Disclaimer : Not mine! I just want to play with the characters ;)
General POV
Arizona knew that it would be difficult. Parenting a tiny human with the woman she loves would have been difficult too because God knows that having a child was the last thing on her mind since her brother's death but that… Parenting a tiny human with the woman she loves and Mark of all people, not only it is difficult, it is also causing her physical pain since the moment Calliope told her she was pregnant with his baby. The worse is the baby is not even here yet.
At first, she truly thought that it would be okay, difficult but okay. Having a child with Mark was like having two kids at once and come on, she looks after at least twenty of them at work so how bad could this get? The answer is pretty bad. Mark is definitely not a child, maybe a teenager, a horny smelly teenager who is nowhere you want him to be and everywhere you don't want him to be.
Maybe she is the problem because let's face it, Callie seems perfectly fine with how things work, with Mark being there for the baby, for her because obviously "he's the father", how can she forget it? Well, she can't, not with Mark here for breakfast because he slept in their bed the night before, not with Mark here when she gets back from work, not with Callie calling Mark every time she gets a pregnancy scare, or when the baby's kicking. All things happening to Callie is an excuse to call Mark, even a simple foot massage. Each decision, even the smaller ones, are a fighting because M. "I'm the father" can't stand being in the minority, stating we're against him every time Callie and she agree on something for the few times that happened.
The only possible thing to do in such situation is of course talking to Callie but after the Africa fiasco, she's scared she'll lose Callie forever because Mark was always there for her, and who is she to make her choose between her baby daddy and her girlfriend? No one, not since she chooses her career on top of the love of her life. She truly thinks she's lucky that Callie accepted to take her back. She can't let herself lose her, not another time. So, she just needs to suck it up for what? The next 18 years? She could do that, she has to.