How do I tell her that I'm having a child?

So tell her that. Don't let this sit for longer than a day or two, or itr will just get harder to say it, and you will look worse for having kept it from her.

You weren't together, and you hadn't been together for a longtime when you had sex with the other woman. The problem is, you didn't use birth control. And now here you are.

Do you know for a fact the pregnant one is keeping the pregnancy?

If she carried it to full term, does she intend to keep it or adopt the child out?

Before you talk to your original GF, you need to deal with the pregnant one, find out what she intends to do, and whether you are looking at becoming a father. Clealry, that is going to matter.

You won't be able to do anything about your standing as the father, until there IS a child born. Until then, you legally have no involvement, no say in the matter, but also no responsibility. Once the baby is born, however,. if she's keeping it, you need to go to court, submit DNA test results, and then make arrangements for custody, a parenting plan, and child support.

Clearly this is going to make a huge difference in your life, if the pregnancy becomes a child of yours. Talk to the mother-to-be and find out where you may stand, and then you will have an idea of what to tell the other girl about your future. If you're lucky and things work out with her, there's no reason you can't be a solid father to the child, and still have a decent relationship with the girl you actually love. Don't marry someone you slept with once, just because she got pregnant. You'll be a better father, if you have a solid relationship with someone you actually know. That is, if you're lucky and the one you want to keep can handle this.

Now, go get some birth control, and don't believe people who say they're on it.

She knows after 3 weeks that she is pregnant? I think you are being duped. Or maybe you're just an idiot.

Don’t dally. Tell her. If she doesn’t know you slept with someone after your break up, start with that. Add that the girl is now 3 weeks pregnant.

Don’t lie to her about anything, including lies of omission. They are just as bad, and just as trust-busting.

If she doesn’t decide immediately to break up with you, she will want to know what you and the girl intend to do about the pregnancy. If you don’t know, tell her that. No BS.

The sooner you tell her the better it will be. As soon as you start hiding it, the worse it will be.