INTRODUCTION: Alterations in the immune system can result in alanine aminotransferase (ALT) flares either during pregnancy or after delivery in women with chronic hepatitis B virus (HBV) infection. The aim of this study was to prospectively assess changes in serum biochemical and virological markers of HBV infection during and after pregnancy in a large North American cohort of pregnant women with chronic HBV.; METHODS: Adult pregnant women enrolled in the Hepatitis B Research Network between 2011 and 2016 were included. Serum ALT values and HBV DNA viral levels were obtained at <28 weeks and >28 weeks of gestation and <16 weeks, 16-31 weeks, and 32-48 weeks postpartum. Outcomes of ALT flares included severity, duration, and initiation of antiviral therapy.; RESULTS: Among the 158 pregnant women with chronic HBV, the median age was 33 years, 73% were Asian, and 63% were hepatitis B e antigen (HBeAg) negative. The median HBV DNA level was substantially higher in the HBeAg-positive vs HBeAg-negative women (1.3 x 10(8) vs 343 IU/mL), but serum ALT levels at their first study visit were similar. Among untreated pregnant women, there was a very mild increase in serum ALT postpartum among both HBeAg-positive and HBeAg-negative women (P < 0.001). Serum ALT flares (range 107-513 U/L) developed in 3.4% (5/149) during pregnancy and in 4.3% (4/92) after delivery. Twenty-two percent were initiated on antiviral therapy. After withdrawal of prophylactic anti-HBV therapy, 17.2% (5/29) developed serum ALT flares (range 107-208 U/L) within 14 weeks of drug discontinuation, and 3 additional women had flares despite continuous anti-HBV therapy during pregnancy or postpartum. Many ALT flares were not associated with significant changes in HBV DNA levels. No flares were severe with elevations of bilirubin or clinical decompensation.; DISCUSSION: Spontaneous ALT flares in untreated pregnant women with chronic HBV are infrequent, mild, and self-limited both prepartum and postpartum. Although flares after the withdrawal of antiviral therapy postpartum are more common, they were also mild and self-limited. Further studies of the immunopathogenesis of pregnancy-related flares are needed, as well as effects on long-term outcome of the mother and infant.