There is not really enough research on cause and effect in cases of bicornuate uterus to say exactly what the chance is of having an early delivery. There is said to be a chance of miscarriage due to the baby implanting on the septum, and a chance of early delivery because of restricted space. But it all depends on where the baby implants, and on the exact shape of your uterus and whether the baby pushes the septum aside, and on how big your septum is. The research is sketchy (I have seen some that had 20 people in their sample group) and the sample of people to study is usually selected by picking women who have had a problem in their pregnancy, which means it is not that useful for assessing risks in the general population. Lots of women with uterine septums have babies, and go through live not even knowing they have a septum. Try not to worry, but stay alert to unusual symptoms as all pregnant women do.
I have septate uterus and was told about risk of premature delivery with my first baby, and carried her full term but had a c-section at 39+2 weeks cause she was breach, depends on exact shape of the uterus if there's enough space, also my ob told me he thinks I can be low on fluid around the baby so I had a couple more sonograms at the end of pregnancy to be sure there's enough fluid.