The woman, who lived in Ha Tay province outside the capital Hanoi, died Saturday after being admitted a week before for treatment, said a source at a Hanoi hospital who asked not to be named.
'Tests were positive for the H5N1 virus,' the source said, adding that the woman had been seven months pregnant.
Her death brings to 45 the number of people who have died of bird flu in Vietnam since 2003.
Two people -- a 20-year-old man and a 28-year-old woman -- died in June of avian influenza, the first fatalities announced since November 2005.
Since May, six human cases have been reported here, three of them fatal.
Communist Vietnam, once the nation worst hit by bird flu, contained earlier outbreaks through mass vaccination campaigns, the culling of millions of poultry and public education initiatives.
But the virus has come back strongly this year, hitting scores of poultry farms in an unusual summertime outbreak.
Outbreaks have been reported since early May across 18 of Vietnam's 64 provinces and municipalities, mostly among unvaccinated ducks and other waterfowl.
In June, Prime Minister Nguyen Tan Dung told local officials to keep a close watch on the situation, warning that they would be held responsible if outbreaks of bird flu devastated their areas.
After authorities stepped up calls for vigilance, the agriculture ministry recently reported that only three provinces were still battling the problem.
As of July 25, the World Health Organisation had recorded 319 cases of bird flu in humans worldwide, 192 of which were fatal. The Geneva-based body has yet to confirm the last three deaths in Vietnam.
Experts fear the death toll could rise sharply if the virus were to mutate and become easily transmissible between humans, leading to a global pandemic with the potential to kill millions.
WHO chief Margaret Chan said in mid-July that about 20 percent of the population in some countries could be affected in the event of such a pandemic.