Syrian raids hit a maternity hospital in the village of Al-Tah as well as a medical outpost in the town of Khan Sheikhun, the Observatory head Rami Abdel Rahman told AFP.
Two other hospitals in the Idlib province were also hit by heavy airstrikes by regime and Russian jets.
"The raids on the maternity hospital in Al-Tah killed an elderly woman who was an employee of the hospital, in the first civilian death in air strikes in Idlib province since May," Abdel Rahman said.
"The attack and airstrikes are the most intense in the area since the de-escalation zone was announced in May," Abdel Rahman told AFP.
According to the medical charity UOSSM, which has provided equipment to the maternity hospital, the woman killed was a cleaner in the facility and other medical staff were wounded.
In a statement distributed to journalists, the charity said two sets of strikes hit the maternity hospital, setting fuel storage tanks on fire and leaving its incubator room "totally destroyed."
UOSSM slammed the strikes and warned that "systematic attacks on hospitals are back despite de-escalation deals."
The Idlib province and some adjacent areas form one of four so-called de-escalation zones agreed in May by rebel backer Turkey and government allies Russia and Iran.
A recent jihadist offensive has led to the heavy regime and Russian bombardment.
Jihadist factions led by a former Al-Qaeda affiliate, which are not included in the de-escalation deal, launched a fierce assault on a string of regime-held villages along the border between Idlib and neighbouring Hama province.
The clashes have killed at least 19 regime fighters and militia, the Observatory said.
Twelve jihadists and two medics who had been working with them also died.
State news agency SANA also reported heavy clashes and said that government warplanes had hit the opposing forces' supply lines.
Agencies contributed to this report.