Most places women give birth in un-natural position like lithotomy position leading to more tears of perineum and also difficult last phase of delivery
Women should birth in the position that feels safe and comfortable to them, and in which they are effectively pushing. Upright, side-lying, and other non-supine positions minimize genital trauma, allow for rotation and descent of the baby, promote shorter second-stage labors, and reduce need for episitiotomies, according to the evidence. Here are a couple of articles that discuss some of these factors.
J Perinat Educ. 2004 Spring;13(2):30-4.
#5: Non-Supine (e.g., Upright or Side-Lying) Positions for Birth.
On hands and knees, it gives relief for backpain and the diameter of the pelvis gets bigger, so the fetal caput can find its way better. Women who gave birth in this position often say that this kind of position is the most tolerable
In physiologic delivery women should birth in the position that feels comfortable. for reduce tears or episiotomy in lithotomy position we can use lubricant with or without massage.
The position for delivery is dependent on both the mother in labor and the midwife/doctor conducting the delivery. With experience maternal position would not be a problem. Both the attendant and the mother should discuss and agree on which position/s are comfortable for both.
I agree with others who have responded and believe that if the woman has not had a full epidural block that inhibits their movement, then any position that they are comfortable in is good- providing they are not flat on their back. If they are on the floor kneeling and don't want to move then it is up to the midwife to get in a position to assist their birth appropriately, using mirrors etc. I believe that a woman who trusts her midwife and can listen effectively to instructions of when to push and not to push, then a hands off approach to birth can succeed with minimal trauma. Water births are brilliant but needs an experienced midwife to guide them to birth with little interference. Ideally if a woman understands her body and can "centre" herself during labour and listen to what her body needs to do, then health professionals should e guided by them and not the other way around. Too many young women can't do this during labour while they are on their mobile phones or watching TV!
Women should be allowed to have it the way they want it as long as the baby would be safe and the risk of trauma to the mother is reduced. Advantages and disadvantages of her preference should be clearly explained to her as a professional responsibility.
It is interesting that some of the comments include the "operator" or those "conducting" the delivery. It is worth considering that the woman will birth the baby and have some instinct as to the best position. This said the medicalisation of birth often means that women conform to recumbent position which suits the medical model. Even when there is intervention such as CTG etc a more woman centred approach can be considered.