First of all congratulations! I wish you an easy pregnancy and want to assure you that it is possible even with diabetes. I just had my first child 3 weeks ago and have been diabetic for 13 years. Thankfully he is perfectly healthy and so am I!
Regarding Splenda, I would be cautious using it since its a chemically processed sugar substitute and is so new to the market that real research on the use of it during pregnancy is probably not be reliable at this point.
If you are look for a calorie-free/ sugar- free alternative, try using Stevia. It is a natural sweetener and is plant derived. You can find it in leaf form (dried) or as a powder, in health food stores like whole foods. It is also available online on sites such as Luckyvitamin.com.
Let me know if you have any questions about it and good luck with your pregnancy!!!
All the best,