Although lumbar disc herniation is reported in only one in
approximately 10,000 pregnancies,3 and fewer than 2% of lumbar disc
herniations are estimated to result in cauda equina syndrome with
severe and/or progressive neurologic deficits, physicians should be
aware that these conditions can in fact occur during pregnancy. They
constitute an emergency and should be diagnosed and treated prompty
without considering the stage of gestation. A poor prognosis is
correlated with the length of time from onset to surgical
decompression and the intensity of saddle anesthesia, a symptom of
cauda equina syndrome.4 In the present case, the treating physician
was reluctant to submit the patient to diagnostic MRI because the
onset of symptoms had occurred at approximately 16 weeks of gestation.
The literature clearly demonstrates that pregnancy at any stage is not
a contraindication to MRI.8 – 10 In this case, according to the
symptoms and signs, pelvic MRI was unnecessary however, the spine MRI
was the definitive diagnostic procedure for establishing lumbar disc
disease.

http://www.ams.ac.ir/AIM/0362/0362146.htm

You can also ask your obstetrician about pregnancy support products made specifically for sciatica and lower back pain.