Eduardo Vela, 85, a former gynaecologist at the now-defunct San Ramon clinic in Madrid, went to hospital after "an episode of pain and dizziness overnight," his lawyer, Rafael Casas, said.
When he took the stand on Tuesday, Vela appeared frail and spoke in a weak mumbling voice.
The court was not able to say when the trial would resume. It can be delayed by up to 30 working days.
"We had to suspend (the trial) and we hope to have more information" to know if the court could resume the trial next week," one of the judges hearing the case, Maria Luisa Aparicio, told the court.
- Pain, shock and anger: Two of Spain's 'stolen babies' speak out
- Spain's first Franco-era 'stolen babies' trial begins
- Mother finds Spain 'stolen baby' 44 years on
Demonstrators holding baby dolls and placards reading "Human rights for stolen babies" as a doctor goes on trial. Photo: AFP
Vela is accused of having in 1969 taken Ines Madrigal, now aged 49, from her biological mother, and giving her to another woman who raised her and was falsely certified as her birth mother.
Prosecutors are seeking an 11-year jail term for falsifying official documents, illegal adoption, unlawful detention and certifying a non-existent birth.
During questioning in court on Tuesday's opening day, Vela said he could not remember details of how the San Ramon clinic, which he ran for 20 years up to 1982, operated and that the signature on Madrigal's birth certificate was not his.
In a dark and often overlooked chapter of General Francisco Franco's 1939-75 dictatorship, the newborns of some left-wing opponents of the regime, or unmarried or poor couples, were removed from their mothers and adopted.
New mothers were frequently told their babies had died suddenly within hours of birth and the hospital had taken care of their burials when in fact they were given or sold to another family.
Baby stealing began in the 1950's after Franco came to power following the 1936-39 civil war pitting left-wing Republicans against conservative Nationalists loyal to the general, as part of an effort to purge Spain of
It was expanded to take newborns from poor families as well as illegitimate babies.
The system outlived Franco's death in 1975 and carried on as an illegal baby trafficking network until 1987 when a new law that regulated adoption more tightly was introduced.
Campaigners estimate tens of thousands of babies may have been stolen from their parents over the decades.