Pregnancy 2: effects on the respiratory and urinary systems

Introduction

Physiological changes in pregnancy support the developing foetus and prepare the mother for labour and delivery. The first article in this series described how progressive growth of the foetoplacental unit and expansion of maternal tissues increases the production of waste products such as urea, uric acid and carbon dioxide (CO2), placing an additional burden on the organs of elimination, which must quickly adapt. In this second article, we explore the anatomical and physiological changes to the respiratory and urinary systems during a normal pregnancy, and some of the potential complications that can result.

Respiratory system

Upper respiratory tract changes, pregnancy-induced rhinitis and apnoeas

Increased blood volume and cardiac output during pregnancy, as described in part one of this series, increases blood flow to the mucous membranes that line the respiratory tract. In the nasal cavity, this hyperperfusion can result in swelling, plasma leakage and mucus overproduction, often causing a continuously runny nose and nasal congestion (Fig 1). This is pregnancy-induced rhinitis (PIR) – also known as rhinitis of pregnancy – and it affects around 32% of women (Gupta and Anari, 2022). PIR is also associated with increased snoring and sleep-disordered breathing problems (Gupta and Anari, 2022; Caparroz et al, 2016).

The exact mechanisms leading to PIR are uncertain, but the problem may be exacerbated by the increased progesterone levels in pregnancy dilating blood vessels in the nasal epithelia and increasing blood flow, while increased oestrogen can enhance vascular permeability leading to fluid exudation (Poerbonegoro, 2019). PIR typically begins in the first trimester and continues throughout pregnancy, but rapidly resolves within 48 hours of delivery of the baby.

Some decongestants may be teratogenic and cause birth defects, so should be avoided where possible; instead, increased exercise and nasal saline irrigation are recommended and have proven effective (Gupta and Anari, 2022).

PIR can reduce quality of life and has been linked with maternal obstructive sleep apnoea (OSA), which can have adverse effects on the foetus, including:

• Intrauterine growth retardation;
• Premature delivery;
• Increased risk of congenital defects (Bourjeily et al, 2017).

The major risk factors for sleep-disordered breathing and maternal OSA during pregnancy appear similar to those for apnoea in non-pregnant women, with obesity, high body mass index (BMI) and older age associated with higher risk (Pien et al, 2014).

The progesterone/oestrogen mediated engorgement of mucous membranes can also be problematic in the nose, pharynx (throat) and larynx (voice box), making breathing through the nose more difficult. It also increases the risk of epistaxis (nosebleeds) and sore throats or voice changes, such as husky voice (Yanamandra and Chandraharan, 2012).

“Shortness of breath, or dyspnoea, is experienced by around 60-70% of healthy pregnant women”

Changes to thorax, lung volumes and breathing

Perhaps the most obvious change to the respiratory system is compression of lung tissue from the upwards displacement of abdominal organs as the uterus expands. Typically, the diaphragm is pushed upwards into the thoracic cavity by around 5cm (LoMauro and Aliverti, 2015). Ovarian hormones, primarily oestrogen and relaxin, loosen the ribcage ligaments, allowing it to expand by around 5-7cm to maintain total lung volume (Yanamandra and Chandraharan, 2012). These changes increase the subcostal angle of the ribs at the xiphoid process from 68.5 degrees to 103.5 degrees at the time of delivery (LoMauro and Aliverti, 2015) (Fig 2).

Although total lung capacity remains relatively constant, other lung volumes and capacities change (LoMauro and Aliverti, 2015; Heidemann and McClure, 2003). The two most significant changes are:

• Residual volume (the air that stays in the lungs after expiration) decreases by around 7-22% due to upwards movement of the diaphragm (LoMauro and Aliverti, 2015);
• Tidal volume (air inspired then expired during normal breathing) increases by 30-45% (Lee et al, 2017). In a typical pregnancy, increased tidal volume of around 200ml is usual (Willacy, 2022).

As the oxygen demands of the developing foetus and changing maternal physiology increase oxygen consumption, this increase in tidal volume allows for additional oxygenation of the blood. At term, oxygen consumption and CO2 production are up to 60% greater than pre-pregnancy (Heidemann and McClure, 2003) and, during labour, oxygen consumption may be up to 75% greater (Bhatia and Chhabra, 2018). Progesterone initiates the relaxation of the smooth muscle of the airway, resulting in bronchodilation, which facilitates enhanced gas exchange throughout pregnancy (LoMauro and Aliverti, 2015).

How pregnancy affects the breathing rate (breaths per minute) is disputed. Increased progesterone enhances the CO2 sensitivity of the chemoreceptors (which measure blood oxygen and CO2 levels), with Heidemann and McClure (2003) reporting increases in respiratory rate from 12-15 breaths per minute. However, other researchers suggest either no change (Bhatia and Bhatia, 2000) or minimal increases of 1-2 breaths per minute (Carrillo-Mora, 2021). Following delivery most respiratory parameters return to normal quickly and oxygen consumption attains pre-pregnancy levels at around 6-8 weeks postpartum (Bhatia and Chhabra, 2018).

Dyspnoea

Dyspnoea (shortness of breath) is experienced by around 60-70% of healthy pregnant women and often regarded as a normal clinical feature of pregnancy (Lapinsky and Bourjeily, 2020). Theories for this include:

• Haemodilution of the blood (see part one of this series);
• Increased CO2 production by foetus and mother;
• Compression of the lungs by the displaced diaphragm;
• Progesterone-mediated increased sensitivity of the chemoreceptors to CO2 and resultant increased drive to breathe (and in some cases, hyperventilate);
• The diaphragm must contract with greater force to overcome resistance from the upwardly displaced abdominal organs below (Lapinsky and Bourjeily, 2020; LoMauro and Aliverti, 2015).

Dyspnoea may also be related to cardiac changes, with Goland et al (2015) finding that pregnant women complaining of dyspnoea had thicker, more muscular, hearts and increased pulmonary artery pressures than pregnant women with no shortness of breath; however, it remains unclear whether these cardiac changes were due to dyspnoea or contribute to it. In reality, the causes of dyspnoea during pregnancy remain uncertain and are probably a cumulative effect of both the anatomical and the physiological changes described above.

Dyspnoea should always be investigated to rule out underlying disease but, in the absence of pathology, pregnant women should be reassured that it is a normal consequence of pregnancy and will disappear soon after delivery (Lapinsky and Bourjeily, 2020; Lee et al, 2017).

Respiratory function in twin pregnancies

Research on differences in lung function between singleton and twin pregnancies showed no major differences in respiratory function (Siddiqui et al, 2014; McAuliffe et al, 2002). These studies suggest that, in the absence of any underlying pathology, a twin pregnancy will not further alter the anatomy and physiology of the respiratory tract.

Pre-existing respiratory disease

Asthma is the most common respiratory disease in pregnancy, and affects 2-13% of pregnant women worldwide (Bonham et al, 2018). Asthma severity, both before pregnancy and in the first trimester, is generally a good indicator of how disease will be experienced throughout pregnancy. Typically:

• Around a third of women with asthma before pregnancy experience a lessening of their asthma symptoms when pregnant;
• A third experience little change;
• A third find their symptoms are exacerbated or feel more severe (Wang et al, 2020).

The progesterone-induced dilation of the airway may explain why symptoms improve for some women, but the upward movement of the diaphragm may exacerbate symptoms in others (particularly women with high BMIs) by contributing to airway closure during an asthma attack.

Women with moderate-to-severe symptoms that are poorly controlled during pregnancy are at greater risk of having babies with a lower birth weight and other complications, such as gestational diabetes and preeclampsia (Wang et al 2020, Bonham et al, 2018). Conversely, most pregnant women with well-controlled asthma experience similar outcomes to those who do not have asthma (Bonham et al, 2018; Nelson et al, 2012).

Asthma medications during pregnancy should, ideally, be reviewed because some have been linked to congenital defects including cleft palate, spina bifida and heart disease (Garne et al, 2016).

Research has linked asthma in pregnant women to increased risk of childhood conditions in their offspring, including:

• Diseases of the nervous system, ear, respiratory system and skin;
• Infectious and parasitic diseases;
• Potentially, diseases of the digestive tract, congenital malformations and endocrine/metabolic disorders (Tegethoff et al, 2013).

Respiratory tract infections

Influenza
Influenza is a common viral infection during pregnancy, and associated with:

• Increased maternal mortality;
• Premature delivery;
• Low birth weight babies.

Pregnant women who have influenza in their third trimester are three to four times more likely to need hospital admission compared with their non-pregnant counterparts (Raghu and Pulivarthi, 2015). Other factors that increase the risk of more severe outcomes in pregnant women who have influenza include obesity, diabetes and asthma (Valenzuela-Méndez et al, 2016).

Covid-19
At the start of the Covid-19 pandemic, it was feared the immunosuppression necessary to carry a baby to term would predispose pregnant women to infection, but it quickly became apparent that pregnant women were no more at risk of contracting Covid-19 than their non-pregnant counterparts (Royal College of Obstetricians and Gynaecologists (RCOG), 2022). However, Covid-19 remains a common respiratory tract infection in pregnancy, and being pregnant increases the risk of severe infection and complications (RCOG, 2022).

Covid-19 has also been associated with approximately double the risk of stillbirth (RCOG, 2022). Being unvaccinated, having a high BMI, pre-existing diabetes and/or hypertension and being of Black or Asian heritage are all additional risk factors, both for infection and more-severe disease outcomes (RCOG, 2022).

Urinary system

Renal blood flow and glomerular filtration rate changes

A rise in cardiac output and blood volume during pregnancy means renal blood flow increases and is up to 80% greater by the end of the first trimester than before pregnancy (Beers and Patel, 2020). The kidneys are extremely vascular, and increased blood flow leads to renal engorgement, typically increasing kidney length by around 1cm and total size by around 30% (Akyüz et al, 2020).

It is a misconception that pregnant women produce more urine. In the absence of disease, daily urine output remains similar to pre-pregnancy (Yeomans and Gilstrap, 2005) but compression of the bladder increases urinary urge so pregnant women need to urinate more often. Although there is no great change in urine output, the kidneys are more active, with the glomerular filtration rate (GFR) increasing by around 50% by the early second trimester (Beers and Patel, 2020). This enhances clearance of urea, uric acid, bicarbonate and creatinine, which is essential to meet the total combined increase in the production of waste products from the mother and foetus.

An increased GFR can overwhelm the ability of the proximal tubule to reabsorb glucose; this means glycosuria (glucose in the urine) has been observed in the urine of around 50% of pregnant women (Akyüz, et al, 2020), although most of these will have normal plasma-glucose concentrations (Akyüz et al, 2020; Heidemann and McClure, 2003). As glycosuria is so prevalent during pregnancy, it is a poor screening tool for gestational diabetes, a condition that will be explored in part four of this series.

Towards the end of the third trimester, protein levels in the urine may have increased by around 100-200mg per day (Akyüz et al, 2020), so trace amounts of protein are commonly detected in the urine. If levels are excessive (>300mg protein in 24 hours), further investigations are essential to rule out preeclampsia – particularly if such proteinuria is accompanied by increased blood pressure (Akyüz et al, 2020; Beers and Patel, 2020).

The complex interplay of different hormones and how they affect renal physiology during pregnancy is still poorly understood but, typically, results in increased sodium and potassium reabsorption in the kidney and retention of around an extra 1.6L of plasma (Cheung and Lafayette, 2013). This increase in fluid and electrolyte retention is offset by the vasodilator effects of hormones such as progesterone; as such, in a normal pregnancy, although blood volume increases (see part one of this series), peripheral resistance to blood flow decreases and blood pressure falls (Beers and Patel, 2020; Cheung and Lafayette, 2013).

Hydronephrosis of pregnancy and urinary stasis

Expansion of the uterus throughout pregnancy can progressively compress the ureters (the tubes carrying urine from the kidneys to the bladder); this, together with hormonal influences, can result in urine collecting in the kidneys. This is referred to as hydronephrosis of pregnancy and leads to the renal pelvises (hollow inner portions of the kidneys) dilating by around 15mm in the right kidney and 5mm in the left (Ciciu et al, 2022) (Fig 3).

Hydronephrosis occurs in >90% of pregnancies and, generally, becomes more pronounced with each passing trimester (Bayraktar et al, 2021). Hormonal changes also contribute to hydronephrosis, with progesterone relaxing the smooth muscle layer in the ureters; this can result in the ureters becoming elongated, looser and convoluted, often forming loops where urine can collect.

Expansion of the renal pelvises and changes to the ureters can result in urinary stasis (static urine), increasing the risk of infection from bacteria that can grow fast in pooled, stagnating urine (Bayraktar et al, 2021; Lowdermilk, 2006). Following delivery, the compression exerted by the gravid uterus on the ureters is removed and the renal pelvises quickly revert to their original anatomical configuration (Bayraktar et al, 2021).

Renal calculi

Renal calculi (kidney stones) are reported to occur in around 1 in 1,500 pregnant women, which is similar to the rate for non-pregnant women (Meher et al, 2014). However, 74% of renal calculi in pregnant women are composed of calcium phosphate, rather than calcium oxalate, which is most often observed in the general population (Meher et al, 2014).

This difference in chemical composition of the stones is thought to reflect changes in renal physiology, the tendency for a more alkaline urine pH and the increased pooling of urine usually seen during pregnancy (Meher et al, 2014). Movement of calcium from mother to baby during formation of the foetal skeleton may also contribute to calcium phosphate stones (as bone is made of calcium phosphate); this mobilisation of calcium is explored in part three of this series.

Due to the progesterone-mediated relaxation and dilation of the ureters during pregnancy, up to 70-80% of renal calculi are spontaneously passed, either during pregnancy or immediately after childbirth (He et al, 2022). However, larger stones may become trapped in the ureters and cause excruciating, unrelenting pain (renal colic) as they scrape and irritate the sensitive ureter linings. Renal colic usually requires immediate hospitalisation and often surgery.

Around half of pregnant women with renal calculi will develop urinary tract infections (UTIs) and some of these may go on to develop sepsis (He et al, 2022). Renal calculi during pregnancy are also associated with:

• Increased risk of miscarriage;
• Premature delivery;
• Need for assisted delivery (Meher et al, 2014).

Standard advice to patients at risk of stone formation – such as remaining well hydrated and reducing salt intake – is often recommended during pregnancy. If renal calculi are suspected, scans involving ionising radiations, such as X-rays, are usually avoided because of the risk to the foetoplacental unit; ultrasound scans are commonly the diagnostic tool of choice (Meher et al, 2014).

Urinary incontinence

Expansion of the uterus in pregnancy progressively compresses the bladder, which increases the pressure in the urinary (urethral) sphincters and forces them open. This leads to urine leaking from the urethra (Fig 4); the condition is called stress incontinence as it is caused by mechanical stress (pressure) on the bladder. It can be defined as involuntary loss of urine on effort, physical exertion, coughing, sneezing or even just laughing and it is the most common form of urinary incontinence (UI) in pregnancy (Syeda et al, 2022).

The second most common UI during pregnancy is urge incontinence, in which urine leaking is preceded (usually for a short period) by a strong urge to urinate, thought to be caused by overactivity of the smooth muscle layer lining the bladder (Jansson et al, 2021). Dinç (2018) found that UI was present in 300 out of 750 (40%) pregnant women; other studies found an even higher prevalence of up to 52% (Syeda et al 2022; Wang et al, 2022).

The major risk factors for UI during pregnancy include:

• History of UI;
• Higher maternal age;
• Previous vaginal birth/s (known to weaken pelvic floor muscles);
• History of UTIs;
• Obesity;
• Coffee consumption;
• Smoking;
• History of childhood bedwetting (Syeda et al, 2022; Wang et al, 2022).

UI can be inconvenient, uncomfortable, embarrassing and cause psychological stress; some women may even mistake it for rupturing of the foetal membranes (waters breaking) (Syeda et al, 2022). Pregnant women should be reassured that UI is nothing to be ashamed of and is a common, expected consequence of pregnancy, particularly in the final trimester when pressure on the bladder is at its greatest.

Pelvic floor exercises undertaken before, during and after pregnancy have varying degrees of success at reducing UI (Woodley et al, 2017).

Increased urinary frequency and nocturia

Compression by the gravid uterus of the bladder reduces its volume and holding capacity. This increases both the frequency of urination and the urge to urinate throughout the day. It can also disrupt sleep patterns at night, with nocturia (waking to urinate) affecting 75-77% of pregnant women (Rajavuori et al, 2022).

UTIs

UTIs are the most common infection during pregnancy (Johnson et al, 2021). Up to 20% of all pregnant women are affected by UTI, although not all will be symptomatic (Balachandran et al, 2022).

Most UTIs are bacterial in origin, with a similar plethora of bacterial pathogens observed in pregnancy compared with UTIs in the general population. E coli is the most common UTI-associated pathogen; other common bacteria include Klebsiella pneumoniae and staphylococcus, streptococcus, enterococcus, and proteus genera (Habak and Griggs, 2023).

Dilation of the ureters and renal pelvises, which leads to urine collecting and pooling, together with bladder compression, increased UI and the immunosuppression necessary for a foetus to survive until delivery contribute to increased risk of UTI during pregnancy (Habak and Griggs, 2023; Balachandran et al, 2022).

Bladder infections typically lead to cystitis (painful inflammation of the bladder lining), which affects around 1-4% of pregnant women (Johnson et al, 2021). Cystitis is characterised by dysuria (pain when passing urine), often perceived as a burning sensation, and is associated with increased urinary urgency, and persistent pain and tenderness in the suprapubic region (Habak and Griggs, 2023; Johnson et al, 2021).

Around 1-2% of pregnant women develop pyelonephritis (kidney infection), a much more serious condition in which a bacterial infection initiates intrarenal inflammation (Johnson et al, 2021). It is often characterised by flank pain, passing foul-smelling urine that tests positive for leukocytes, protein and blood. In more serious cases, bacteria enter the blood, leading to septicaemia, commonly associated with pyrexia (fever) and chills. Some individuals may deteriorate quickly, developing sepsis and septic shock, which are associated with high foetal and maternal mortality and morbidity (Habak and Griggs, 2023).

As many UTIs are initially asymptomatic, checking pregnant women’s urine for asymptomatic bacteriuria (ASB) is recommended in many countries as 20-30% of patients showing ASB are at risk of subsequently developing symptomatic kidney infections (Balachandran et al, 2022).

UTIs during pregnancy are usually treated with antibiotics, but this needs careful selection as many common antibiotics are potentially teratogenic, triggering birth defects, particularly in the embryonic stage of prenatal development when the major organ systems are forming. However, as many safer antibiotic options are available, an estimated two-thirds of pregnant women with a UTI are treated with antibiotics (Johnson et al, 2021).

Conclusion

Pregnancy is associated with anatomical and physiological changes to the respiratory and urinary tracts. Nurses and midwives must be aware of these, along with common adverse effects and complications that may be experienced as a result.

• The next article in this series will explore the major pregnancy-associated changes to the musculoskeletal systems and skin.

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