Normal Physiology of Pregnancy

Introduction

Respiratory System

• The maternal respiratory system needs to adapt to optimise maternal and fetal oxygenation and to facilitate the removal and clearance of CO₂ from the fetus.
• Many women complain of shortness-of-breath (SOB) during pregnancy. As the abdomen grows, the diaphragm elevates 4cm and the ribs are pushed up and out. This creates a decreased expiratory reserve volume , and although this is physiologically balanced out by an increased tidal volume this still gives the sensation of being SOB. Normally good explanation and reassurance is all that is needed.
• Box 1 shows how lung function tests change in pregnancy, note that the respiratory rate remains unchanged, but the tidal volume increases, thought to be a result of increased progesterone levels.
• Box 2 shows how blood gasses change in pregnancy. If we remember how diffusion works, substances move from an area of high concentration to an area of low concentration. The reason why the CO₂ falls in the mother is so that CO₂ is drawn out of the fetal blood into the mothers, where it can be breathed out in the lungs. You would have thought that the opposite would apply and that there would be an equally large rise in the oxygen levels in the maternal blood, however this is not necessary as the haemoglobin in the fetus (fetal haemoglobin) has a higher oxygen carrying capacity than adult haemoglobin.  Remember because the bicarbonate levels also decrease, pregnancy gives a state of COMPENSATED RESPIRATORY ALKOLOSIS.
• There are some changes in the lungs during pregnancy that can be observed on a chest x-ray (although extreme care must be taken during the x-ray and it is very rare that one is performed on a pregnant lady). Firstly there may be an increase in the amont of vascular soft tissues visible. Secondly small pleural effusions may be visible, especially soon after delivery of the baby.

Cardiovascular System

1. The fetus grows rapidly and has a huge oxygen consumption
2. Many maternal tissues undergo hypertrophy and again need more oxygen
3. The muscular work of the mother increases to support and move the additional weight.

• Progesterone decreases vascular resistance, which lowers blood pressure. Therefore Cardiac output  increases by 30-50%. Blood pressure does, however return to normal later in pregnancy, cardiac output does not, and in fact increases by another third during labour!
• As a result of the drop in blood pressure, the renin-angiotensin system is activated, which causes Na+ and H2O retention. This increases blood volume by 40%. However most of the increased blood volume is plasma, and as such many pregnant women are described as having a physiological anaemia.
• As well as this there is constriction of the peripheral circulation. This could explain why some pregnant ladies complain of symptoms of Raynaud’s Syndrome.
• In pregnant ladies there may often be a systolic ejection murmur, as well as a third heart sound which is the ventricles filling with blood.
• As the diaphragm raises, the heart rotates up and out, which gives changes not only in x-rays but in ECGs. It is important to note that these changes can be seen in completley healthy pregnant ladies, and before ordering any test you should make sure the person doing/interpreting the test knows that the patient is pregnant.
• Pregnancy also increases the chance of varicose veins forming.

Urinary System

• The kidneys increase in size by 1cm
• In early pregnancy, renal blood flow increases by 40%. The raised glomerular filtration rate (GFR) leads to increased reabsorption and as such plasma urea and creatinine levels are reduced. The increased GFR also leads to increased levels of aldosterone being secreted to maintain adequate fluid balance. One substance that is not fully readsorbed however is glucose, and as such 15% of normal pregnant women have some degree of glycosuria.
• Increased progesterone levels relax the smooth muscle in the bladder and ureteric walls. As a result of this the ureters elongate and become both dilated and tortuous. Unfortunately this can lead to urine stasis, increasing the risk of urine tract infections (UTIs).
• One of the noticeable features of pregnancy is the increased frequency of urination. In the first part of pregnancy this is all down to increased urine production, however as pregnancy progresses and the uterus gets larger, mechanical pressure on the bladder plays a larger and larger part.

Dermatological Changes

• Increased oestrogens may lead to Palmar erythema and Spider Angioma formation.
• Increased oestrogens combined with increased levels of melanocyte stimulating hormone often leads to hyperpigmentation of the umbilicus, nipples, face and linea nigra.

Musculoskeletal System

• Shifts in posture and gait are often observed as additional weight of pregnancy goes on.
• Another consequence of the additional abdominal weight is that lower back strains are common.
• The increased progesterone levels soften critical ligaments, so that bones may separate slightly during labour. The best example of this is the pubic symphsis.

Gastrointestinal Tract

• Nausea (morning sickness) occurs in more than 70% of pregnancies. It normally resolves after 16-18 weeks.
• It has already been mentioned that progesterone  relaxes smooth muscles, and the smooth muscle in the GI tract is no exception. This leads to decreased gastric motility (therefore constipation) and reflux.
• The decreased smooth muscle activity in the gallbladder increases the risk of gallstone formation. Cholesterol stones are most likely to form.
• Because the mothers body is trying to provide the fetus with a constant supply of glucose, pregnancy also increases the chance of a diabetic state developing.

Genital Tract

• Before pregnancy, the uterus weighs 100g, and at term it weighs 1100g. The huge increase in weight is achieved by hypertrophy, not hyperplasia of the myometrial cells. The hypertrophy occurs up to 20 weeks after conception, after that the increase in uterus size is purely stretching from the fetus and surrounding fluid.
• To go with the increase in weight, there is an increase in blood flow to the uterus, the vast majority (85%) of which goes to the placenta.
• In later pregnancy oestrogens cause the cervix to become softer and erosions begin to form in it. The uterine ligaments lengthen and thicken and there is a marked increase in vaginal secretions.

Immune System

Endocrinology

References

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