Though they aren't known for in general for their planning, forethought or constructive communication, Ana and Christian did discuss the subject of children before, amazingly enough, maybe in 50 Shades Darker and definitely earlier in 50 Shades Freed. In a rare moment of concord, they agreed that they wanted children, but not for a while.
Ana reacts to the news of her pregnancy with great alarm and anxiety. Here's a direct quote from page 409:
"Shit, shit, shit, shit, shit. How could I have let this [skipping a Depo shot] happen...again? I suddenly feel sick and offer a silent prayer. Please no. Please no. It's too soon. It's too soon. It's too soon."
Ana's abject terror makes little sense at first glance. Why yes, she has indeed missed some birth control shots, leading to her unplanned pregnancy. However, she lives in the U.S. in the present day, so she has safe and legal alternatives to raising a child that she does not want at this point. She can either have an abortion or give the child for adoption. Sure, she will incur unforeseen expenses, decisions and, especially in the case of adoption, long-range emotional effects, but that's okay. Life goes on. It's not the end of the world.
Or is it? What does Ana really fear? Let's dig a little deeper... In their aforementioned fight over surnames, Ana initially wants to keep hers, but capitulates to Christian after he throws a temper tantrum of insecurity and entitlement. She most likely fears that Christian will do something similar in this case. In fact, in the beginning of Chapter 20, on page 410, she says the following:
"I gape at Dr. Greene, my world collapsing around me. A baby. A baby. I don't want a baby...not yet. Fuck. And I know deep down that Christian is going to freak."
Ah hah -- she doesn't have an objection to being pregnant as much as she does to feeling Christian's wrath when she acts in a way that he doesn't approve of. I bet that Ana fears that Christian will browbeat her into keeping the baby. After all, he's intimidated her into compliance by staging shit fits before.
Well, in that case, no wonder Ana experiences terror when being told she's pregnant. Though marriage ideally provides love, safety and protection, she realizes on some level that she is not safe with her spouse. Ana swears up and down that she loves and trusts Christian, and I'm sure she does, but the violent extremity of her panic suggests that, on some primal, limbic level, she also fears for her life.
Anna believes that Christian might kill her.
This can't end well.