HIV-infected pregnant women face complex decisions about whether and how to disclose their serostatus. Previous studies have shown that HIV disclosure is associated with better care engagement, emotional adjustment to the disease, and reduced risk of HIV transmission, but women face both real and perceived barriers to disclosure. We examined patterns and predictors of HIV disclosure in a cohort of 200 women diagnosed or confirmed to have HIV during antenatal care in the Kilimanjaro region of Tanzania and followed participants to three months postpartum. Twenty women also completed qualitative in-depth interviews during pregnancy and three months postpartum. During the pregnancy period (at least 30 days post-diagnosis), 79.5% of women had disclosed to at least one other person, with disclosures generally restricted to the father of the child and/or a small number of close family members. By three months postpartum, 11.9% of women had still not disclosed to anyone. Women who presented to antenatal care with an established HIV diagnoses and married women were more likely to report disclosures. Social support was positively associated with disclosure. In qualitative interviews, women pointed to community gossip and stigma as barriers to disclosure. Those who had not disclosed to the father of the child noted fears of abandonment during the vulnerable pregnancy period. Despite expressed fears, participants reported overall positive experiences of disclosure that led to increased support. Taken together, these results point to the need for comprehensive, flexible, and culturally informed interventions that assist pregnant and postpartum women in deciding when and how to disclose. Such interventions should acknowledge and explore common barriers to disclosure, including fears of public stigma and personal consequences. Given the strong associations between disclosure, social support, and community stigma, interventions for disclosure should be nested in broader efforts of public education and HIV stigma reduction.