Usher is reportedly demanding a paternity test to confirm he is the father of his youngest son with estranged wife Tameka Foster - because the former couple's sex life was virtually non-existent when the tot was conceived.

The R&B superstar filed for divorce last month, revealing he had been living apart from the stylist since July 2008 and stating that there was "no reasonable hope of reconciliation".

According to the National Enquirer, Usher was stunned to learn Foster was pregnant last year - because the passion in their marriage had fizzled out months before little Naviyd Ely Raymond was born on December 10.

A source tells the tabloid, "The first thing Usher said was, 'How did this happen?' - because he was gone a lot, and their sex life was very, very limited.

"People have been telling him that around the time the little boy was conceived, Tameka got together with another man."

But Foster is adamant the boy is the Yeah! hitmaker's, and claims the tot was conceived during a rare night of intimacy.

The insider adds, "She says that although they weren't spending much time together, they did get together for one night of passion. And that's when she got pregnant."

In new court documents filed in Atlanta, Georgia on Monday, Foster insists the divorce came as a shock to her because they were "intimately together" just days before he began proceedings for a legal separation.

Her attorney continued, "(Foster) had every reason to believe her marriage was intact."

Foster is seeking child support for the former couple's two children Usher Raymond V, 19 months, and Naviyd, almost seven months, in addition to legal costs.

Usher is seeking joint custody of his kids.