Physiology of the Mother

1. Endocrine System
2. Cardiovascular System
3. Renal System
4. Respiratory System
5. Musculoskeletal System
6. Gastrointestinal System
7. Immune System
8. Quiz

There are many changes that occur in a woman’s body during pregnancy, known as the maternal physiological adaptations of pregnancy. These adaptations include:

• Endocrine - increased amounts of progesterone, oestrogen and relaxin (a hormone that loosens ligaments) throughout pregnancy cause a wide range of changes. Increased oxytocin and prolactin affect breast milk production and let-down (the release of milk when the baby feeds). Thyroid stimulating hormone (TSH) decreases in the first trimester (the first three months) because human chorionic gonadotrophin (hCG) stimulates the TSH receptors on the thyroid. This raises thyroxine, which suppresses TSH by negative feedback.
• Cardiovascular - in pregnancy, plasma volume increases proportionally more than red blood cell mass, causing a reduction in haematocrit and the physiological anaemia of pregnancy. Blood pressure falls in the first and second trimesters, while heart rate and cardiac output rise. The left ventricle adapts with a mild hypertrophy (an increase in muscle mass). Venous blood can back up later in pregnancy, causing varicose veins (swollen, twisted veins) and haemorrhoids (swollen veins in and around the anus).
• Renal - dilation of the blood vessels in the kidneys increases renal blood flow and the glomerular filtration rate (GFR). The higher GFR leads to increased creatinine clearance (faster removal of creatinine from the blood) and a lower normal serum creatinine in pregnancy, and contributes to urinary frequency (needing to pass urine more often).
  • Pressure from the growing uterus on the ureters, and progesterone relaxing the smooth muscle of the ureters, cause physiological hydronephrosis and hydroureter (widening of the kidney’s urine-collecting system and of the ureters). The resulting urinary stasis (slow-moving urine) increases the chance of urinary tract infections in pregnancy. Kidneys also release more renin (which leads to salt and water retention), erythropoietin (which stimulates red blood cell production) and calcitriol (the active form of vitamin D) in pregnancy.
• Respiratory - progesterone release causes an increase in tidal volume, and a decreased pCO₂ (partial pressure of carbon dioxide in the blood), causing a mild respiratory alkalosis (the blood becomes slightly less acidic than normal). Peak flow (the fastest rate at which air can be blown out) should remain the same during pregnancy.
• Musculoskeletal - progesterone and relaxin release loosens the ligaments of the sacroiliac joints (between the sacrum and the pelvic bones, at the back) and the pubic symphysis (the joint at the front of the pelvis) to aid vaginal birth. This can cause a waddling gait (way of walking) and pain at the pubic symphysis.
• Gastrointestinal - pregnancy can cause constipation, bloating, reflux and heartburn. Pica, cravings for non-food substances, may also develop.
• Immune - production of TH1 cytokines falls and production of TH2 cytokines rises. Cytokines are signalling proteins: those from T helper 1 (TH1) cells drive cell-mediated immunity, and those from T helper 2 (TH2) cells favour antibody responses. This immune modulation (adjusting the immune response rather than switching it off) contributes to relative maternal immune tolerance of the fetus.

There are many changes that occur in a woman’s body during pregnancy, in most of the systems that exist in the body. This article details the maternal physiological adaptations of pregnancy.

Endocrine System

There are many hormonal changes which occur in pregnancy.

In early pregnancy, the corpus luteum (the structure left in the ovary after ovulation) produces oestrogen and progesterone until the placenta takes over in hormone production. Inhibin is also released to inhibit FSH (follicle stimulating hormone) release, to prevent more follicles (fluid-filled sacs in the ovary, each containing an egg) from maturing. With in vitro fertilisation (IVF), fertility hormones stimulate the ovaries to produce several eggs, which are then collected from the follicles with a needle. Corpora lutea (the plural of corpus luteum) still form, but after this ovarian stimulation they may make too little progesterone. Progesterone is therefore commonly given until placental hormone production is established. This is called luteal phase support (the luteal phase is the part of the cycle when the corpus luteum is active).

The different hormones each have specific effects during pregnancy:

• Progesterone - its main action is to relax smooth muscle. This dilates blood vessels and causes some common symptoms of pregnancy, such as constipation and heartburn (see the Gastrointestinal System section).
• Oestrogen - breast tissue growth, water retention and increased protein synthesis.
• Relaxin - softens the cervix (the neck of the uterus) and pelvic ligaments for birth.
• Oxytocin - major role in uterine contractions in labour, and is responsible for milk let down.
• Prolactin - stimulates breast development in pregnancy and promotes/maintains lactation postpartum (after birth).
• Thyroid Stimulating Hormone (TSH) - levels decrease in the first trimester due to high hCG (the hormone that is used in pregnancy testing). The hCG stimulates TSH receptors, which increases thyroxine (T4) levels and so reduces TSH by negative feedback.

Cardiovascular System

Pregnancy is a high volume state: blood volume increases by about 45%, which is roughly 1.2 to 1.6 litres more than before pregnancy. Both plasma volume and total red-cell mass increase, but plasma expands proportionally more. The resulting dilution lowers haemoglobin concentration and haematocrit (the proportion of blood volume occupied by red cells), despite the increase in total red-cell mass. This causes the physiological anaemia of pregnancy. This physiological anaemia is a normal response to pregnancy but can cause classical anaemia symptoms such as fatigue. Iron supplementation such as ferrous sulphate (an iron tablet) is only indicated if iron deficiency is present, as physiological anaemia of pregnancy alone does not require treatment.

Even though there is a higher volume of blood during pregnancy, blood pressure decreases. This is because several hormones, including oestrogen, relaxin and progesterone, cause dilation (widening) of blood vessels. This decreases vascular resistance, which decreases the afterload on the heart (the resistance the heart pumps against). To compensate, cardiac output rises by about 40%, mainly through a larger stroke volume and partly through an increase in heart rate. Blood pressure is lowest in the first and second trimesters and rises back to non-pregnant levels in the third trimester. New high blood pressure (140/90 mmHg or above) after 20 weeks could be a sign of pre-eclampsia: new high blood pressure with protein in the urine or other new problems, such as kidney or liver involvement (check our article on diseases of the mother in pregnancy).

Heart rate increases in pregnancy by approximately 10 to 20 beats per minute. This is done to increase cardiac output, needed because there is increased volume of blood which in itself increases preload further contributing to cardiac output. This leads to a mild hypertrophy of the left ventricle which is normal in pregnancy and will return to normal after pregnancy. This can lead to a number of changes, including a physiological S3 beat of pregnancy, an added heart sound due to rapid ventricular filling in early diastole associated with increased blood volume, and ECG changes. These changes can include ventricular ectopic beats, small Q wave and inverted T wave in lead III and a left axis deviation due to the hypertrophy. The apex beat also shifts as the diaphragm is displaced by the fetus.

The gravid uterus (the uterus containing a fetus) can press against the pelvic veins in the later months of pregnancy, which can cause backing up of the venous blood. This can cause varicose veins, haemorrhoids, and lower leg oedema. If the inferior vena cava is compressed by the gravid uterus, blood return to the right atrium is reduced, reducing the preload of the heart, causing hypotension. In severe cases this can lead to collapse of the mother. Relieving this pressure on the vena cava improves venous return to the heart. Pregnant women should therefore lie on their side rather than flat on their back where possible, and a collapsed pregnant woman who is breathing can be placed in the left lateral position. If a woman more than 20 weeks pregnant is in cardiac arrest, she stays on her back so that chest compressions can be given, and the uterus is pushed manually to the left (manual left uterine displacement) as early as possible.

Women with pre-existing cardiovascular medical conditions are at particular risk during pregnancy, because their heart has to cope with the larger blood volume and cardiac output.

Renal System

Dilation of the blood vessels in the kidneys (renal vasodilation) increases blood flow to the kidneys. This increases glomerular filtration rate (GFR), and the larger plasma volume raises GFR further. Combined with the gravid uterus pressing on the bladder, this increased GFR leads to the urinary frequency seen commonly in pregnancy. This increase in GFR also increases creatinine clearance, meaning that baseline serum creatinine is lower in pregnancy. As a result, significant renal impairment may be present before creatinine rises outside the non-pregnant reference range, which can make acute kidney injury more difficult to recognise. Plasma osmolality (the concentration of dissolved particles in the plasma) also shows a small reduction, of about 10 mOsm/kg. This happens because the thresholds for thirst and for release of antidiuretic hormone (ADH, the hormone that makes the kidneys hold on to water) are reset to a lower osmolality. Peripheral oedema (tissue swelling) is common in pregnancy, and pressure from the gravid uterus on the pelvic veins contributes (see the Cardiovascular System section).

During pregnancy, there is an increase in size of the calyces and renal pelvis (the parts of the kidney that collect urine) and of the ureters, causing a physiological hydronephrosis and hydroureter. This widening is caused by the uterus compressing the ureters and by progesterone, which reduces the tone and peristalsis of the ureters. It is often more marked on the right. The resulting urinary stasis in the widened ureters and kidneys contributes to the increased urinary tract infection (UTI) risk in pregnancy.

During pregnancy, the kidneys also release more renin, erythropoietin (EPO) and calcitriol. The renin causes retention of fluid (contributing to the increased circulatory volume and physiological anaemia of pregnancy). The erythropoietin release is a response to try to combat the physiological anaemia of pregnancy by increasing red blood cell production. The calcitriol increases the absorption of calcium from the gut, which doubles from about 12 weeks, to supply calcium for the growth of the fetus.

Respiratory System

Later in pregnancy, the gravid uterus pushes the diaphragm upwards, by about 5 cm at its resting position. This reduces the functional residual capacity (the volume of air left in the lungs after a normal breath out). However, the diaphragm still moves at least as far with each breath, so vital capacity (the largest breath that can be breathed out after a full breath in) does not change. Progesterone stimulates central ventilatory drive (the brain’s drive to breathe), increasing tidal volume and minute ventilation (the volume of air breathed in each minute). This causes a decreased pCO₂, which causes a mild respiratory alkalosis. This lower maternal pCO₂ actually enhances the gas exchange with the placenta. Normal arterial pH in pregnancy is about 7.40 to 7.47.

Despite these changes in the respiratory system during pregnancy, peak flow and FEV1 (forced expiratory volume in one second: the volume of air blown out in the first second of a forced breath out) should remain unchanged. So in a pregnant woman with asthma, a fall in peak flow or FEV1 cannot be explained by pregnancy itself, which is important when managing her asthma.

In the upper respiratory tract, oestrogen causes increased vascularisation and capillary engorgement (more blood vessels, swollen with blood), leading to nasal stuffiness and congestion. Nosebleeds are also more common in pregnancy.

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