Young couple (wife is pregnant) goes to live in a village which turns out to be a coven of witches. Husband is recruited; the coven wants the baby and he helps them try to kill her and take the baby. She survives and he is killed. Final scene shows her in a park with the baby, and a young horse-mounted policeman helps her with the baby carriage. We realize he is also a witch because of the way he ties a knot -the witches all tie things with a special knot. Seen on tv in the early seventies.
This is Crowhaven Farm from 1970. From the IMDb synopsis:
Maggie discovers she's pregnant at some point, and the witches are after her baby. The movie is available in 5 parts on Youtube and in part 5 (around 12:30) you can see the final scene you've mentioned where the horse-mounted cop ties a knot on her baby's carriage in a park: