Rita and John's Marriage Certificate
The marriage license for Rita's first marriage to John Ackerman is shown briefly onscreen. It shows Rita's birthdate as 04/19/1989 and the marriage date as 08/16/1989. John's birthdate is shown in 1984. If this were true, Rita would have been approximately 4 months old at the time of marriage and John would have been about 5 years old. Since the marriage was known to Dexter to have taken place prior to Rita's second marriage and children with Paul, it's likely that neither her birthday or the first marriage date are realistic for the timeline of the show.