Spears married childhood friend Jason Allen Alexander on January 3, 2004 in Las Vegas, Nevada.[41] The marriage lasted fifty-five hours, ending with an annulment granted on January 5 which stated that Spears "lacked understanding of her actions to the extent that she was incapable of agreeing to marriage because before entering into the marriage the Plaintiff and Defendant did not know each other's likes and dislikes, each others' desires to have or not have children, and each other's desires as to State of residency".[42][43]
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Months after the Las Vegas incident, Spears embarked on The Onyx Hotel Tour which was cancelled in June, after Spears injured her knee during the filming of the video for the single "Outrageous". An MRI showed floating cartilage in her knee.[44] Throughout this time she began a relationship with Kevin Federline and in July 2004, Spears announced her engagement three months after they met. Federline had very recently been in a relationship with actress Shar Jackson, who was at the time pregnant with his second child.[45] These initial stages were chronicled in Spears' first reality show Britney & Kevin: Chaotic, which aired on UPN during May and June of 2005.[46] On the night of September 18, 2004, Spears married Federline in a surprise, non-denominational ceremony at a residence in Studio City, California, filing legal papers on October 6, 2004.[47][48]

After the marriage Spears announced via her website that she would be taking another career break in order to start a family.[49] Her artistic career did remain low profile only releasing her first hits collection, Greatest Hits: My Prerogative. The album debuted at number four on the U.S. charts and featured three new songs, including "My Prerogative".[50] Britney stood by her intent of starting a family; and seven months after her marriage she chose her website to announce her pregnancy writing, "the time has finally come to share our wonderful news that we are expecting our first child together.”[51] Britney gave birth to her first child, son Sean Preston Federline, on September 14, 2005 in Santa Monica, California by a scheduled caesarean section.[52]

A few months after giving birth to Sean, speculation about Spears being pregnant for a second time began to swirl.[53] In May 2006, she announced her second pregnancy with an appearance on The Late Show with David Letterman.[54] Spears appeared on Dateline NBC the next month to discuss tabloid rumors about an impending divorce, and motherhood. She addressed an incident which occurred in February 2006 when photos revealed her driving with her son unrestrained in her lap,[55] explaining, "I see a bunch of photographers and I’m scared and I want to get out of the situation... They’re coming up on the sides of the car which is a scary situation for me... so I get my baby out of the car and I go home.”[56] The month following the televised interview, Spears posed nude for the August 2006 cover of Harper's Bazaar. In the accompanying interview, Spears stated that she is not ready to retire from performing.[57][58]

Just two days shy of Sean's first birthday, Spears gave birth to her second son Jayden James Federline on September 12, 2006 in Los Angeles, California.[59] Spears filed for divorce from Federline November 7, 2006 citing irreconcilable differences and asking for both physical and legal custody of their two children, with visitation rights for Federline.[2] Laura Wasser has been hired to represent Spears in the case.[60] According to a representative for Federline's lawyer, the divorce filing "caught Kevin totally by surprise."[61] Finally, on March 29, 2007, Spears and Federline reached a settlement, "All parties signed a global settlement agreement on all issues concerning their marriage and custody of their two children," said Michael Sands, spokesperson for Federline's lawyer, Mark Vincent Kaplan.[62]