SPD is your pubic bone not flexing causing a lot of discomfort. This is obviously in the front above your vagina. Sciatica runs down your back, through your butt, and down your leg causing, for some, extreme pain. Really it’s just the different in where the pain is coming from and what is causing it. Sciatica can be treated by stretching the muscles of your back/hips and seeing a chiro whereas SPD is an issue with the cartilage of your symphysis pubis.