If, after your 16 weeks maternity leave, you feel you are not ready to resume your work activities, you can take extended maternity leave, called ´Excedencia por cuidar de hijos´ in Spanish. Unpaid leave, maximally until your child turns 3.
It doesn´t depend on the collaboration of your employer whether you can take this leave or not, it´s a right as established in the law, but you do need to give him advance notice, of course, with a minimum of 15 days, unless your Convenio stipulates a different advance notice period, so please check.
As you will not be working, you will not be entitled to the paid ´absence for feeding´ that working mothers could take in bulk if they desire, following their regular maternity leave. Nor will you build up any holidays, or be entitled to extra payments (13th month in December for instance). You will be entitled to any formation the company organises in your absence and should be invited to join, especially if relevant to your re-incorporation.
During the first year of this extended maternity leave, your employer is obliged to keep your job open for when you return, after that, he is not, but if your own job has been filled by another worker, he has to offer you a similar, equivalent job. This does not depend on the company having any vacancies or not, it´s your entitlement.
This ´job reserve´ period extends to 15 month if the mother is part of a ´familia numerosa – general category´ and 18 months if part of a ´familia numerosa – special category´.
Source and reference for your employer: The Statute for Employees, article 46.3.