Doctors at the Ruber Clinic brought "the condition under control," a royal spokesman told Hellomagazine.com.
Letizia, 33, is eight months pregnant. After getting the all-clear from the hospital, her husband Crown Prince Felipe took her back to Zarzuela Palace.
If the royal baby is a girl, she stands to become the first Spanish princess to have the same rights of accession as a male heir, Hellomagazine.com said.
Spain's ruling Socialist party is reportedly preparing a constitutional amendment changing the system that favors males.