By the time your child is 4, he/she (sonogram, eh?) will be too heavy for a child seat, so you might as well make the investment now and get more years of use out of it. My child was too heavy by 4 even though she is a lightweight and the seat was rated to 40lbs. It *looked* wobbly to people even if it was technically bolted on.
Plus, if your child is young enough to fit in a bike seat, you will definitely need emergency supplies (drink, milk, diapers, change of clothes -- for both of you, heh-heh). None of that is easy to carry with a seat ("Here kid, hold this bag, OK?").
If you buy a trailer, consider getting one of the new ones that makes room for the *back* of the child's helmet.
If you have hills, make sure your breaks are in good order. When you are breaking, you will have to stop not just your own mass but that of the child and trailer (about 70lbs).
About visibility, I have this theory. Car drivers put everything in two categories: "Dangerous", and "Mostly Harmless". They prompty forget anything that they have labeled as "Mostly Harmless". Yellow jersies do not label you as "Dangerous". Hauling a trailer with a child does. Like other bikers have suggested, I've thought of putting a large doll in the trailer so that people will cut me more slack. Or wearing an Osama Bin Laden t-shirt. Something to make them look, fear, and remember.
Good luck! Be sure to report back in 4 years when you've started to get regular sleep again.
