Maternity leave is always a tricky issue - not just because of the federal Pregnancy Discrimination Act (which applies to businesses with 15 or more employees), or California's Pregnancy Disability Leave (which applies to all businesses).
The additional issue is 'reasonable accommodation'. While you as a manager must treat pregnant women equally, it is advisable that you treat them with utmost courtesy and accommodate their needs as much as practicable.
Although each situation is always difficult, consistency in treating your employees is essential.
This article, via CePro, is a good start.
And get your policy in writing right way!