Are you leaving a career, or just a job? Will you be able to pick up where you left off? What will it take to keep your skills and industry knowledge current?
Talk to colleagues who have taken time off and returned to careers in your field. How long did they stay out? How did they get back in?
If you do choose to stay home after maternity leave, stay in touch with your coworkers, and keep up on all the happenings in your field through online networking, industry news sites and professional associations. That way, when you're ready to go back, you'll be prepared.