Update 2019. With the new decree on equal rights for fathers and mothers with regards to ´parental leave´(in force since 1.4.2019), the situation for 2-parent families to whom twins or triplets are born or who adopt them, has changed, as the extra leave has to be shared by both, so in practice, it will be one week extra for each instead of 2 for the mother.
When you are working on a contract and pregnant with twins or triplets, you are entitled to 2 weeks extra maternity leave, so 18 instead of 16. For triplets, total of 20 weeks.
And if the twins or triplets are born prematurely or for whatever other reason have to remain in hospital after the birth and they have to be hospitalised for longer than 7 days the maternity leave will be amplified with the days they have to stay in hospital, with a maximum of 13 weeks.
Here´s a link to the Seguridad Social webpage, in English.
The one hour/day (breast)feeding time a monther is allowed, with pay, until the child is 9 months old, is doubled for twins, or tripled for triplets. You can also take this leave ´in bulk´ but you need to inform your employer with at least 15 days notice if you wish to do so.
You are also entitled to an extra one-off payment, to help you deal with the extra costs involved in having twins or triplets.
If the links do not show in English, please use the option to the top right of the page to change the language.
Best make an appointment with the Seguridad Social beforehand, on how and when to inform them, once the twins or triplets are born, so you´ll be prepared.