Beyond the normal fears of worrying that the baby might not be okay physically, one of my greatest fears about having a child was that I would not know when she was sick, when to take her to the doctor, when to feed her, what to feed her, how to feed her, etc. I was concerned that I would do things like ignore her symptoms when she was sick and then she'd be suffering and I would just let her without taking her to the doctor until things were really bad.
Or I was concerned that she'd be hungry for solid food when she got older and I'd forget to feed her. Or that I would forget to change her or put her in new clothes and she'd be in gross dirty ones for days. I was concerned I wouldn't remember to bathe her and the list goes on. So I read every parenting book I could and took classes to learn how to care for a baby because I felt completely unprepared for actually caring for another human being.
The classes and books definitely helped me be prepared, but, as it turns out, it wasn't quite as difficult to remember to do all these things as I thought. For one thing, I am a creature of habit who thrives on routine. Once we worked out a routine for bath, bed time, changing clothes and diapers, food, etc. it became easier to remember to do things. Also, having a routine and structure helped me to know what my baby needed. I could deduct since she hadn't eaten in awhile that she was probably hungry when she cried. If she cried at a strange time I could assume she might be sick.
It turns out that mothering and remembering to care for your child is quite instinctual. It's the first thing I think about more often than not. As the routines have changed over time - more time between feedings, switching over to solid food instead of milk - it did take some time to get used to the new routine. For example, when we started to switch her to eating more solid foods we had to really experiment with when, how much, and what to feed her. Now, though, we have a breakfast, snack, lunch, snack, and dinner routine. But it took a month or so to get into it (and to remember to do it!).
I'm glad to know that my fears were mostly unfounded. It's pretty easy to tell when your child is sick. It's obvious when she gets red spots all over her body that weren't there before or when she feels warmer than usual or has a cold and snot is pouring out of her nose. And the advice nurses are so helpful at helping you know what to do in those situations--whether you need to bring her in, what to watch for. So you don't have to know everything. We still have a routine and I still read about parenting, but now it's more specific to what we are experience in this new stage of her life and I feel so much more confident that I know what she needs, since now I know my child pretty well and what she is most likely crying about.