• Reproductive System
  • Physiology

Physiological Changes In Pregnancy

  • Reading time: 6 minutes, 32 seconds
  • Reproductive System
  • 2020-07-05 04:34:12

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 i


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