On this page of the Assassin's Creed Valhalla guide, you'll find a detailed walkthrough of The Goddess of Birth side quest.
Reward for completing the mission: 1200 XP.
In the place marked in the picture above, you will meet a woman who has just gone into labor. The woman will ask you to help her get to a nearby hill.
Get on the horse, and the woman will take a seat behind you. Now you need to go to the temple ruins shown in the above picture. After a short cut-scene the mission will end.