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Next Lesson - Diseases of the Mother in Pregnancy
Abstract
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 throughout pregnancy cause a wide range of changes. Increased oxytocin and prolactin affect breast milk production and let down. TSH decreases in the first trimester due to hCG binding to the TSH receptors and causing 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 decreases and heart rate increases, causing a mild cardiac hypertrophy. Venous blood can back up later in pregnancy, causing varicose veins and haemorrhoids.
- Renal - increased blood volume in pregnancy causes increased blood flow to the kidneys. This increase in glomerular filtration rate leads to increased creatinine clearance and a lower normal serum creatinine in pregnancy, and contributes to urinary frequency.
- The fetal head on the ureters as well as less mobile ureters causes physiological hydronephrosis and hydroureter, which increases chance of urinary tract infections in pregnancy. Kidneys also release more renin, erythropoietin and calcitriol in pregnancy.
- Respiratory - progesterone release causes an increase in tidal volume, and a decreased pCO2, causing a mild respiratory alkalosis. Peak flow should remain the same during pregnancy.
- Musculoskeletal - progesterone and relaxin release loosens sacroiliac ligaments and pubic symphysis to aid vaginal birth. This can cause a waddling gait and symphyseal pain.
- Gastrointestinal - pregnancy can cause constipation, bloating, reflux and heartburn. Pica, cravings for non-food substances, may also develop.
- Immune - reduction in TH1 cytokine production and increase in TH2 cytokine production. Pregnancy is associated with immune modulation, including a shift towards TH2 cytokine responses, contributing to relative maternal immune tolerance.
Core
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.
There are many hormonal changes which occur in pregnancy.
In early pregnancy, the corpus luteum produces oestrogen and progesterone until the placenta takes over in hormone production. Inhibin is also released to inhibit FSH release, to prevent more follicles from maturing. However, in in vitro fertilisation (IVF), ovulation still occurs and a corpus luteum is formed, so progesterone is produced. Progesterone is commonly supplemented because luteal phase support may be inadequate following ovarian stimulation, until placental hormone production is established.
The different hormones each have specific effects during pregnancy:
- Progesterone - its main action is to relax smooth muscle which causes most of the symptoms of pregnancy and blood vessel dilation. Gastric emptying is also slowed.
- Oestrogen - breast tissue growth, water retention and increased protein synthesis.
- Relaxin - softens the cervix 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 T4 levels thus reducing TSH by negative feedback.
Pregnancy is classed as a high volume state. This means that blood volume increases from around 5L to around 7.5L in an average woman. However, this is an increase in plasma volume and not an increase in red blood cell count, meaning that haematocrit decreases (haematocrit is a measure of the concentration of red blood cells). This causes the physiological anaemia of pregnancy. It is a normal response to pregnancy but can cause classical anaemia symptoms such as fatigue. Iron supplementation such as ferrous sulphate 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 progesterone causes dilation of blood vessels. This decreases vascular resistance, which decreases the afterload on the heart. This causes the heart rate to increase. Because blood pressure should decrease during pregnancy, any increase in blood pressure could be a sign of pre-eclampsia (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. Therefore when coming across a collapsed pregnant patient a useful first step can be to place her in a left lateral position, or manually displace the gravid uterus to the left, to encourage venous return to the heart.
Women with pre-existing cardiovascular medical conditions are at particular risk during pregnancy.
The increased blood volume in pregnancy increases blood flow to the kidneys. This increases glomerular filtration rate (GFR). 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. The increased GFR also means that more solutes are filtered out of the blood into the urine, causing a reduction in plasma osmolarity which can cause peripheral oedema.
Due to the extra work of the kidneys during pregnancy, there is an increase in size of the calyces, renal pelvis and the ureters, causing a physiological hydronephrosis and hydroureter. This coupled with the urinary stasis caused by an increase in progesterone and the uterus compressing the ureters can contribute to the increasing 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 calcium stores in the body to allow for proper growth of the fetus.
Later in pregnancy, the gravid uterus presses up on the diaphragm splinting it, making it harder to breathe deeply. Progesterone release causes ligament relaxation in the chest, increasing the tidal volume. This causes a decreased pCO2, which causes a mild respiratory alkalosis. This actually enhances the gas exchange with the placenta. Normal pH range in pregnancy is 7.40 to 7.45.
Despite the changes that do occur in the respiratory system during pregnancy, peak flow and FEV1 should remain unchanged. This is important in the management of asthmatics during pregnancy.
In the upper respiratory tract, oestrogen causes increased vascularisation and capillary engorgement, leading to nasal stuffiness, congestion and frequent nosebleeds.
Progesterone and relaxin hormones act to loosen the ligaments in the sacroiliac joint and pubic symphysis to increase the size of the pelvic floor, which assists in childbirth. However, this can cause a ‘waddling’ gait and pain in the pubic symphysis and other ligaments such as those between the ribs.
Progesterone release causes smooth muscle relaxation and decreased peristalsis, which can lead to constipation and bloating. It also causes relaxation of the lower oesophageal sphincter, resulting in reflux and heartburn, and delays gastric emptying, contributing to bloating and ‘morning sickness’. This occurs throughout the day despite the name, and normally occurs most severely in the first trimester. Taste changes may occur, the extreme version of this being pica, the cravings for non-food things, like soil.
To prevent rejection of the fetus, there is a reduction in cell-mediated immunity and TH1 cytokine production. This is balanced by an increase in TH2 cytokines and humoral immunity. This means that pregnancy can improve TH1 mediated conditions, like psoriasis, and worsen TH2 mediated conditions like eczema. Overall, these changes result in a state of relative maternal immunomodulation, rather than global immunosuppression.
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Thomas Burnell
In this article
There are many changes that occur in a woman’s body during pregnancy, known as the maternal physiological adaptations of pregnancy.
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