While she happily announced this week she and ex-fiance Ken Urker are expecting, there's a law in the Bayou State that adds another layer to the story. It says if the mother of a child is married to someone who isn't the biological father when the child was conceived -- or has been divorced for less than 300 days at the time of birth -- then her ex, or estranged husband, is presumed to be the father.

So, in the eyes of Louisiana, Ryan's name would go on the birth certificate as the daddy ... even though Gypsy says Ken is the biological father.

Under the law, Ken would have to establish paternity through a DNA test.

Of course, Ryan and Gypsy haven't finalized their divorce yet ... and, the bun's already in the oven -- so even if GRB and RA got the divorce finalized today, they'd still need to go through the paternity test.

This isn't difficult to do -- all 3 parties (Gypsy, Ken and Ryan) just need to sign a document and Gypsy and Ken need to provide a paternity test proving he's the father ... and it'll all be sorted out.

Sources close to Gypsy tell TMZ ... Gypsy's aware of the law and believes moving the divorce along will help matters, though she knows they'll also have to perform a DNA test.

ICYMI ... Gypsy and Ken announced the pregnancy earlier this week, posting photos with ultrasound pics of their soon-to-be newborn. According to her caption, she's due in January.

We're told she gave Anderson a heads-up before announcing the pregnancy news which came as a huge blow to him because she says he still had eyes on a possible reconciliation.