No need to tell them now. It is illegal for them to use this in a hiring decision unless you have a physical strenuous job and are asked upfront if anything will prevent you from lifting 75 pounds or standing all day, and that is a question they must also ask male employees.
They are not allowed to ask any medical questions at all. They can't ask if you're diabetic or arthritic or bipolar or pregnant.
You were right not to add it as a discussion topic during interviews. It does not belong there. If their opinion would be clouded, that's their issues, and it's exactly why it became illegal to discuss it!
You tell them when you are getting ready to take a parental leave, and you give them a month's notice or so so that task shifting and temporary retraining can be done. If something should happen and you need to go on bed rest, it's the same as if you were to fall and have a back issue, or any other unforeseen condition. Then you tell them - when they need to know and when it affects your ability to do your job or their ability to schedule temp help. You do not need to apologize for this or make excuses for it.
When the time comes, then you tell your direct supervisor first, not the old boss. The recruiter is out of the picture. The only reason to tell them now is if you think it would affect their decision about hiring you - but women have fought for 50 years for it NOT to matter, so that should be your position! Telling them now is virtually saying that you think they might rescind their offer and discriminate. They'll probably take that as insulting. So you take the high road, the professional road, and assume that they will too. It's no different than if someone gets an offer and then gets pregnant 1 week or 1 month or 1 year later. It is not a factor in the hiring decision, and shouldn't be.
If you have to go into the office once in a while for a meeting, when you're obviously showing, someone will probably comment. Then you can address it sooner if necessary. But the discussion should be about when you would be taking a leave, not whether you should have told them earlier or whether you feel badly for not mentioning it. Don't be sheepish or embarrassed. Be totally professional. Once you are employed, you will probably get an employee handbook which will detail the policies and procedures for all kinds of things, from vacation to parental leave to sick time.