Antiphospholipid syndrome occurs when the immune system mistakenly creates antibodies that make the blood much more likely to clot. This can cause dangerous blood clots in the legs, kidneys, lungs and brain. In pregnant women, antiphospholipid syndrome also can result in miscarriage and stillbirth. By: https://www.mayoclinic.org/diseases-conditions/antiphospholipid-syndrome/symptoms-causes/syc-20355831#:~:text=Overview,%2C%20kidneys%2C%20lungs%20and%20brain.