It could be the separation anxiety. The kid is 24*7 in front of your eyes, your priority- rather- the only work you entrust upon yourself when you take this break. And even the thought of not being able to see the kid as much as one has been doing sends shivers down the spine.
It could also be the anxiety towards the kids needs not being met or care not as much as a mother would do. Giving him time to evolve, to learn, to stimulate his gray cells, to teach him new things actively- all might not match the existing scale.
It could also be the anxiety towards the workplace, work and the outcome. The confidence in your skillset, the added stress, the exhaustion that might set in, the quality and the outcome of the work that one does- everything adds up to mount the challenge.
It could also be the doubt about losing the personal-professional balance. The stress and tiredness reaching the kid, and the upbringing and devoting time to kid leading to sub-par quality of work -both are dangerous and dreaded.
All in all, it is a challenge. Overcoming mental blocks, preparing physically for the time to come and "taking this step" will only solve the riddle and devise new ingenious solutions.