"Save the Children supported maternity hospital in #Idlib bombed, casualties reported - numbers unconfirmed," it tweeted.
The Syrian Observatory for Human Rights also said that the hospital in the rebel-held town of Kafar Takharim was heavily damaged and left barely operational.
The monitoring group did not specify if the raid was carried out by Syrian regime aircraft or warplanes of its Russian allies.
The bombing "hit the entrance to the hospital, which is the biggest in the area, serving over 1300 women monthly and carrying out over 300 deliveries a month", Save the Children said in an email.
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.