The Syrian Observatory for Human Rights said a jihadist from Al-Nusra Front, which has changed its name to Jabhat Fateh al-Sham, was killed in the raid.
"Save the Children supported maternity hospital in #Idlib bombed, casualties reported - numbers unconfirmed," the charity tweeted.
It said the bomb hit the entrance to the hospital.
"This is the only hospital specialising in maternity and children in the northern western side of rural Idlib," a statement said.
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The Britain-based Observatory, quoting local sources, said a jihadist was killed in the raid that struck the hospital in the rebel-held town of Kafar Takharim, leaving it heavily damaged and barely operational.
"It was him that was targeted. He went to visit his wife who had just given birth when the bombing happened," said the monitoring group's head, Rami Abdel Rahman.
The bombing came a day after the Syrian branch of Al-Qaeda, Al-Nusra Front, said it was breaking ties with the global terror network and changing its name to Jabhat Fateh al-Sham.
"The hospital has six incubators for premature babies, and an outpatient clinic for supporting pregnant women and providing after delivery care," it said.
The Observatory did not specify if the raid was carried out by Syrian regime aircraft or warplanes of its Russian allies.
Amnesty International said the strike "appears to be part of a despicable pattern of unlawful attacks deliberately targeting medical facilities" across Syria and a "potential war crime".
"Deliberate attacks on hospitals and medical facilities are serious violations of the laws of war and can never be justified," said Amnesty's Philip Luther.
Last week, air raids struck four makeshift hospitals and a blood bank over a 24-hour period in the regime-besieged eastern sector of the city of Aleppo.