Nearly 20 others were injured including four children.
An interior ministry official said the blast went off at dusk in Ur, a mainly Shiite residential district on the outskirts of Sadr city.
Sadr city was also hit by a blast just a day earlier.
"A terrorist left a car parked on the street corner and, when the bridegroom's family arrived in a convoy, he set off a bomb," he said.
A defence ministry official confirmed the blast, but said the attack was thought to have been a suicide bomb rather than a remote-controlled device.
Baghdad is in the grip of a deadly sectarian conflict between rival Sunni and Shiite factions which kills scores of people every day.
US boosts troops
The US has approved a proposal to increase the size of the Iraqi security forces beyond 325,000 and accelerate their training.
Defence Secretary, Donald Rumsfeld, would not say how many more Iraqi security forces that have been recommended.
"I am very comfortable with the increases they have proposed and the acceleration in achieving some of those targets that they have proposed," Mr Rumsfeld told reporters.
"And I understand that the Iraqi government is as well."
The current number of troops stands at about 310,000.
CBS News reported that the US Commander in Iraq, General George Casey, was expected to recommend an increase in the size of the Iraqi force of up to 100,000 troops.
Warning to neighbours
A senior Iraqi security official issued a veiled threat to neighbouring countries whom he accused of sponsoring insurgents.
"We don't want to send booby-trapped cars for the ones that are sent to us. We are capable of that but this is not the nature of the new Iraq," national security advisor Muwafaq al-Rubaie told reporters in neighbouring Kuwait.
"Neighbours are sending death vehicles, terrorists, financial and logistical support and also plotting," he said, declining to point the finger at any particular country.
"We will give them more time to reconsider their calculations ... We don't want to open fire on any country ... but Iraq's patience has a limit," he said on the sidelines of an Iraq donors' conference.
A senior official from a neighbouring country will visit Iraq in the next few weeks to discuss a political solution, Mr Rubaie added without elaborating.
Iraq's Shiite-led government has repeatedly accused Sunni-ruled neighbours Jordan and Saudi Arabia of not doing enough to stop the flow of Islamic militants wishing to join the Sunni Arab insurgency still plaguing Iraq.
The government's US backers have in turn accused Shiite Iran and its regional ally Syria of seeking to torpedo Western ambitions for the new Iraq.
Mr Rubaie warned that terrorism in Iraq would spread to neighbouring countries if left unchecked.
"Even if neighbouring countries build fences of concrete, terrorists will move there. It's a cancer that will spread all over the region if it is left to grow in Iraq," he said.