This is a good question to explore, particularly if you are trying (and struggling) to conceive.
Sometimes even when you want a baby so badly on a conscious level, there can still be unconscious resistance to it. The unconscious mind is extraordinarily powerful and typically has the final say in terms of what we are able to manifest on a physical level.
Let's say that the messages you received growing up (either from your own mother, or culturally) were about how difficult it is to be a mother. How you'll never have time for yourself, or you won't be able to balance your family and career, or that it will mean sacrificing your relationship with your spouse. Those messages, if unexplored and unprocessed will become internalized.
Often the messages that we internalize aren't ever spoken aloud. Maybe it came from seeing your mom always putting herself last. Perhaps it was clear that her needs were never met. Maybe she never fully bloomed in her own life or career because she was too busy caring for the entire family. These messages become part of our unconscious too and can have profound impacts when consciously we are ready to move forward in certain areas of our lives.
There are several modalities that can help explore and break through this type of unconscious conditioning and resistance. The one I use most frequently in my practice is NET (neuro emotional technique). As we explore and release these unexamined parts of ourselves we are able to transform and heal on a deeper level.