Objectives The proportion of women delivering with known HIV status in sub-Saharan Africa is not well described. Risk of HIV transmission to newborns is a major concern, but there may also be increased risks for other adverse pregnancy outcomes.

Design Hospital registry.

Setting North East Tanzania (1999–2006).

Population Singletons (n= 14 444).

Methods Births were grouped by maternal HIV status and socio-demographic factors predicting HIV status, and associations between status and pregnancy outcomes were studied.

Main outcome measures Maternal HIV status, perinatal mortality, prematurity, small for gestational age (SGA), birthweight and low Apgar score.

Results The proportion of mothers with known HIV status increased from 7% before 2001 to 78% after 2004. Single motherhood, rural residence, low maternal education, maternal and paternal farming and higher paternal age were associated with unknown HIV status. About 7.4% (95% CI 6.7–8.1%) of women were HIV infected, with increased likelihood of infection with higher gravidity, single motherhood, rural residence, maternal business or farming occupations and paternal tribe. Compared with HIV-uninfected women, the untreated HIV-infected women had a higher risk of SGA births (adjusted risk ratio [ARR] 1.6; 95% CI 1.1–2.4), preterm birth (ARR 1.8; 95% CI 1.1–2.7) and perinatal death (ARR 1.9; 95% CI 0.95–3.8). Women with unknown HIV status had moderately increased risks. Treated HIV-infected women had a risk similar to that of the HIV-uninfected women for all outcomes, except for low Apgar score.

Conclusion HIV testing and infection were associated with socio-demographic factors. Untreated HIV-infected women had higher risks of adverse pregnancy outcomes, and risks were also increased for women with unknown HIV status. There is still a need to increase availability of HIV testing, education and adequate therapy for pregnant women.