Objective: To investigate if maternal asthma is associated with an increased risk of maternal and placental complications in pregnancy. Methods: Electronic databases were searched for the following terms: (asthma or wheeze) and (pregnan* or perinat* or obstet*). Cohort studies published between January 1975 and March 2012 were considered for inclusion. Forty publications met the inclusion criteria, reporting at least one maternal or placental complication in pregnant women with and without asthma. Relative risk (RR) with 95% confidence intervals (CIs) was calculated. Results: Maternal asthma was associated with a significantly increased risk of cesarean section (RR = 1.31, 95%CI = [1.22–1.39]), gestational diabetes (RR = 1.39, 95%CI = [1.17–1.66]), hemorrhage (antepartum: RR = 1.25, 95%CI = [1.10–1.42]; postpartum: RR = 1.29, 95%CI = [1.18–1.41]), placenta previa (RR = 1.23, 95%CI = [1.07–1.40]), placental abruption (RR = 1.29, 95%CI = [1.14–1.47]) and premature rupture of membranes (RR = 1.21, 95%CI = 1.07–1.37). Moderate to severe asthma significantly increased the risk of cesarean section (RR = 1.19, 95%CI = [1.09–1.31]) and gestational diabetes (RR = 1.19, 95%CI = [1.06–1.33]) compared to mild asthma. Bronchodilator use was associated with a significantly lowered risk of gestational diabetes (RR = 0.64,95%CI = [0.57–0.72]). Conclusions: Pregnant women with asthma are at increased risk of maternal and placental complications, and women with moderate/severe asthma may be at particular risk. Further studies are required to elucidate whether adequate control of asthma during pregnancy reduces these risks.