Policies regarding sudden resignation of women employees

Greetings! My friend is an HR in a startup company. She has been put in an unexpected situation where many women employees are putting their papers out-of-the-blue on the grounds of being pregnant and are not ready to serve the notice period (which is for 3 months). My friend does understand that it is difficult to work for 3 months during pregnancy but the employees are not ready for a notice buy out and are asking for their experience letter and resignation acceptance from the company. I just wanted to find out if there is any workaround for this situation or any policy that can be effectuated for such situations since the company will have to search for new employees to replace the women who are quitting suddenly? What policy should be incorporated by the company to ensure that these women at least serve a minimum notice period until the new employee joins in (they need to transfer knowledge to their counterpart) and that the projects are not affected because of the exit of women employees. Thank you.