Pregnancy doesn't prevent divorce, the grounds for divorce are:
Iowa Divorce Grounds
The only ground for Iowa divorce is as follows: the breakdown of the marriage relationship to the extent that the legitimate objects of matrimony have been destroyed and there remains no reasonable likelihood that the marriage can be preserved.
Now, the husband can ask for DNA test to dis/prove paternity, so be prepared to have the bio dad tested also because STBXX in not likely to want to pay child support for someone else's child. Wouldn't it be better to get the divorce first?