Physiological Changes during pregnancy

Pregnancy comes with many physiological and anatomical, biochemical, systemic or local changes in the body in order to meet the needs of the developing fetus in the uterus.  These changes tend to maintain a healthy environment for the fetus without compromising the mother’s health. They also help to prepare for the process of delivery and care of the newborn.

These changes are caused by hormones involved in pregnancy. However,  most of the systemic changes return to  pregravid state 6 weeks after delivery.

I will discuss these changes system by system for detailed information and easier understanding as proper understanding helps to distinguish normal changes from

coincidental disease processes:

Reproductive system:


Prior to pregnancy, uterus is a small, almost solid, pear shaped organ about 7.5x5x2.5cm and weighing about 60 g, volume 10ml. At the end of pregnancy, it measures about 28x24x21cm and weighs approximately 1100g at term. 

During pregnancy, the uterus is divided into two functional portions either above or below the isthmus and are called  upper and lower uterine segments respectively.

 Its capacity increases from about 10ml to 5,000ml (5litres) or more at term. 

These are due to enlargement (hypertrophy) of the myometrial cells due to estrogen and distension by the growing fetus and build up of new cells via hyperplasia .

The upper part,fundus and body become the upper uterine segment.

 Fibrous tissue in the muscle increases markedly.

Oviducts (fallopian tubes) are lifted with the uterus as it grows. 

The enlarging uterus, placenta and growing fetus requires additional blood flow.

So by the end of pregnancy, one sixth of the total maternal blood volume is contained in the vascular system of the uterus. 

Braxton Hicks contractions also occur in the uterus intermittently Throughout pregnancy. They are irregular painless contractions that may be felt through the abdominal wall from about the fourth month. 

In later months, Braxton Hicks contractions become uncomfortable and may be confused with true labor contractions.


Estrogen stimulates the glandular tissue which increases in cell number and becomes hyperactive.

Gravid cervix is soft and purple while the non-gravid cervix is firm and pink. 

The endocervical gland secretes thick, sticky mucus that forms the mucous plug, which seals the endocervical canal and prevents the ascent of the organism into the uterus. The mucuous plug   is expelled when cervical dilatation begins, known as “show”.

Lower part of the uterus,  cervix and isthmus  become the lower uterine segment.

The hyperactive glandular tissue also increases the normal physiological mucorrhea resulting in profuse discharge.

Increased cervical vascularity also causes both the softening of the cervix (Goodell’s sign) and its bluish discoloration (Chadwick’s sign).


They stop producing oval during pregnancy, but the corpus luteum continues to produce hormones until about weeks 6-8 gestation.

Progesterone secreted by the corpus luteum maintains the endometrium until about the seventh week of pregnancy when the placenta assumes the task. Then the corpus luteum begins to disintegrate slowly.


Oestrogen causes the thickening of the vagina/mucosa, a loosening of the connective tissue and an increase in vagina secretion, known as leukorrhea of pregnancy .

Vagina  increases in capacity and length secondary to the hypertrophy of the lining epithelium and muscle layer.

Increased glycogen content in the wall secondary to the effect of estrogen. This may favours yeast infections.

The secretions of vagina during  pregnancy are white, thick and acidic (PH 3-5 to 6.0). The acid PH prevents infection but favours yeast organisms. Thus, the pregnant woman is more susceptible to monilial infection.

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The supportive connective tissue of the vagina loosen throughout pregnancy that by the end of pregnancy, the vagina and perineal body have relaxed enough to permit the passage of the infant.

Increased vascularity (blood flow) may show the same purple/bluish colour (Chadwick’s sign) as the cervix. Osiander’s sign: which is the pulsation of fornices.


Changes in breast are noticed as early as 4-6 weeks of pregnancy especially in primigravida. There is an increased vascularity of the breasts in pregnancy. 

Oestrogen and progesterone cause many changes in the breasts. The breasts enlarge and become more modular as the gland increases in size and number in preparation for lactation.

Nipples become more erectile and the areola pigmentation(darkens). There is also a prominent superficial vein. 

Montgomery’s follicles (sebaceous glands) enlarge. Striae (reddish stretch marks) may develop.

Colostrums, an antibody yellow secretion leak or expressed during last trimester. Colostrum is present from the 16th week of pregnancy. Few days after childbirth, colostrums gradually convert to mature milk.

Change in sexual desire during pregnancy Most women experience sexual desire changes at least to some extent during pregnancy. Fear that coitus would result in early labour and loss of desires due to the increased level of oestrogen

During the early pregnancy most women report decrease in libido because of nausea, fatigue, and breast tenderness that follow the first trimester of pregnancy. While during the second trimester, as blood flow to the pelvic area increases to supply the placenta, libido and sexual enjoyment drastically increases. During the third trimester, sexual drive may remain high or it may decline due to difficulty in finding a comfortable position and increasing abdominal size.

Changes in Respiratory System during pregnancy:

There is an increased oxygen requirement. Volume of air breathed increase 30% to 40%. Progesterone decreases airway resistance, leading to progressive increase in oxygen consumption(15-20% above non pregnant level by term), increased carbon dioxide production.

The enlarging uterus presses upward and elevates the diaphragm and the lungs as well.Breathing changes from abdominal to thoracic due to elevated diaphragm.

Capillary dilatation occurs in the respiratory route (Nasopharynx, larynx, trachea, bronchi) which makes breathing difficult through the nose.

Nasal stuffiness and nose bleeds (epistaxis) because of estrogen induced edema and vascular congestion of the nasal mucosa.

Lower ribs flare out and do not fully return to its normal position after delivery.

Functional respiratory changes occur in which there is a slight increase in respiratory rate. These changes include  

50% increase in minute ventilation, 40% increase in minute tidal volume and there is also increased blood flow.

 Cardiovascular changes in pregnancy:

Blood volume increases in the first and second trimester and slows in the third trimester.

Blood flow increases to organ systems with an increased workload.

Cardiac output begins to increase early in pregnancy.

Pulse rate may increase and systemic blood pressure declines slightly.  

There is little change in SBP but DBP decreases by 5-10 mmHg from 12-26 weeks, then increases to  pregravid level by term.

Venous pressure: No change in the upper body but increases in the lower extremities.

Decrease venous return to the heart. The enlarging uterus puts pressure on pelvic and femoral vessels –interfering  with returning blood flow, causing stasis of blood in the lower extremities leading to leg edema; varicose veins of the legs vulva and rectum (hemorrhoids) in late pregnancy.

 When the woman lies supine, the uterus may press on the vena cava reducing blood flow to the right atrium lowering blood pressure, causing dizziness, pallor and clamminess, may also press on the aorta, this condition is known as supine hypotensive syndrome or vena cavalry syndrome or aortocaval syndrome, corrected by the woman lying on her side or pillow under the hip.

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Plasma’s volume increases (50%) more than erythrocyte increase (30%), thus haematocrit level (measures red blood cells in plasma) decreases, this decrease referred to physiologic anaemia of pregnancy (pseudo-anaemia).

Hematological system during pregnancy :

– Increase in blood volume – most striking change

– The change occurs until term and the average increase in volume is 45-50%

– The mechanism for increasing the volume of blood is not properly understood (aldosterone related factors during pregnancy may contribute to this effect) increase water and salt retention.

– RBC increased by 33%

– Iron need increases because of increase in red blood cell mass. This is why Iron supplementation is necessary during pregnancy.

– WBC total count usually increase

– Platelets increase in production

– Clotting factors – Several factors increase- F- I, F-VIII mainly

– To lessen extent, F-VII, IX, X and XII

– Decrease- F- XI, F-XIII

Changes in gastrointestinal system during pregnancy:

Nausea and vomiting are common in the first trimester because of elevated human chorionic gonadotropin (HCG) level and discharged carbohydrate metabolism.

Nutritional requirements including  vitamins and minerals are increased; usually mother’s appetite increases too.

Pregnant women tend to rest more often, conserving energy and thereby enhancing fetal nutrition.

Gum tissue becomes hypertrophic,  soften, hyperemic  and bleed easily because of  increased systemic estrogen.

Saliva secretion may increase and even become excessive (ptyalism).

Gastrointestinal mobility:Delayed or slowed gastric emptying and decreased peristalsis due to relaxing effect of progesterone thereby decreased the hormone motline  that stimulates smooth muscles in  gastrointestinal tract.  

The woman complains of bloating and constipation due to increased water absorption caused by oestrogen.

The enlarging uterus displaces the stomach upwards. The anatomical position of small and large intestines as well as appendix will shift  due to this enlarging uterus.

The cardiac sphincter also relaxes and heartburn (pyrosis) may occur due to reflux of acidic secretion into the lower oesophagus. 

Enlarging uterus, slower emptying time, increased intragastric pressure, acidity and  gastric reflux.This makes heart burn to be common in pregnancy.

Hemorrhoids may occur due to constipation and pressure on vessels.

Gallbladder: She is predisposed to gallstone formation due to prolonged emptying time of the gallbladder because  of decreased motility resulting from the relaxing effect of progesterone.The stasis of bile will subsequently lead to gallstone formation and infection. This stasis of bile is gestational cholestasis and usually resolves by itself once the period of pregnancy is over, i.e. postpartum.

 Liver:No morphological changes but functional changes.Decreased plasma protein (albumen). a globulin (synthesized by liver) increases serum alkaline phosphatase activity.

Changes in urinary tract system during pregnancy:

Each kidney increases in length and weight. During the first trimester, the enlarging uterus is still a pelvic organ and presses against the bladder, producing urine frequency.

The enlarging  uterus displaces the bladder upward and anteriorly thereby increases pressure which in turn causes urinary urgency and frequency. The progesterone and relaxin cause the bladder muscles called detrusors to relax and thereby increase its capacity.

 During the first trimester, the symptoms decrease when the uterus becomes an abdominal organ and pressure against the urinary bladder lessens.

Urinary frequency reappears in the third trimester when the presenting part descends into the pelvis and again presses on the bladder.

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Renal pelvis and ureters elongate and dilate leading to stasis of urine and subsequently increased risks for urinary tract infection and stone formation.

Renal function:Changes occur due to increased maternal and placental hormones in order to handle both waste products of metabolism generated by mother and the fetus.

(ACTH, ADH, cortisol, etc.) increase plasma volume. Glomerular Filtration Rate increases by 50% (beginning early and lasts up to term).

Renal blood flow rate increases by 20-25% (early to mid-trimester) after the end of the second trimester remains constant.

Urine volume does not increase although glomerular filtration rate increases because of reabsorption.

Creatinine and BUN decrease due to their increased clearance rate.

Glycosuria is not necessarily as normal. 

Proteinuria changes little during pregnancy.

Changes in Skin and Hair (Integumentary system) during pregnancy

Changes occur in skin pigmentation caused by increased estrogen, progesterone and melanocyte levels.

Pigmentation increases in areas that are already hyperpigmented; the areola, the nipples, vulva and perineal areas.

The skin in the subumbilical midline of the abdomen known as may develop a pigmented line called linea nigra (extends from the pubic area to the umbilicus or higher).

Facial(forehead, cheek) chloasma (Melasma gravidarum) also known as the “Mask of pregnancy” may develop darkening of the skin over the forehead and around the eyes; fades after child birth.

Sweat and sebaceous glands are hyperactive due to increased blood supply to the skin and hence pregnant women sweat profusely. 

Striae gravidarum or stretch marks may appear on the abdomen, thighs, buttocks and breast.

The enlarging abdomen causes stretch on collagen fibers of the skin and effect of ACTH.

Changes in musculoskeletal during pregnancy

The joints  of the pelvis relax during pregnancy because of hormonal influence, relaxin and oestrogen.

The result is often a waddling gait. Lumbar spinal curve increases leading to lordosis and posture changes.

Pressure of the enlarging uterus on the abdominal muscle may cause the rectus abdominis muscle to separate producing diastasis recti.

If the separation is not regained postpartally, subsequent pregnancies will not have adequate support leading to pendulous abdomen.

Metabolic changes in pregnancy

Most metabolic functions increase during pregnancy because of the increased demand of the growing fetus.

Weight gain-adequate nutrition and weight gain is important.

The average pattern of weight is 1.6 to 2.3kg in the 1st trimester and 5.5 to 6.8kg during each of the last trimester.

Extra water is needed for the fetus, the placenta, amniotic fluid and the mother’s increased blood volume, and enlarged organs.

Protein and carbohydrate needs equally increase.

Changes in endocrine system during pregnancy

The thyroid gland is enlarged.

Anterior pituitary gland produces follicle-stimulating hormones that stimulate the ovum and luteinizing hormones that bring about ovulation.

Prolactin is responsible for initial lactation.

Posterior pituitary gland secretes vasopressin and oxytocin

Vasopressin causes basic instruction that results in increased BP.

Oxytocin promotes uterine contraction and stimulates ejection of milk from the breasts (the letdown reflex) in the postpartum period.

Insulin needs are increased and the pancreatic islets of Langerhans are stressed to meet this increased demand. Hence, the woman may show signs of gestational diabetes.

Central nervous system:

Psychological instability or emotional imbalance often accompany pregnancy. Anxiety, fear and even depression are equally seen in pregnancy sometimes.  This is caused by circulating pregnancy hormones.