You can start a divorce when you're pregnant, but you won't be able to finalize the divorce until the baby is born or the pregnancy otherwise ends. Texas Judges won't finalize your divorce as long as you're still pregnant because they want to establish child support obligations. They're going to act on what's in the best interest of the child.
You can always file for divorce and be ready to finalize it soon after the baby is born. Keep in mind that you'll still have to wait 60 days.