In a pregnant female, there is decreased requirement of the spinal anaesthetic agent because of all of the following except –
A smaller dose of local anaesthetic is nedded to produce the same level of neuraxial block in pregnant women compared with nonpregnant women.
The causes may be :-


Compression of the inferior vena cava by the pregnant uterus causes "distension or engorgment" of the epidural venous plexus. This decreases the volume of the epidural space and also the volume of the CSF per spinal segment. Therefore a given dose of epidural or intrathecal local anaesthetic will produce a greater degree of demiatomal involvement.
Pregnant women exhibit increase susceptibility to local anaesthetics.


Apex of lumbar lordosis cauded to thoracic kyphosis is reduced in supine position. This may enhance the cephalad spread of CSF.
Exaggerated lumbar lordosis
Decreased volume of subarachnoid space
Engorgement of epidural veins
Increased sensitivity of the nerves to anaesthetic agent