Boston — The figures are becoming familiar by now: * More than half of the women in the United States are part of today's paid work force.
* Eighty percent of these 46 million working women are in their prime childbearing years.
* More than half of these women are mothers of preschool children.
What is less often heard, however, is the story behind the statistics: How are so many women combining - or hoping to combine - work and motherhood?
For most, careful advance planning helps to make approaching parenthood an especially happy time. Although pregnancy traditionally has been considered a disability at the workplace, no different from other medical problems, the growing numbers of pregnant working women are prompting new options for individualized maternity leaves.
When her son was born three years ago, for example, Meg Wheatley found herself in an enviable position. She worked for a small firm, and her boss was a woman who had shared the care of her own child with her husband. Ms. Wheatley was able to arrange for a nine-week maternity leave, with the added benefit of being able to bring her son to the office when she returned to her job.
''My situation was quite easy, and the leave was worked out in a straightforward manner,'' Ms. Wheatley notes. ''But that seldom happens. Most corporations tend to do only the minimal when it comes to maternity leaves, and most women still find that they have to negotiate their own leave provisions.''
A specialist in human resources management, Ms. Wheatley does research and consulting for corporations, universities, and private foundations. She also is co-author of the recently published ''Managing Your Maternity Leave'' (Houghton Mifflin, Boston), a practical guidebook for women who want to take a carefully planned first step from career to parenthood and back again.
''The key to making maternity leave work is to present a sensible plan to your employer,'' says Ms. Wheatley's co-author, Marcie Schorr Hirsch. ''You can't present an employer with a problem - you've got to give at least one good alternative for covering your work for the time you'll be away.''
Ms. Hirsch did just that when she applied for maternity leave from her teaching duties at Brandeis University a year ago. She not only came up with a plan to combine her leave with additional time off to finish a doctoral thesis, but also was ready with a carefully selected replacement.
With a wealth of personal and professional experience to draw on, authors Wheatley and Hirsch explore three stages of maternity leave in their book: planning the leave, staying visible and involved while on leave, and reentering the office. They offer valuable advice, including the following:
* Before announcing your pregnancy, resolve your own questions about your leave. If you're in line for a raise or promotion and think your announcement might affect a decision, wait a while. When you do announce, be aware of office protocol and the order in which you break your news.
* Know your legal rights: The 1978 Pregnancy Disability Act basically extends to pregnant women the same benefits already in place for other workers.
* In negotiating your leave, assess your worth and individual talents. Consider carefully the tone of your request.
* Listen for your boss's unspoken concerns and try to talk them out.
* Don't accept a ''suspended animation'' of your job, putting work on hold until you return.
* Negotiate a minimum of three months time off. Evidence now shows that returning to work after six weeks is not good for you, your baby, or your organization.
* Begin to train your replacement two to three months before the approximate date of your departure, and plan to stop work two weeks before your expected delivery date.
* While you're on leave, arrange for a regular means of communication and stay on routing lists for office memos.