Labor, particularly at the second stage, is characterized by a further increase in cardiac output by 40–50% through a pain-induced sympathetic response and through auto-transfusion by uterine contractions. Moreover, soon after delivery, decompression of the inferior vena cava increases the preload. Again, these changes put women with LVOT obstruction at a higher risk of pulmonary edema during the later stages of labor and shortly after delivery. Finally, changes in the extracellular matrix of the tunica media of the aorta can occur, leading to increased vascular compliance. The latter, in combination with cystic media necrosis (as seen in patients with a bicuspid aortic valve or in Marfan’s syndrome), clearly increases the risk of aortic dissection, even several months after delivery.

Risk of Pregnancy in Congenital Aortic Stenosis

In earlier observations, mortality among pregnant women with severe AS was estimated to be 17%, whereas fetal mortality occurred in up to 30%.^10,11 Two small cohorts confirmed this burden of maternal and fetal adverse events, especially in cases of moderate to severe stenosis, with an emphasis on the functional decline of the mother and a greater incidence of intra-uterine growth retardation and prematurity versus matched controls^.12,13 As a consequence of these high mortality rates and the potential risk of chest pain, heart failure, and collapse, as well as sudden death during pregnancy and delivery and shortly after delivery, most physicians discouraged patients with significant AS from becoming pregnant or proceeding with existing pregnancies.

In the past few decades, there has been increasing interest in pregnancy in women with underlying heart disease, and researchers have sought to calculate a risk score that could guide physicians regarding appropriate counseling tailored to the individual patient. Clark et al. developed a mortality risk prediction based on the underlying condition (see Table 1).^8,14 This table represents a synthesis of maternal mortality estimates for various types of heart diseases, divided into low-, intermediate-, and high-risk groups. Group 1 includes conditions that should have negligible maternal mortality (less than 1%) if proper care is provided. Group 2 contains a spectrum of diseases with an estimated risk of maternal mortality of 5–15%. Mild to moderate AS, regardless of either functional class or symptoms, is situated in this category. The authors consider the risk acceptable in individual cases with appropriate counseling and care and a tight follow-up regimen. Patients in group 3 have a mortality risk of more than 25% and should be advised to avoid or to terminate pregnancy. Severe AS and poor functional class (New York Heart Association (NYHA) classes III or IV) are in this group. In contrast, in a large prospective trial of 599 pregnancies in Canadian women with varying underlying heart disease (74% congenital, 22% acquired, and 4% arrhythmic), a scoring system has been developed and validated, irrespective of the underlying heart disease.^15,16 The risk factors that predict independently adverse maternal events (pulmonary edema, sustained arrhythmia, stroke, cardiac arrest, or cardiac death) are summarized in Table 2. The most significant predictor was left ventricular dysfunction (defined as an ejection fraction below 40%). The other risk factors were: poor functional class (NYHA classification more than II); cyanosis before pregnancy (oxygen saturation less than 90%); pre-conception cardiac events such as stroke, transient ischemic attack, pulmonary edema, and symptomatic arrhythmia; and, finally, left heart obstruction at the level of the mitral valve or the LVOT. The risk for cardiac complications in pregnancies with no risk factors, one risk factor, and more than one risk factor was 5%, 27%, and 75%, respectively.¹⁵

According to Siu et al., a moderate aortic valve stenosis (valve area <1.5cm) is already a predictor for a higher incidence of both maternal and fetal adverse events. However, a more recent observational study of the same group found substantial problems only when the aortic valve stenosis was defined as a valve area <1cm; they reported 6% maternal events and 12% fetal events, as defined previously.¹⁷ No deaths were found among their series. The problems related to AS seem to occur not only during pregnancy, during labor, and after delivery, but also in the postpartum period. In a series described by Silversides et al., a high incidence (31%) of valve surgery during a follow-up period of nearly three years after the pregnancy was reported.¹⁷ This should be taken into account at the time of pre-pregnancy counseling.