Sciatica during pregnancy comes about for two different reasons:
This is normal spinal alignment, with a forward sloping sacral angle of about 50 degrees
1. Irritation of the facet joints caused by increased arching of the lower back
During the final months of pregnancy the increasing size and weight of the abdomen pulls the lumbar spine forward into a deepening arch that alters body alignment and postural forces acting through the base of the spine. This dramatically alters the load-sharing across the lower lumbar segments, in effect making the facet joints at L4 and L5 bear much more that the optimal 16% of load. This is a very common cause of sciatica during pregnancy.
By late pregnancy the sacral angle is much increased with much greater hollowing of the lumbar curve
Since the facet joints (unlike the discs, which should bear most of the load) are highly innervated or 'wired for pain' it is quite common to develop pain in the buttock, down the back of the thigh and into the calf. This form of sciatica during pregnancy is usually a referred pain, where pain is felt in the leg although the inflammation itself is of the facet joint in the back.
2. Direct squashing trauma to the sciatic nerve inside the pelvis
Pregnancy and sciatica often go hand in glove, even without a pre-existing lower back problem. Sciatica during pregnancy is commonly caused by the the bulk of the baby bearing down and literally squashing the sciatic nerve inside the pelvis.
Sciatica during pregnancy often raises fears of a clot in the calf, although this is rarely the case. From the graphic above you can see that the trunk of the sciatic nerve, made up of a bundle of nerve roots emerging from the spine at the lower five lumbar levels, lies right across the bony bowl of the inside of the pelvis. It is situated directly under the growing baby. For this reason, sciatic pain is more common towards the end of term as the baby gets larger.
During the final stages of a pregnancy the baby (usually) turns upside down and moves lower in the abdomen, as the head engages in the pelvis. This makes the mother more comfortable as the biomechanics become easier. It also gives the diaphragm more room to move, easing both breathing and digestion (you almost feel not pregnant again!), not to mention sleeping! However the hard presence of the baby's skull can bring pressure to bear against the inner bowl of the pelvis and this can cause a nasty case of sciatica during pregnancy.
Pregnancy sciatica is more likely to be related to a pre-existing lower back problem, since the nerve itself retains a degree of sensitisation and is more easily annoyed by the weight of the baby pressing upon it. Alternatively, there may be no sciatica during pregnancy, only after delivery which indicates the sciatic nerve was injured during the birth Sciatic pain after labour is again quite common and directly attributable to the massive forces generated by the expulsive powers of the uterus. It is more likely to develop after a prolonged labour.
This right-angled stretch is an ideal way to relieve sciatica during pregnancy. It takes the pressure of the baby off the sciatic nerve and also helps relieve the pain and discomfort of swollen ankles. Most important here to read Sarah Key's 7 of the best sciatica stretching exercises during pregnancy. Better still, watch the video on everything you need to know about sciatica and what to do.
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