Violence by an intimate partner, including violence during pregnancy, is an important human rights and public health issue. This paper presents the findings from large household surveys conducted in Mbeya and Dar es Salaam, Tanzania, in 2001-2002, as part of the WHO Multi-Country Study on Women's Health and Domestic Violence against Women. Seven (n=88) and twelve per cent (n=147) of ever-partnered, ever-pregnant women in Dar es Salaam (n=1,298) and Mbeya (n=1,205), respectively, reported being physically assaulted during pregnancy by their partner. Of those experiencing partner violence during pregnancy, 38% (n=33) and 23% (n=34) reported blows to the abdomen. More than a third of women experiencing the violence in each setting reported that it started during pregnancy. In both settings, the violence was significantly associated with adverse maternal health behaviours and outcomes, including drinking during pregnancy, having had a child that died and the partner preventing or discouraging attendance for antenatal care. Factors significantly associated with higher likelihood of partner violence during pregnancy included being currently unmarried, having had children from different fathers, partner's unfaithfulness and his refusal to use contraception. While interventions on partner violence during pregnancy have been tested in antenatal services in some developed countries, effective solutions for how to intervene in low-resource settings like Tanzania are still needed.