MS is one of the most common lesions found during pregnancy [1]. Pregnancy is a state that increases a woman’s intravascular volume by 30–50 % and cardiac output by 70 % [2, 18]. This haemodynamic disturbance exacerbates the pathophysiology of MS causing a gradual increase in the mitral valve pressure gradient and left atrial pressure, which may cause a relatively mild, asymptomatic MS to become decompensated and present during pregnancy [2, 18]. Without intervention, women with MS and mild heart failure (NYHA class I or II) have a maternal mortality of 0.4 %, rising to 6.8 % to those with severe heart failure (NYHA class III or IV).

This usually results from progressive heart failure, particularly during the second and third trimesters, and acute pulmonary oedema [19]. Obstetric risk mostly results from the risk of acute heart failure during delivery or immediately afterwards, with the risk to the fetus increasing with NYHA class but usually resulting from prematurity, intrauterine growth retardation and stillbirth [19]. For mild MS in pregnancy, symptoms can be managed with medical therapy alone and are unlikely to cause serious problems [18]. ESC guidelines recommend that pregnant women with severe symptomatic disease should be considered for active intervention with PBMV if the procedure can be carried out by a skilled operator using minimum radiation and with abdominal and pelvic shielding [19]. In terms of timing, the procedure should be delayed until after between 12 and 14 weeks to prevent radiation exposure during organogenesis and preferably performed after 20 weeks [8].

PBMV for symptom relief during pregnancy has been shown to have high procedural success rates and excellent short- and long-term maternal and fetal outcomes, with results comparable between pregnant and non-pregnant women [1]. Incidence of major complications is low with immediate symptomatic improvement and almost no adverse effect on the outcome of pregnancy [20]. A 17-year follow-up study of children born from mothers who received PBMV agreed with the previous literature and found that development was normal in all cases with no long-term radiation-induced or haematological disease [20]. As pre-pregnancy symptoms predict the likelihood of serious adverse outcomes, it is advisable to counsel women with an established diagnosis of MS, even if asymptomatic, against pregnancy and consider routine, pre-pregnancy intervention [18].

Atrial fibrillation (AF) is commonly associated with MS, occurring in between 40 and 75 % of patients with symptoms [1]. Its pathophysiology in rheumatic MS is unique and likely stems from persistent rheumatic inflammation, left atrial fibrosis and remodelling in addition to an increased left atrial size and left atrial hypertension resulting from the MS itself [1, 18]. A large left atrial size is an independent predictor of systemic embolism, death and development of AF regardless of MVA or the mitral valve pressure gradient.

The issue with AF in MS is twofold. One, it predisposes to thrombus formation in the left atria due to blood stasis and hypercoagulability resulting in thromboembolic complications [18]. Two, it reduces cardiac output and precipitates symptoms, reducing exercise capacity and increasing morbidity [1]. It indicates the beginnings of a more severe, symptomatic phase of disease, and the AHA/ACC guidance recommends that in asymptomatic patients with new-onset AF, the need for PBMV should be considered as expectant management [8]. Those with AF have worse immediate and long-term outcomes after PBMV, likely due to AF being a marker for unfavourable clinical and morphological features [21]. The presence of AF is an independent predictor for long-term major adverse cardiovascular outcomes after PBMV.

Selection of an appropriate balloon size is important in ensuring adequate splitting of the commissures without extensive damage, which may lead to iatrogenic mitral regurgitation and thus poorer outcomes [2]. With the final MVA associated with long-term outcomes, a delicate balance is required. It has been previously suggested that an echocardiographic measurement of mitral valve diameter may provide a more accurate way of selecting balloon size [2]. Indeed, the MVA is routinely assessed in the workup for PBMV anyway. In a randomised control trial comparing current methods with novel, echocardiographic methods, the authors concluded that echocardiographic balloon sizing is a reasonable method that results in good PBMV outcomes and a good post-procedural MVA and may decrease the risk of iatrogenic mitral regurgitation [22]. It is important to mention, however, that the literature on balloon sizing is limited, and the study was not adequately powered to make a definitive conclusion due to its small sample size (n = 86). In addition, women may have a larger pre-procedural MVA than men [11, 12], and this may not be taken account of in the current balloon reference size method.