Paige is 3 and a half months old. A little small for her age as she was a month premature. The romper is in one piece and comes from a whole book of rompers with sweaters, hats and booties to match. I did not make the hat or booties for this outfit. The book is by Leisure Arts, called So-Bright Romper Sets. I got it on clearance from Wal-Mart. I used Simply Soft scraps that I had around for the color bands and it worked really well. It says the sets are for Newborn to 3 months, but the sweaters are made in 3-6 months to fit over the rompers. Hope this helps.
I think you can make pants for a baby, but let me tell you, it's a lot harder to change the diapers! The romper has buttons that you sew in the legs and at the back of the neck. That was the most time consuming part of the project. She liked it, and got lots of compliments on it.