The idea of using a suction cup to extract the foetal head was introduced by Younge in 1706. The modern vacuum extractor was invented by Malmstrom in 1954.
On the European mainland the ventouse is the intervention of choice, however in the U.K. and U.S. forceps are preferred.
Where different sizes of cup are available, the largest cup is used according to the dilatation of the cervix.
The only force that can be applied is tension, not torsion, thus the operator relies on the shape of the birth canal to rotate the baby. The amount of tension is limited by the quality of the vacuum; it is relatively difficult to damage the baby by overpulling.
The baby's head is left with a "chignon" - an iatrogenic caput succedaneum. This may take as little as 2 hours or as long as 2 weeks to resolve. A chignon is associated with a higher incidence of neonatal jaundice.
The advantage of this technique is that it is less damaging to the maternal vaginal tissues.
This should NOT be considered a manual for undertaking a ventouse delivery, but as a brief summary to how/why such deliveries may be undertaken.
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