Two-thirds of those with HIV live in sub-Saharan Africa
Men have double the risk of catching HIV if their partner is pregnant, say researchers.
A study of more than 3,000 couples in Africa also backed previous research showing women are more susceptible to HIV infection when they are pregnant.
The researchers speculate that changes in a pregnant woman's immune system may contribute to the increased chance of her partner becoming infected.
A UK charity said the findings showed the importance of antenatal testing.
The findings have been presented at the International Microbicides Conference in Pittsburgh, alongside a separate study showing a microbicidal gel is safe to use during pregnancy to prevent HIV transmission.
Young women of reproductive age are among those at greatest risk of HIV infection in countries with high levels of disease.
Several studies have suggested that during pregnancy women are more at risk of catching HIV from an infected partner, but this is the first time researchers have shown that men are more susceptible to infection if their partners are pregnant.
The study, carried out in Botswana, Kenya, Rwanda, South Africa, Tanzania, Uganda and Zambia, involved 3,321 couples in which one partner was HIV-infected and the other not.
Over two years there were 823 pregnancies, and analysis showed that pregnancy increased both male-to-female and female-to-male infection.
For women, it seemed that factors other than pregnancy contributed to the increased risk of HIV infection.
But in men, the link between pregnancy and their risk of infection was much clearer, even after accounting for other factors, such as having unprotected sex.
The findings also showed that measures of HIV infection such as viral load and CD4 count had no bearing on the man's risk of acquiring it.
Study leader Dr Nelly Mugo, from the University of Nairobi and the University of Washington in Seattle, said it could be that biological changes during pregnancy make a woman more infectious.
Jason Warriner, clinical director for Terrence Higgins Trust, said: "This shows why it's so important to have antenatal screening programmes in place.
"One of the biggest challenges we face in the UK is that one person in four with HIV doesn't even know they have the virus, because they haven't been tested.
"Along with promoting the use of condoms, screening programmes for at-risk groups would help bring down high levels of undiagnosed HIV and significantly reduce onward transmission."