Well, May is a good time to get a puppy (nice weather makes walks and toilet training easier), as Ashbee says, you will have time to house train it before the baby comes and by the time the baby is born your pup will be around 6 months so starting to settle down a bit.
I would consider the breed though - is it a large breed dog? What are its excersize and training needs etc. Collies are very high maintenance dogs and do best with lots of exercise and mental stimulation (for example), so personally I wouldn't want the added responsibility of a young Collie with a new baby. Same goes for other 'working breed' dogs like Springer Spaniels etc. As they could easily start acting out, showing destructive behaviour etc if they don't get enough attention.
You also need to consider how easy it will be for you to walk it, especially in winter with a newborn. If your husband/partner can walk it, or you are happy to pay a dog walker, that might work.
Generally speaking if it were a smaller, calm breed of dog I'd definitely say go for it. If it's a larger or more 'high needs' breed then maybe consider backing out unless you know you have the ability to give it what it needs as well as look after your baby.
I have 2 dogs but I have had them a long time, Buster is 4 and a half, got him in November 2011 aged 9 weeks, he's a medium sized mix breed (part labrador and lab sized), The other dog is a long haired chihuahua who is 5 and a half, I got him in April 2012 and he was nearly 2 then.
I will see how I go when the baby is due, if I'm struggling to get out with them I'll employ a dog walker. Although as I'm due in July it'll probably be easier to get out then, I can see it bothering me more in the winter when it's wet and cold and windy and I don't want to take the baby out in bad weather to walk them, that's when I'll probably pay someone else to walk them for me lol.