andrea9255,
If you are asking as to whether or not having an L4-5 fusion will affect your ability to carry a baby to term, the answer is no. It will not affect a pregnancy.
There are many women who have had one level fusions, and for that matter,
extensive scoliosis fusions, that have carried babies to term without difficulty, several babies in many cases.
You will be at higher risk for back pain during the pregnancy, as most women will have some back discomfort somewhere along the way. But, usually it is manageable.
You should speak with your orthopedic spine surgeon, but it should not be a problem from the back standpoint. Again, women who have had essentially their whole spine fused for severe scoliosis when they were an adolescent, have been able to carry and delivery healthy babies.
Good luck.