Syria Relief, the aid agency that manages the hospital, said those killed were relatives of patients.
Save the Children said the bomb, from an air strike, hit the entrance to the hospital in rural Idlib province.
Images show part of the building in Kafer Takhareem destroyed. It is not clear who carried out the attack.
Save the Children says the hospital is the biggest in the area, carrying out more than 300 deliveries a month.
UK-based monitoring group the Syrian Observatory for Human Rights said a nearby civil defence building was also damaged.
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