I fell when pregnant, on my back. Will baby be ok>12 wk?

Yes. The baby is very well protected by your pelvic bones at this stage of pregnancy. A fall on your back would do nothing to "jar looses" a normal pregnancy.
Most likely. It is best to see your ob/gyn or family doctor to be sure. An ultrasound can ensure the baby is doing fine. Babies are resilient and the uterus and amniotic fluid serve as a buffer that is very protective. Any concerns at all, yes, see your doctor. And all this depends on how far you fell and how hard...