lactogen. This hormone is secreted by syncytiotrophoblast. Its levels increase when the level of HCG starts to drop .

HPL has no effect on the fetus but has an effect on :

  1. The breast:
    • Mammary growth during pregnancy.
    • Produce of colostrums.
    • Milk production during lactation
  2. proteins:
    • HPL stimulates protein synthesis at cellular level.

3. carbohydrate:

    1. Stimulate insulin secretion.
    2. Inhibit insulin action.


HPL mobilize fat from body store (lipolysis) lead to increase maternal blood glucose and maternal tissue can not utilize the glucose so the glucose will be available for fetus.


Estrogen is produced by corpus luteum in early pregnancy. it is produce by placenta in late pregnancy.

Role of estrogen:

On connective tissue: estrogen leads to polymerization of mucopolysaccharides of the ground substance leads to loose connective tissue mainly in the cervix.

On the protein: estrogen stimulates directly RNA synthesis leads to protein synthesis.


Its production same as estrogen. progesterone has an effect on smooth muscle leads to a decrease in muscle excitability leads to muscle relaxation mainly in the uterus.

Thyroid function

During pregnancy there is:

  • An increase thyroid-binding globulin.
  • Increase bound form of T3,T4.
  • No change in free form of T3, T4.

So no evidence to support what previously thought to be physiological such as increase in the size of the thyroid gland, increase BMR, body temperature, heart rate.

Metabolic changes

Total metabolism is increased due to the needs of the growing uterus and the fetus. Basic metabolic rate is increased to the extent of 30% higher than that of the nongravid women.

The metabolism increases in proportion to the weight gain.
Other than supplying the fetus with its needs, the energy is utilized for the resultant increase in heart rate, respiratory rate, and liver function.
The increase in metabolism is aided by the increase in thyroid hormone secretion caused by the thyroid-stimulating hormone from the pituitary gland.
Often, the thyroid gland hypertrophies as a result and may appear enlarged in about 70% of individuals.


Daniel Ogera

Medical educator, passionate about simplifying difficult medical concepts for easier understanding and mastery by nursing and medical students.

Article details

About this article:
  • Topic:Physiology
  • Duration:10 minutes, 42 seconds
  • Subtopic:Reproductive System

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