A birth stool is a great addition to your options when having an active birth. It's probably one of the oldest birth tools around, as even the Goddess Isis is depicted in ancient carvings as giving birth on a birth chair, or stool.

 In fact, the modern origin of giving birth in the Lithotomy Position (ie. flat on back, legs apart) was the result of a certain King who found it to be the optimal position for his mistresses to give birth in for his viewing pleasure. As the role of birth moved from being the realm of women and midwives to that of the male doctor it was used for the doctors benefit only.

Before this, all depictions of women giving birth have been ones of them in upright positions that normally involve squatting or kneeling. Not only does this position allow some help from gravity but prevents the full force of the babies head being directly on the perineum and therefore reduces the chance of severe tearing.

A birth stool helps the woman maintain a squatting position, without having to support all her weight on her legs. Her partner or Doula can also sit behind her in order to provide something for her to lean against and rest between contractions.

You can hire your own birth stool from BirthWorks - http://www.birthworks.co.za .
Here is a great video that shows how a birth stool can be used during birth: