Syria Relief, the aid agency that manages the hospital in Kafer Takhareem, said those killed were relatives of patients.
"We're heartbroken & outraged by the attack on our partners' maternity hospital in #Syria. Children must be protected," Save the Children tweeted.
Save the Children said the bomb hit the entrance to the hospital, which is located in rural Idlib province.
It is not clear who carried out the attack.
The hospital is the biggest in the area, carrying out more than 300 deliveries a month and assisting over 1,350 women, according to Save the Children.
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