During pregnancy, a number of structural and functional changes occur within the renal system, some of which continue into the postnatal period. Blackburn (2007) summarises the changes, some of which are discussed below.

During the first trimester, the renal calyces, renal pelvis and ureters begin to enlarge, resulting in physiological hydroureter and hydronephrosis becoming more pronounced during the second half of the pregnancy. During the last trimester, the enlarging uterus displaces the ureters laterally; they elongate, becoming more tortuous. The volume of the ureters increases, possibly up to 25 times, resulting in up to 300 mL of urine being stored in the ureters (Blackburn 2007). This has implications for the accuracy of 24-hour urine collections and increases the risk of urinary tract infection (UTI).

Progesterone relaxes the smooth muscle of the bladder, resulting in decreased tone; oestrogen predisposes vesicourethral valve incompetence (and therefore reflux of urine) and the glomerular filtration rate increases by 40–60% (Blackburn 2007). Bladder capacity doubles by term, holding up to 1000 mL. The bladder mucosa becomes more oedematous, predisposing it to trauma or infection. McCormick et al (2008) cite that the incidence of UTI in pregnancy is 8%. The National Institute of Health and Clinical Excellence (NICE 2008) suggests that all women should be routinely offered mid-stream urine (MSU) screening in early pregnancy (booking visit) to screen for asymptomatic bacteriuria. UTI in pregnancy affects morbidity and can lead to preterm labour.

During the first trimester, the enlarging uterus compresses the bladder, increasing the desire to micturate, resulting in urinary frequency. During the second trimester, the bladder is displaced upwards, allowing bladder capacity to return to normal. However, during the third trimester, pressure from the presenting part, particularly following engagement, can once again result in urinary frequency or stress incontinence. As the bladder is displaced into the abdomen the urethra is elongated and bladder emptying is affected. Nocturia may also occur during pregnancy due to increased excretion of sodium and water occurring when the woman lies down (Blackburn 2007).

Pressure may be exerted on the sacral plexus by the presenting part during its descent through the pelvis, resulting in increased frequency or retention of urine; this is also associated with an occipitoposterior position.

Retention of urine occurs when the pressure on the sacral plexus results in inhibition of impulses. The bladder fills but there is no associated desire to void urine, compounded by the distension-inhibiting nerve receptors within the bladder wall. Pressure from the descending presenting part is exerted on the bladder and urethra, particularly at their junction. The resulting compression prevents the passage of urine, even with the desire to void. Lack of privacy and poor posture also contribute to retention of urine. Women should be encouraged to void urine every 1–2 hours during labour to minimise these risks, and particularly at the onset of second stage.

During the early postnatal period, a marked diuresis occurs. Between the second and fifth postnatal day, up to 3000 mL of urine may be produced daily, with 500–1000 mL being voided at a time (Blackburn 2007). Proteinuria may be evident as a result of autolysis (Abbott et al 1997). The structural changes that occurred during pregnancy slowly return to normal during the puerperium, although in some women this may take longer (up to 16 weeks).

Women should pass urine within 6–8 hours of delivery (Blackburn 2007). However, some women may experience a delayed sensation to void urine. The risk of partial or complete inability to void urine is increased in the presence of:

Incomplete emptying of the bladder and urinary stasis increase the risk of UTI.

Stress incontinence may also occur following delivery as a result of damage to the perineal branches of the pudendal nerves (Abbott et al 1997). If this persists beyond the puerperium medical attention should be sought.

An important part of the midwife’s role and responsibilities is record keeping, especially if the woman is experiencing difficulty with micturition (dysuria). Records should show the amount of urine passed and the frequency, with any symptoms associated with dysuria, e.g. stinging (NMC 2005).

A baby’s bladder is an abdominal organ, it being too large for the small pelvis to accommodate it. Consequently, a full bladder can compress the diaphragm and affect respiration. Micturition is an involuntary process with no control over when and where to void urine. Babies usually pass urine within the first 48 hours of life, with approximately 66% of babies passing urine in the first 12 hours, 93% by 24 hours and 99% by 48 hours (Blackburn 2007). It is important that the midwife records that the baby has passed urine following birth (NMC 2005). Urinary output is variable, depending on gestational age, fluid and solute intake, the ability of the kidneys to concentrate urine and perinatal events. Urinary output increases during the neonatal period; for example, breastfed babies pass around 20 mL of urine during the first 24 hours, increasing to 200 mL by the tenth day (Johnston et al 2003). Usually, small amounts are passed on a frequent basis, and by the second week of life the baby may produce up to 20 wet nappies a day.